By on June 17, 2013

2012 Jeep Patriot Latitude, Exterior, trail rated badge, Photography by Alex L. Dykes

Four wheel drive, all wheel drive, 4WD, AWD, full-time, part-time, 4Hi, 4Lo, 4×4. There are many names and just as many ways of motivating every wheel a vehicle has on the ground. What’s the difference between four-wheel-drive and all-wheel-drive? In one word: Marketing. Want to know more? Click past the jump as we dive in the most controversial topic since “Dodge vs Chevy.”

Motivating four wheels in a car isn’t new, we’ve been doing it for over 110 years. If you thought this was a recent affectation, you’re not crazy. Over the last 30 years there has been an explosion in the number of vehicles powering a quartet of tires. There has also been a similar explosion in the number of ways we power four wheels. At the same time the way systems are designed, marketed and used have converged and with them the terms AWD and 4WD have have practically merged. Of course, the SAE does have a definition “an all-wheel-drive vehicle is one that has an on-demand feature that occasionally sends power to the non-primary powered wheels.” But what that means has changed a great deal over time.

The Good Old Days

Let’s set the way-back-machine to 1970. Trucks and “Jeeps” had 4WD aka 4×4 systems. The system had to be engaged manually once you were on a loose surface because they “locked” the inputs of the front and rear differentials together making turning difficult on high traction surfaces. Engaging AWD on pavement could result in damage to the systems, or at the very least strange road manners. These systems were found on vehicles that would otherwise be RWD like trucks and truck-based “things.” Frequently the transfer case featured a reduction gear for more severe situations. 1970 Land Rover Range Rover, picture courtesy of Land Rover

Then came the 1970 Land Rover Range Rover (above), the self-proclaimed “first mass-produced vehicle with full-time AWD.” (Note they didn’t call it 4WD until later.) The system used a lockable center differential that allowed the front and rear axles to spin at different rates on pavement allowing the system to be engaged at all times. The system was designed with off-roading in mind, so the transfer case had a low range like like the rugged truck based systems at the time in addition to the full-lock feature.

Then came the AMC Eagle. AMC jammed a new NP119 transfer case made by New Process Gear behind a Chrysler transmission. The unit featured a viscus coupling to the front axle that would allow power to flow to both axles simultaneously while still allowing them to turn at different rates. But this AMC wasn’t a truck, didn’t have a low-range and had an independent front suspension. Not knowing what to call it, AMC called it 4×4. So much for standards.

16 - 1989 Dodge Colt 4WD Down On The Junkyard - Picture Courtesy of Murilee Martin

Then Audi released the Quattro in 1980, but despite featuring a manually lockable center differential, Audi chose to call it “all-wheel-drive” or AWD. (Later Quattros were automatic.) The AWD vs 4WD differentiation was born. Soon everyone was getting into the four-wheel-motivation game but nobody agreed what to call the systems. In 1982 Fiat introduced the world to the first four-wheel-motivated vehicle with a transverse engine layout and a transaxle (the Fiat Panda 4×4). It was the start of a revolution. Some car companies followed Audi’s suit and referred to car systems as AWD while the  Toyota Tercel, Dodge Colt and others sported 4WD or 4×4 labels. This was the start of the “that’s not four-wheel-drive, that’s all-wheel-drive” argument.

By the ’90s SUVs started to roam the land. The box-on-frame creatures borrowed their drivetrains from  truck parts bins and brought with them 4×4 and 4WD monikers. (And a bevy of full-time and part-time systems.) Meanwhile, the proliferation of AWD systems exploded and we soon started seeing them in everything from Chrysler minivans to the Porsche 993. Despite the proliferation, the industry had more-or-less settled on calling longitudinal “truck” systems 4WD/4×4 and “car” systems (especially transverse systems) AWD.

2012 MINI Countryman, Exterior, rear, Photography Courtesy of Alex L. Dykes

The 21st Century

Crossovers happened. In truth the crossover was born in the 20th century, but the era of the “modern crossover” dawned within the last 15 years. In 1995, crossovers were a microscopic segment composed of jacked-up station wagons. By 2005 the non-truck utility vehicles accounted for more than 50% of the segment. At the dawn of the 2014 model year there are few “traditional” SUVs left, especially in the volume mid-sized segment. Those that remain account for a minority of sales.

Back to the marketing. Now, more than ever, the lines between truck and car are being blurred by marketing speak. Ford calls their Explorer AWD while Nissan is claiming the Pathfinder had 4WD and Chrysler says the Jeep Patriot is a 4×4. The truth is all three drivetrains operate on the same general design as that 1982 Fiat Panda: the transverse AWD system. The system Fiat called “4×4″ in the 1980s is now thought of as “AWD” by Fiat in this decade. What gives?

2012 Volvo S60 T6 AWD R-Design, Exterior, Photography Courtesy of Alex L Dykes

The Current State of Affairs

This brings us to the present. Now that we know the AWD vs 4WD vs 4×4 battle is a war of marketing speak, and we have a bit of history under our belts. Let’s talk about how AWD systems work. Why? Because it’s more important to know how the systems work than what they are called. Let’s go over them one by one. Since I’m not a graphic artist I’ll toss in a rough power-flow diagram to show how each system works.

Part time locking AWD System, Drawing Courtesy of Alex L. Dykes

Part time locking systems with a longitudinal layout

In the picture above we have a traditional “truck” system, the one that some people will call a “real 4×4.” There is no center differential so the system shouldn’t be used on-pavement because the front and rear axles cannot spin at different speeds. The system has to be engaged by the user in some manner, either with a lever or a button. Most systems use a chain drive to connect the front and rear axles so power flow is (in theory) locked 50/50 front/rear. If one rear wheel is freely spinning, the front wheels will still have grip. If one front wheel and one rear wheel freely spin, the vehicle won’t move. To solve that problem the systems usually include some form of locking or limited slip differential in the rear or both rear and front axles. The systems are typically very rugged and if the system employs fully-locking axles on the front and rear power is exactly 25/25/25/25 percent wheel to wheel and if three wheels lost traction the remaining wheel can consume all 100% of available power. Some systems integrate a low-ratio reduction gear into the transfer case.

Full time locking RWD based AWD System, Drawing Courtesy of Alex L. Dykes

Full time systems with a longitudinal layout

Based on the part-time systems we just talked about, Land Rover was the first company to use an existing idea to improve their new luxury off-roader and added a center differential after the reduction gear. This system became all the rage after AMC brought it to the mainstream in 1979 for the 1980 Eagle. These systems can take a variety of different forms. The “center differential” can be a simple open unit, a limited slip, a Torsen that apportions power unequally (i.e. 75% rear, 25% front unless slip occurs) or a simple viscous coupling which isn’t technically a differential at all. Each type of stem has benefits and drawbacks depending on your application. Open diffs apportion power equally, but if the front or rear wheels loose traction the car can’t send power to the other axle. Limited slip systems (including manual or auto-locking units) can connect the front and rear together, thus operating like a part-time system when the unit is fully engaged. If the system engaged on pavement however you can get a “binding” feel in tight turns. Torsen units are primarily used in performance oriented systems like high-performance variants of SUVs where you want added traction but a decidedly RWD bias.

You’ll find full-time systems of some description in the current Audi Q7, Jeep Grand Cherokee/Wrangler, Mercedes ML/GL/GLK/G, BMW X1/X3/X5/X6, GM’s full-size SUVs, Dodge Durango, Infiniti EX/FX, Land Rover LR4/Range Rover/Range Rover Sport, Lexus GX/LX, Nissan Armada, Porsche Cayenne, Volkswagen Touareg, Subaru Forester/Tribeca/Outback/XV, Toyota FJ/Land Cruiser/4 Runner/Sequoia.

Is that a long list? Yes. However that a complete list (insofar as I know) of SUVs currently sold on our shores with this type of a system. Why did I bother to list them all? Because it shows how few of this type of system there really are in the utility vehicle segment. Just a few years ago this number was higher and the market share of this system was higher still.

Subaru AWD Comparison, Courtesy of Subaru

Subaru and Audi you ask? Yes indeed. Audi’s longitudinal systems and Subaru’s AWD systems claim to be different or superior to the competition, but in reality the only difference is that they merge the center and front differentials into the transmission housing resulting in a space savings, but not necessarily a weight savings. (Mercedes claims 4Matic will take a scant 150lb toll in 2014, 50lbs lighter than Quattro.) This also means that the Subaru systems share design elements with traditional rugged body-on-frame SUVs, something that Subaru owners seem to rarely know but might want to brag about.

Front Wheel Drive Biased Transverse AWD System, Drawing Courtesy of Alex L. Dykes

Transverse engine based systems

British Motor Corporation popularized transverse engine front-wheel-drive systems in 1959 with the launch of the original Mini. The drivetrain layout has been so popular that the same basic design is used by 16 of the top 20 best-selling vehicles in America. (Everything but the full-size pickups on the top-20 list.) This drivetrain layout represented a challenge to AWD development, so it wasn’t until 1982 that Steyr-Daimler-Puch produced a four-wheel motivation system based on a transaxle. (For that Fiat Panda.)

What’s a transaxle? Excellent question. A transaxle is a transmission that integrates a front differential into its casing. That’s an important thing to keep in mind because the transaxle is why FWD layouts are preferred for fuel economy. In a transverse transaxle the power doesn’t have to “turn” 90 degrees to spin the front wheels. HOWEVER, in a transverse transaxle based AWD system, the power has to make two 90 degree turns on its way to the rear wheels. First power leaves the transmission, then heads to an angle gear which sends it to the back. Then power flows to the rear differential which turns power 90 degrees to the wheels. This is part of the reason that transverse full-time systems that always send power to the rear are [in general] just as efficient as longitudinal “RWD based” AWD systems. (This is why most of them disconnect the rear wheels whenever possible.)

V70R_AWD_System

While there are exceptions to this rule, 99% of transverse FWD systems have a fundamental difference from longitudinal systems because of the integrated front differential. Instead of creating a purpose built AWD transaxle, what car makers do is just extend the power output of the transmission (before the differential) out of the transmission case and into the angle gear that sends power to the back. (See the diagram above.) This means that the input to the front and rear differentials are tied, just like a part-time locking system that we discussed above. To keep the system from binding and improve fuel economy a clutch pack or a viscus coupling is placed between the angle gear and the rear differential. This allows the rear wheels to be uncoupled, but does nothing about the front wheels. Systems like this are incapable of sending more than 50% of the power to the rear unless the front wheels have zero traction. Acura’s SH-AWD system takes things one step further and uses an “acceleration device” aft of the clutch pack to make the rear wheels spin faster than the front wheels thereby giving the vehicle a slight rear “bias” even when the front wheels have traction.

Transverse systems come in many different flavors so it’s important to know what you’re buying before you sign on the line. Some systems on the market are “slip-and-grip” systems like the Honda CR-V which won’t lock the center clutch pack unless front wheel slip occurs. Then we have systems like the Ford Explorer which usually sends some power to the rear, locks the coupling during hard acceleration and varies it depending on vehicle dynamics. The Honda Ridgeline allows the center coupling to be locked in first gear while Jeep’s Patriot allows the center coupling to be almost fully locked at all speeds.

Jeeo Cherokee Front Wheel Drive Biased AWD System, Drawing Courtesy of Alex L. Dykes

2014 Jeep Cherokee Trailhawk

Perhaps the ultimate hybrid and head scratcher will be the 2014 Jeep Cherokee. Chrysler has yet to release complete details about the system, but what we can glean from the spec sheets and interviews is a system that meets all the criteria of a “traditional” 4×4/4WD system but has a functional layout similar to the systems “real” off-roaders would laugh and point at. We have a 9-speed automatic, nothing unusual there, but next we get something new for a transverse vehicle: a 56:1 (I4) or 47:1 (V6) reduction gear positioned after the transmission but infront of the differentials. (That’s lower than the Grand Cherokee and not too far off the 71:1 in the Wrangler.) Like the other systems, inputs for the front and rear diffs are mechanically tied and a clutch pack is used to connect or disconnect the rear axle from the transmission. Unlike many of the systems however, the 2014 Cherokee can fully lock the center coupling and Jeep tossed in an electronic locking rear differential.

I’ll close by posing a question: If my 2001 GMC Envoy (GMT360 SUV) with its two-speed transfer case and locking center differential can be considered a 4WD/4×4 vehicle. What is the Cherokee? AWD or 4WD? With 4-Low range and a locking rear differential it meets all the traditional requirements, but under the hood you’ll find a four-cylinder or V6 engine sitting sideways. This author’s humble opinion is that the name doesn’t matter if the vehicle does what you expect of it. That Cherokee? We’ll have to wait and see but I suspect it will be as capable as a Grand Cherokee mostly thanks to a substantially lighter curb weight.

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138 Comments on “Alphabet Soup: 4×4 vs 4WD vs AWD Where’s the Differential?...”


  • avatar
    NormSV650

    I’d take none of the above with rear wheel drive and a limited slip differential. From about 40 mph on up there wasn’t a truck that keep up on packed powder, or a couple of inches of dry stuff, with my C5 Corvette on Dunlop M3′s.

    Fwd on snows are the best two wheel drive with the weight of the engine and transmission over the wheels. Not sure if throwing a few hills changes things as Ohio is relatively flat.

    • 0 avatar
      Scoutdude

      Good steep hills make FWD suck.

      • 0 avatar
        mkirk

        The 93 and up Land Cruisers got the full time, locking center diff Transfer case but they also had a limited slip function. This set up chews up snow pretty well. I drove a Miata in Watertown NY (way, way upstate) with a Torsen year round. Even taking the ground clearance issues out of play (the diff would get snow packed under it and high center if the road wasn’t freshly plowed) there is no way I’d go back. I made it work but the first winter I had all season tires and getting it to turn often involved using the handbrake. The next winter I got snow tires and it worked, but no where close to the Cruiser which has never worn winter tires.

      • 0 avatar
        krhodes1

        Other than starting off from dead stop on a slippery LEVEL surface, FWD sucks in the snow in general. RWD with 50:50 balance is better. Steer with the wheel or your right foot as needed. Of course, AWD is the best, right up until the sense of false confidence it inspires causes you to slide so far off the road they don’t find you until Spring. :-) 2x the go, but the same amount of stop and turn.

        I’m also not wild about traditional limited slip diffs in the snow. Too much of a double-edged sword. They can bite you hard. Better is Volvo’s autolocking/unlocking setup, or a modern traction control setup.

        As to the subject at hand, my ’02 Jeep Grand Cherokee can do RWD, AWD, 4WD and lo-range 4WD. Something for everyone!

        • 0 avatar
          Scoutdude

          With no center diff or the center diff locked driving all wheels actually helps you turn as the rear would normally track a smaller circle so having the front and rear wheels trying to turn as the same speed forces the rear end around providing a little rear steer.

        • 0 avatar
          CJinSD

          Your experiences do not have much to do with mine. Anything is possible in a light FWD car on narrow all season tires right up until the snow is higher than the front bumper. My BMWs were worthless without the best winter tires available. One of them had an LSD and the other ASC+T, but they were completely useless on all season tires. With winter tires, I could have lots of fun and keep moving, but a hill could stop them without, let alone starting on a hill. I’ve never even had snow tires for one of my FWD cars, and I never had any trouble getting places in deep snow with such hot-rods as a Horizon, a Spectrum, a Festiva, a Sundance, a Jetta, or a Rabbit. The 2003 Mini Cooper was pretty bad on unplowed roads because of its lack of ground clearance. It would wind up with all its weight supported by the snow and its wheels barely touching anything. Not all FWD cars are created equally, but there are reasons that underpowered Saabs and Minis won all those winter rallies back in the day.

          • 0 avatar
            Scoutdude

            There is the big difference the FWDs you are talking about barely had enough torque to open a beer bottle so they couldn’t spin the tires even on snow while the BMW actually had horsepower and torque.

          • 0 avatar
            CJinSD

            I’ve driven 2002s and 320i’s in the snow too. They weren’t anywhere near as capable as FWDs with similar power and the same sized tires. We used to do some fairly irresponsible things on snow days, and those cars couldn’t keep up. Besides, it isn’t like there’s a gas pedal for controlling how much torque you send to the wheels in a more powerful car or anything. Most of the cars I listed could spin their tires on slippery surfaces if that was your intention anyway.

            I’ve got no illusions about 3,500 lb FWD cars with aerodynamic doohickies practically scraping the pavement under their chins and 225/45R18 tires being effective rally machines, but the idea that RWD is remotely equal to FWD because of what they can do with electronic stability control, traction control, and mirco-siped winter tires is complete and utter fantasy. That’s a testament to tire technology and safety nets. Just try driving one on all seasons and you’ll see reality. A FWD car on all seasons in the snow just makes it prudent to slow down. At least you’ll be moving.

          • 0 avatar
            krhodes1

            I would no more consider using “all season” tires in the winter than I would walk over a bed of nails in bare feet. Sure you can theoretically do it, but it’s stupid. The issue isn’t the going – it’s the stopping and turning.

            I’ve had plenty of low-powered FWD cars, Jettas, a FWD Subaru, and Saabs. I will still take a well-balanced RWD car in the snow any day and twice on Sundays. Add in modern traction and stability control and it is a complete non-issue.

          • 0 avatar
            Summicron

            ‘using all season tires in the winter’

            Excellent way to go sideways a lot and turn your bowels into water.

            For balanced RWD, 30 years and no wrecks tells me that 4 or 5 eighty pound bags of salt over the rear axle of a RWD pickup will safely get you through anything up to the door sills.

            But of course going slow is still the best traction control no matter the vehicle.

          • 0 avatar
            Carlson Fan

            Snow covered roads are ieasy to drive on. It’s ice that will get you. And snow tires or salt bags over the rear wheels won’t help you there. I can’t count how many times I have seen people slide into the ditch in RWD and/or FWD vehicles when they hit an icey patch while my PU locked into 4High covers the same stretch of road and never misses a beat.

          • 0 avatar
            OliverTwist

            My mum’s hobby car, a 1986 Citroën 2CV Charleston, has one of the skinniest tyres ever for a car (no wider than my forearms at their widest) and a tiny two-cylinder motor with ridiculously low output (29 horsepowers from 602cc).

            This car is really delightful to drive during the snow storms. The skinny tyres really cut through the snow. Ironically, the front-drive Citroën ‘outperformed’ our family car, 2003 Mercedes-Benz, with all of traction control and likes, on snow…

    • 0 avatar
      NormSV650

      I’m pro snow tire anything after this winter as the Verano seldom sees the salt and the all-seasons are managable on flat, packed snow.

      But I spent most of the winter in a new Encore AWD. This thing just claws in the direct you turn the wheel and is very sure footed. Unlike the GF’s Forrester that rotates with any steering input the Encore AWD just goes. Of course having the Tirfect Tune, downpipe, and intake help once the tires stop spinning vs the 2.5l Subaru engine.

      Though I wish for than 50% rear bias, the short wheel base can be fun until 30 mph when the electrics are forced back on. The AWD setup is from Haldex and is magnetically coupling the rear end. At a stop it is 100% coupled and decreases until 37 mph when it is uncoupled. The even is seamless at any mph and is a blessing for highway cruise fuel economy.

  • avatar
    udham

    Couple of points -

    1. The Jeep Wrangler is a part time system.
    2. Almost all part time transfer cases tend to have a reduction gear. In the last 2 decades, the only single speed part time transfer case I am aware of is the Jeep Nitro (NP 143)

    For a nearly complete history of New Venture Gear (or New Process Gear) transfer cases, please see http://en.wikipedia.org/wiki/New_Venture_Gear

    There are some single speed transfer cases (i.e. transfer case without a reduction gear) that are not on the list, otherwise it is fairly complete.

    • 0 avatar
      ezeolla

      “1. The Jeep Wrangler is a part time system”

      Glad I am not the only one who caught this. I have been under my TJ a few times and it sure looks like that first diagram

  • avatar
    Scoutdude

    Sorry but you missed the Jeep Quadratrac system used in the mid 70′s that had a clutch type limited slip center differential that could also be manually locked and the NP203 used by the big 3, in the mid 70′s, in their full size trucks that was a “full-time” 4wd system with a lockable open style center differential.

    Many of the systems that are marketed today as AWD are nothing more than electronically controlled 4wd systems and once they get the wheels back to the same speed disengage themselves. I will not own such a system as they create dangerous handling characteristics when the other axle is disengaged in the middle of a corner on a slippery surface. Lots of owners of Fords that have a Auto, 4×4, and low 4×4 switch modify them just so they can’t automatically engage and then disengage the system due to the dangerous handling that it causes. Personally I just chose the Mercury with a center diff meaning the wife didn’t have to think if she should or shouldn’t be messing with the switch. It also means that full throttle launches regardless of road conditions always go smoothly.

    When Ford first offered the ability to drive the rear wheels on their Tempazes they badged the vehicle AWD even though it had no center diff and you had to push a button to engage it, for the final year they badged them 4wd after hearing the criticism that the systems were not AWD.

    • 0 avatar
      Luke42

      Having owned one of these AWD systems (2002 Ford Escape), I found its handling to be far better than the traditional 4×4 (2004 F-150) in the snowy flat road-centric conditions that I encounter.

      The Ewcape’s AWD system does depend on the powertrain computer, so if you don’t trust the computer then the system isn’t for you. But I found it to react more quickly than any human or traditional 4wd, and I found it did the right thing at the right time on a surface that alternated between pavement and slippery stuff, so the computer controlled AWD provided the best of both worlds.

      I can’t speak to serious off-roading, but I did some drive-offs with the Escape and the F150 last winter when they both were in my driveway, AWD the a clear winner in the situations I encounter.

      But you can take all of this with a grain of salt because I don’t really need AWD or 4WD, and decided to dump ‘em both for a Sienna. The AWD in the Escape was better than FWD in the van, but the van is plenty good in the snow (except for the doors freezing shut), and it also burns less gas and hauls more people/stuff. So, for my purposes: FWD van wins, AWD is my second choice, and traditional 4WD gets 3rd place.

      • 0 avatar
        Power6

        Escape is not the same as what he referred to, the old Explorers had part time systems that would literally shift into and out of 4WD based on slip data from ABS sensors. Pretty funky if you ask me.

        The Escape has a transverse drivetrain, its like a car based AWD, which engages with a clutch, much more smooth and controlled than the old Ford auto-4×4. The older Escapes had a lockable center diff too, very nice.

  • avatar
    Lie2me

    (Scratches head)… I was corrected the other day when I referred to my own vehicle as AWD, it’s 4WD according to the manufacturer, who by the way, refers to another one of it’s models with the same set-up as AWD according to this article. This leads me to believe that it’s really more marketing semantics then any real industry-wide designation based on certain criteria. Though, I’m more inclined to follow Jeep’s definitions given there history except now calling the Cherokee a 4X4 will surely “wrangle” the purists… *sigh*

    • 0 avatar
      Luke42

      You have an Escape, right?

      Ford marketing dressed it up like an Explorer and called it 4×4, in to sell it to people who want to say they have a 4×4. But, for all practical purposes, it’s Ford’s AWD answer to the AWD Subaru Forester.

      It’s also a very sure footed vehicle in slippery conditions.

      • 0 avatar
        Lie2me

        Yeah, it’s great in all sorts of conditions, having owned several Jeeps prior, I’m always surprised at how good it is, but it’s a AWD and I’m fine with that. Ford doesn’t have to pretend it’s something else, besides the Explorer has the same set-up, so it doesn’t make any sense.

  • avatar
    CJinSD

    In the ’80s, anything with a center differential was considered AWD. Anything without one was 4WD, which was why the early Subarus and Colts were labeled as such with their part time systems. Has Audi started using capital letters when writing quattro? Back in the day, Quattro was only applied to the original AWD turbo coupe, while quattro was what the badges said on other models when equipped with AWD. The first production car with AWD that I can think of was the 1966 Jensen FF.

    • 0 avatar
      Scoutdude

      The Colt Vista wagons had a viscous coupling in the rear drive shaft that allowed their full time engagement of the rear axle while providing slip to prevent bind from occurring on surfaces with good traction.

  • avatar
    Autopassion

    So, is it true that if you have to replace one tire on a Subaru (or an Audi)you have to replace them all – otherwise the marginal difference in tire circumference will wreck the drive train?

    • 0 avatar
      Robert Gordon

      That is bollocks. Unless you replacing one tyre with a tractor tyre and the other with a mini tyre.

      • 0 avatar
        Scoutdude

        Sorry but it is true. Save a little on tires and end up spending thousands on drive train components.

        • 0 avatar
          Robert Gordon

          Yes it’s true.. and by the way I have a bridge for sale.

          Compared to the huge torque difference during braking/accelerating, cornering – not to mention off road use – the difference in tyre size between worn and new is insignificant. For heavens sake, many of these cars even have space saver spares – riddle me that?

          • 0 avatar
            Power6

            The spare tire is sized to be the same circumference as the stock tires. On older Subarus (pre-VDC) they told you to put the spare on the front if possible, to protect the rear Viscous LSD. The whole point is to protect the V-LSD, which is rotation sensetive. A long highway trip with a large enough rotation speed difference can overheat and lock up the viscous unit.

            The automatic Subarus without diffs, they have you simply pull the “AWD” fuse if you put the spare on, to run in FWD mode.

            I guess Subaru is a bit conservative with their guidelines but this has heppend in the more extreme cases (different brand or size of tires on each axle)

    • 0 avatar
      Scoutdude

      Absolutely, if the difference in diameter is too great you will damage things.

      Some manufacturers have set perameters for matching the tires used on their four-wheel drive and all-wheel drive vehicles.

      Audi: As published in their vehicle owner’s manual, “rolling radius of all four tires must remain the same” or within 4/32-inch of each other in remaining tread depth
      Porsche Cayenne: Within 30% of the other tire on the same axle’s remaining treadwear
      Subaru: Within 1/4-inch of tire circumference or about 2/32-inch of each other in remaining tread depth

      For that reason you also need to keep up on tire rotations as the FWD biased systems will still wear out the front tires twice as fast as the rears.

      Thankfully when you do need that single tire you can get a tire from tirerack shaved to match the remaining 3. However that only makes sense your existing tires are between aprox 30-80% or their original tread depth depending of course on the mfgs recommendation for max variance. Less than 30% or so and it really isn’t worth paying an extra $25 to take off the majority of the tread so you can get the remaining life out of the other 3, unless you are selling it very soon in which case you’d be better off to check the local used tire places and E-bay for a close match.

      • 0 avatar
        tankd0g

        Your vehicle is always turning wheels at different speeds unless you are traveling in a perfectly straight line. The recommendation to keep tires within 30% has nothing to do with wearing drive line components. It is to ensure that ABS functions properly. As long as you stick with the same brand/model tire, and your other tires are not down to the wear bars, you are well within the design limit of the ABS system.

        • 0 avatar
          Scoutdude

          Sorry but no the ABS doesn’t care about different tire sizes as it looks at the rate of deceleration to determine if the wheels are about to lock up. Traction control on the other hand does care since it does compare the speeds of the front vs rear wheels. When I bought my Marauder I specifically bought one of the early production units that doesn’t have traction control. That allows me to run any size combo of tires instead of just the same stagger. The ABS doesn’t care at all that I put the same height tires front and rear while the guys with the later models need to maintain the stagger with in about 3% or turn off the traction control every time they start the car. I’ve also ran staggered tires on my other Panthers w/ABS and w/o traction control w/o any problems.

          The fact that you make turns doesn’t really matter for example Subaru loosens up the center clutch when the steering wheel is not straight and the reality is you don’t constantly turn.

        • 0 avatar
          Power6

          The recommendation to replace all or shave the replacement tire is to save the center/rear diff. In Subarus the diffs are viscous, i.e. viscous LSD not the viscous coupling Alex referred to. The viscous unit doesn’t engage in normal circumstances driving straight ahead. If you have different size tires it introduces a slight rotation difference which may heat up the viscous units over a long high speed trip. A slight risk but Subaru isn’t about to say its perfectly fine.

    • 0 avatar
      Jacob

      Actually, replacing just one tire is a bad thing on any 4×2 car as well. At very least make sure you don’t put different tires on the drive axle.

    • 0 avatar
      d002

      On some cars, for some reason, replacing just one tyre can make the steering wheel shake like crazy and end up ripping the engine mounts and pulling out the exhaust pipe. I had a Mitsubishi Sigma that did this. Apparently Morris Marinas had the same problem. It had something to do with where the rear struts are mounted.

      Its also common on rwd cars with single link IRS, like a Datsun 1600. On torsion beam suspension (fwd) it doesn’t seem to.

  • avatar
    Robert Gordon

    The Range Rover in the picture has a permanent four-wheel drive through a dual-range transfer box with viscous-coupling central diff lock, not a manually locking unit. So you should either change the text or the picture.

    • 0 avatar
      mkirk

      Thought the Rovers were like the cruisers…Manually locking AND a viscous coupler.

      • 0 avatar
        Robert Gordon

        Other than the Road Rover prototype I am not aware of any Rovers with locking diffs, either viscous or manually actuated – in fact I can’t even recall that there has ever been a 4wd Rover (apart from the aforementioned Road Rover).

      • 0 avatar
        Robert Gordon

        I presume you must mean Range Rovers. As a rule of thumb Pre-1990 Range Rovers have manually operated diff locks whilst post 1990 are viscous (this only applies to RR Classic and P38s – I have no idea what they do now).

        However, there are many Landrover Special Vehicles (LSV) products fitted with Viscous couplings well prior to this date, and they even flirted with some standard fit ones about the 1982 period if I recall correctly.

        Much of the development work on the VC as a workable concept for off-road vehicles was done on the Range Rover by Tony Rolt (it is ridiculous that such men aren’t revered as highly as they should be – I urge anyone with a bit of time spare to do a google search on this man’s remarkable life) at FF Development.

        As someone already pointed out the viscous coupling on the Jensen FF precedes the AMC by nearly two decades.

        • 0 avatar
          investable

          I had a 1994 Land Rover Disco SE. It had a lever for the diff lock and a second for low/high range- I currently have a 2003 Land Rover Disco 2 hse, it still has the high/low but no seperate action to lock the diff.

          • 0 avatar
            Robert Gordon

            Discovery I and II use the LT230 transfer box (LT stands for Leyland Transmissions – a comforting thought) – with a manual locking centre diff. They simply deleted the lever from the cockpit on later cars since Hill Descent and Traction control made it obsolete. Some folk retrofit lock controls on later discos.

  • avatar
    Robert Gordon

    The car in the photo is a 1990 Range Rover CSK not a 1970 model as suggested btw.

  • avatar
    Styles79

    Add in the JDM Nissan e.4wd system for small cars that features a conventional transverse front wheel drive system supplemented with a generator and rear electric motor……

    4wd is very popular in the northern regions of Japan, in everything from small cars to sports sedans.

    http://www.nissan-global.com/EN/DOCUMENT/PDF/ENVIRONMENT/TECHNOLOGY/en_tech_e4wd.pdf

  • avatar
    tankd0g

    My Rav4 is AWD except when it’s not. You press a button to lock the center differential and it becomes 4wd. No fancy electronic controls or viscous couplings. I wish they still built them that way.

  • avatar
    bumpy ii

    Couple of random points.

    The top diagram, minus the rear axle and driveshaft is how the Honda/Acura Legend and Vigor, and the Chrysler LH cars were powered.

    The domestics didn’t offer 4×4 on their pickups until the mid-60s. Before that, you had to get a Napco conversion done.

    • 0 avatar
      Styles79

      Yeah, I had a KA8 Legend, and could never fathom why they did that. Why not just carry the shaft back and make it RWD? It’s not like there were any weight or cabin space savings……

    • 0 avatar
      Scoutdude

      Maybe from the big 3 but International Harvester offered factory installed 4wd starting in the 1955 on their 3/4 ton S-120 models.

      In addition to the LH and Vigor, FWD Subarus, the original Tercel, older Eldos & Toros, older Audis, VW Dasher, the LH’s predecessor and a couple others used a similar longitudinally mounted engine on their FWD set up. The LH and Tercel were purposely done that way so they could make it AWD/4WD like the way Subaru did their original 4wd system or in the case of the LH make it RWD. Unfortunately Chrysler didn’t have the money to tool up to do all that so they were FWD only.

    • 0 avatar
      PonchoIndian

      Dodge had factory 4×4′s starting in 1946.

    • 0 avatar
      joeaverage

      Were the AWD Honda and Acura cars like the CR-V with standard FWD and a light duty front to back driveshaft, rear diff and rear axles? I have an early CR-V and the system has been very robust even past 265K. The U-Joints in the front-back driveshaft are tiny though. They lasted about 235K miles but I doubt they would have lasted as long carrying RWD loads.

      Looking at the system I think Honda would have needed to add significant weight to the car (couple hundred pounds anyhow) to beef up the rear part of the driveline to do RWD full time. The existing system is very easy to sell as FWD or AWD, just leave out parts assemblies.

      Without comparing front axle and rear axle curb weights, I’m not sure the CR-V (or Accord or Acura) would have been good with RWD. I expect it would be too light in the rear end and might be a handfull on wet roads.

      AWD would come on during oversteer possibly but still – who wants a grocery getter with oversteer issues? Also had a 1st gen Mustang years ago and that car was very prone to snap oversteer. Fun when out for a ride but a pain in the rain, snow, etc. b/c I didn’t want to slip and slide everywhere I went. It was a traction issue, not a horsepower problem b/c I had the six cylinder. My aircooled Beetle had better rear-end traction than the Mustang although the Beetle had it’s issues too.

      • 0 avatar
        Snowrocket

        I cannot speak too much in the way of Honda/Acura systems except for the Real Time system. Early Civic Wagovans (station wagons) had a part time 4WD. You’d press a button, and a dog clutch would engage the rear axle. Very old school Subaru-like part time 4WD, and it worked.

        The Real Time AWD used since then (on the Civic wagon, and later on all CRVs)has a few distictions.
        1. The name has never changed; up until just recently, it’s ALWAYS been called Real Time. I think the newest CRVs say just “AWD”.
        2. It’s always been a reactive system needing front wheelspin to engage the rear axle. These types of systems can leave you stuck on really slippery surfaces/hills. Why? Because breaking traction at the “primary” axle leaves you with less traction to THEN work with AT the primary axle.
        3. It does not and will not transfer a lot of power to those rear wheels. That is for four reasons that I can think of: fuel economy, weight, “retain FWD handling”, and cost. The downside is that in any YouTube comparison test, flooring one up a steep gravel or dirt incline from a stop produces NO rear wheelspin. Not enough torque to the rear means the rear is not “doing its share” to help you up the hill. For this last reason in particular, I’ve always felt that Real Time AWD was among the worst engineered AWDs available. CRV: good vehicle, bad AWD.

  • avatar
    PrincipalDan

    I guess I’m one of those traditionalists that smiles every time I see the 4×4 badges on a Patriot or a Compass. I know the non-trail rated systems have more in common with Subaru’s system than the system on a Wrangler.

    Having said that, as long as you can lock the center differential, I don’t care what you call it. When I thought I might end up with a job that required me to have a long all-weather commute the first thing I thought of was Patriot or Compass because of the capability of the 4X4/AWD system.

    • 0 avatar
      Alex L. Dykes

      Actually the Patriot’s AWD system has very little in common with the Subaru system. The Subaru system is thematically similar to what the Grand Cherokee and Wrangler use.

      • 0 avatar
        cgjeep

        The Subaru’s system is nothing like what the Wrangler uses. The GC is nothing like what the Wrangler uses. The Wrangler uses a part time system that has to be manually engaged. It locks it 50/50. The Subaru’s system might be similar to the GC.

        • 0 avatar
          Alex L. Dykes

          “similar” in that they are using a longitudinal engine layout, transfer case and front and rear diffs. Just because the Wrangler is a part time system and the GC is a full time system doesn’t mean they aren’t related. “Related” is a very encompassing term and it was used in that way. Similarly the Subaru uses a longitudinal engine layout, transfer case and front and rear diffs, they just jam it all in the same transmission housing. My point was that the Patriot’s transverse FWD based system is totally different.

          • 0 avatar
            Power6

            Right on, the innovation that Audi came up with (and Subaru copied??) is the hollow driveshaft, which eliminates the need for the front driveshaft to run outside the transmission, it simply runs through it.

            Conceptually Audi and Subaru transverse AWD is the same as the old part-time systems, doesn’t matter whehter it drives a diff or locked shaft, plenty of Jeeps have either, and so did old Subarus.

  • avatar
    Greg Locock

    “and if three wheels lost traction the remaining wheel can consume all 100% of available power.”

    No. Absolutely not. If all the diffs are traditional diffs then the wheel with traction will remain virtually stationary, whilst the others spin up.

    The only tractive torque available to the non slipping wheel is equal to the friction on the other wheels.

    The power to each wheel is T*rpm/5252. T is constant for each wheel as they are open diffs. The spinning wheels have greater rpm than the non spinning wheel therefore they absorb (uselessly) more power.

    • 0 avatar
      Alex L. Dykes

      You misread that line, I said “if they front, center and rear diffs are all locked.” If everything is locked, then yes one wheel will get all the power if the other three are in the air.

      • 0 avatar
        Power6

        Alex, you completely glossed over that, at least you didn’t get into a “how many wheel drive” as the typical clueless Internet debater gets into.

        The hard part to understand, is that a locked diff can distribute anywhere from 0-100 to 100-0 power split, compeltely dependent on the traction of each tire. The relationship between rotational speed (What any diff except Torsen addresses) and power-split is the concept that 90% of car guys will miss.

        • 0 avatar
          Alex L. Dykes

          Well put. That is something most people don’t comprehend.

        • 0 avatar
          PonchoIndian

          Actually a locked differential is exactly what it sounds like, it is locked. That means an equal amount of power is sent to either side of the differential.

          • 0 avatar
            Flipper35

            Correct, locked means it no longer acts as a differential. For example, a Detroit Locker or similar.

          • 0 avatar
            Power6

            No its not…now you see what it is so hard to understand! Think of it this way:

            RWD truck, single rear axle locked diff. Put right wheel in air left on ground, 50lb-ft applied, truck moves forward. Power split is 100-0 left-right. Now put left wheel in air right on ground. 50lb-ft applied truck moves forward again. No torque split is 0-100 left-right. You can’t apply 50lb-ft to thin air, it has to go to the wheel that moved the truck forward.

            You are caught up in rotational speed which is always 50-50 when the axle is locked. But torque split does not follow rotational speed. HArd to wrap the brain around it.

          • 0 avatar
            Greg Locock

            Flipper35- No, each shaft rotates at the same speed. The power transmitted by each axle shaft will depend on the torque on that wheel, which could be anything from zero up

            Alex-sorry yes i missed that you were talking about locked diffs

          • 0 avatar
            joeaverage

            Like this?

  • avatar
    NMGOM

    Alex,

    Great article on a complex topic. Thanks. You obviously put a lot of work into it.

    To show you what a mess of misleading and often conflicting terminology surrounds this topic, about five years ago, I recommended that my friend (a lady) buy a 2004 CPO Honda CR-V. Its drive system was called “Realtime AWD”. Without studying the workings (which in that era Honda seemed to have guarded jealously), I assumed this was something like Subaru’s sure-footed “Full Time AWD” .

    Well, it wasn’t and isn’t. In my view, Honda’s terminology was just a little bit misleading, because when the first wet snow fall came, the “slip & grip” nature of the CR-V came through loud and clear. The front wheels were spinning, but then the rear wheels engaged suddenly, causing the vehicle’s back end to sway back and forth. Scared the dickens out of her! We solved this problem by getting Dunlap SP60 aggressive All-Season tires so that the the front wheels rarely lose traction and the back ones don’t have to “snap on”. In other words, we have essentially reduced the CR-V to a FWD car for peace of mind!

    She used to have a 1991 Isuzu Trooper, which had a real 4WD system with two locking differentials. It was very tenacious in snow AND you knew exactly what the vehicle was doing: each of 4 wheels had a job to do and they did it. None seemed to have been greatly phasing into and out of the traction regimen, and none slammed on suddenly. She really misses that Trooper, but gas mileage was poor and it rusted out badly, so she had to get rid of it.

    —————–

    • 0 avatar
      Scoutdude

      The original “Real Time” system was a big joke. It used a center clutch that was powered by a pair of hydraulic pumps connected to the front and rear outputs. They formed a loop and if pressure built up between the two pumps it would engage the clutch. There was a little built in leakage so that not too long after it locked up it would unlock. Forget to rotate the tires or replace only 2 and you’d blow up that clutch in short order.

    • 0 avatar
      Prado

      I bought a 99 CR-V new with the Realtime 4WD. It should have been called RealBad 4wd. A total crap ‘reactive’ and useless system that left me stuck when making a Uturn on a banked soft shoulder. Fool me once. It was replaced with a REAL 4WD 4Runner. Absolutely love the system in the 4Runner. High/LO with a lockable center Torsen Diff. Great for both on and off road. A lockable rear would have been nice but I honestly haven’t had the need for that yet.

    • 0 avatar
      corntrollio

      Yes, most of the transverse systems are reactive rather than proactive. When the front wheels slip, then the coupling or clutch engages the rear axle.

      People had similar criticisms of earlier full-time AWD systems in some cases — e.g. the Ford Explorer AWD system (not Control Trac) comes to mind, but all those systems got better over time too. Then again, the Ford Explorer V6 could switch from the Auto AWD system to a locked part-time 4WD hi or lo as well, sort of like some of the GM SUVs, and I believe the Expedition allowed the same (I think the V8 was AWD 35/65 with no low-range, but it’s been so long that I’m not entirely sure).

      • 0 avatar
        Power6

        The newest systems are getting pro-active, I think the haldex enggages based on acceleration foprces, throttle angle etc. intead of waiting for slip.

        The issue always was that with no center diff like a FWD clutch system, you want to be disengaged normally. Subaru has been doing the proactive part of that for some time with their MPT system. 90-10 normal but in say 1st gear acceleration is goes to 50-50 proactively.

        • 0 avatar
          Frownsworth

          I think of them as predictive. The computer only knows so much, and when a prediction is wrong, it is all down to slip sensors again.

          Some calls Honda’s SH-AWD – “Synthetic Handling” AWD, and after having driven their SH-AWD on slushy, icy and snowy terrain, I have to agree. The behavior of the AWD is highly unpredictable. When you downshift (with paddles), the vehicle exhibits FWD pull to one side in the front, but when you turn a corner, the computer first over speeds the outer rear wheel to emulate a RWD feel, then slips because of lack of traction, then immediately engages the fronts harder to compensate.

          I still think longitudinal+Torsen is king for this kind of thing.

          • 0 avatar
            joeaverage

            I’ve owned a 5 speed CR-V for 14 years. The system works well enough for what it is. It is not 4WD. It’s really 2WD b/c of open-differentials but that said, it does its job well enough. It is not flawless though.

            If the driver takes off hard and spins the front wheels hard, the system engages abruptly. Its a dump hydraulic pump and clutch pack.

            Done hard enough it can break loose all four wheels causing the vehicle to slide sideways – not good if you are nearly stuck sideways on a hill somewhere. Apply power gently which is wise in slippery conditions anyhow.

            What the system does well is that gentle transition from FWD to AWD b/c you are driving on slick surfaces at surface street speeds, the system will come and go as necessary. There won’t be enough torque to cause the system to engage abruptly.

            I can feel the system engage – it does not engage 100% so the front axle is still rotating at a speed slightly faster than the rear axle. The steering gets light and the AWD system is giving up some traction this way. Not a problem in my application. Climbing hills in Siberia might be a problem. ;)

            If the rear wheels have more traction than the front axle – it can push the front end during a hard parking lot turn i.e. bad understeer. Not unlike 4WD in this way.

            Again, start slow and it’ll get the hard turn done.

            The vehicle is primarily FWD with an extra push from the rear axle. There are better AWD or 4WD systems out there but this one isn’t the worst. In the small CUVs of the late 90s, the RAV4 had a better system b/c the system could be engaged and kept engaged. of course I see old CR-Vs, and I don’t see as as many old RAV4s. Good or bad AWD systems – I want a car that lasts. ;)

            Despite all-season tires, our ‘V has carried up across soft fields, through the snow, across the ice, off-road through the woods and we’ve forded several creeks over the years with water to the top of the tires. You have to match your expectations to your vehicle’s limitations. 265K miles and it continues to do daily duty, on the original clutch as well. We towed 750 lbs Saturday for about 50 miles – it continues to do what we need a vehicle to do. Your mileage may vary… ;)

      • 0 avatar
        Scoutdude

        Later years of BOF Explorers had the choice of 2 transfer cases. One a traditional part time 2sp with an electronic shifter and an Auto 4wd mode that engaged a coupling when it sensed slip. It had no center diff. The other option offered was a single speed AWD transfer case with a center differential that was the only option in the Mercury and short lived Lincoln versions. The Expedition offeres a selectable automatic 4WD system like the Explorer with an Auto 4×4 mode as well as locked 4hi and 4lo settings. No single speed nor AWD transfer cases were offered in the Expedition.

  • avatar
    Kyree S. Williams

    Gee…you’re right. There are hardly any midsized body-on-frame SUVs. The only ones left (that I can think of) are the Toyota 4Runner, Toyota FJ Cruiser and Lexus GX 460…all of which are on the same fundamental platform. Then there’s the Jeep Wrangler (which is more of a truck than an SUV) and the Mercedes-Benz Geländewagen. And none of those sell in terribly-significant numbers, least of all the Mercedes-Benz.

    Seven-seaters like the Pathfinder, Explorer and Durango have all gone to unibody platforms, and GM refuses to bring the new body-on-frame TrailBlazer to the U.S. That just goes to show you how few people needed truck-based SUVs in the first place…

  • avatar
    Hummer

    You didn’t cover Torsen gear reduction in the hubs, which is definitely one of the best.

    And actually predator motors sports sells a transfer case that has options for…

    2wd, AWD, and 4wd

    But if your good at brake modulating…. Well…

  • avatar
    oldowl

    Given all the informative and sometimes conflicting information given here, what are good current choices–by brand or systems within brands–for someone who wants…

    Occasional moderate off-road ability: Dirt, sand, gravel, snowy, or uneven roads (but not boulder fields, foot deep ruts, grille-deep snow, or hub-deep mud)?

    Or at these less demanding levels, are differences in systems and/or brands not significant enough to seriously influence a buying decision?

    • 0 avatar
      Snowrocket

      The following good choices are my opinions, gleaned from reading road tests and talking with LOTS of people over the last 30+ years:
      * Most Important Point: Buy from a manufacturer who has been TOTALLY committed to 4WD/AWD in most, if not all of their offerings. They put WAY more research and money into ensuring their systems work effectively. This GENERALLY will mean cars from Subaru and Audi, and trucks/SUVs from Jeep and Rover. Noteworthy exception: Mitsubishi Lancer Evo. The Evo is heralded worldwide for its fun, grip, and handling on any surface at any speed. It is by far Mitsu’s best product.
      * Tires matter; have two sets of four. One for winter, one for summer.
      * Due to torque split and chassis handling balance, I’m REALLY BIG on manual tranny Subarus and longitudinal engined Audis. The AWD on these are seemless, fun, and good at starting out from a standstill on a steep icy hill.
      * To a degree, you get what you pay for. I would generally rate an Audi’s AWD superior to a Subaru’s, but you ARE paying for more sophistication.
      * For most people, most places, most of the time, 6″ of ground clearance is all you need. That negates the “need” for big tires/lots of ground clearance/truck/truck-based SUV. In the snows we get here near Akron, Ohio, USA, my ratty old AWD Impreza 2.2 liter passes everyone. It has open front and rear diffs, four good snow tires…and 6″ of ground clearance. It’s NOT sophisticated, but it IS effective…up to about 8″ of fresh snow of any consistancy.
      * Look at comparison videos on YouTube and consider the SOURCE of the video. The Subaru wins the video produced by Subaru. Well big surprise there!! Most consistent winners: Subaru, Evo, Audi, Jeep, Rover.
      * Learn to drive better in all circumstances; you’ll go farther in EVERY vehicle. I learned from my dad who was WAY better than average, Car and Driver magazine, and rally drivers.

  • avatar
    JaySeis

    Maybe someone here can explain how the Explorer AWD system actually works? As I understand, it’s based on the Haldex system where normally, a vicious clutch pack apportions most power to the front (say 70-80%). If the rear wheels spin, the clutch pack engages and (this part I’m not sure about) power is sent to the rear at a proportion of 50:50?

    Ours is the Adrenalin version and if you push it in a curve, it normally feels like a FWD (that’s front) car (think big tall wagon with IRS) with a hint of oversteer. The first time the clutch pack engaged on wet grass I’d though I’d broke something. Now I’m used to it and it rarely engages..wet grass, snow, ice, etc will do it.

    • 0 avatar
      Lie2me

      Actually the system engages all the time, you just don’t realize it. During acceleration it can get to almost 50% rear, at normal cruise it’s almost 100% front.

  • avatar
    th009

    I believe your list of longitudinal layout SUVs/CUVs with centre differentials is missing the Audi Q5.

    • 0 avatar
      corntrollio

      Yes, although the Q5 has different components from the Q7. Audi uses quattro to cover a variety of different systems (and yes, as stated above, it’s lower case to differentiate from the ur-Quattro). Most of the front-engine longitudinal systems use a Torsen differential (although three different types have been used), although the original Quattro’s quattro system did not, and the latest longitudinal quattro system in the RS5 also does not (it uses the “crown gear” instead). The Audi V8′s system had two Torsen differentials — center and rear. Starting with the RS4

      The Q7 has a transfer case made by Borg Warner, which includes a Torsen differential (type 3), and biases power to the rear at about 40/60.

      The other front-engine longitudinal Audis (A4/A6/A8/Q5) use what Alex said and now all bias power to the rear (although they were 50/50 in the past before type 3).

      The transverse Audis have all used Haldex clutches. I believe older transverse VWs used viscous couplings.

      Audi has used viscous couplings only on one car I believe — the longitudinal mid-engined R8. The R8 is still heavily rear-biased, as is the intention with a mid-engine car, but the viscous coupling sends some power to the rear based on conditions (something like 15-30% I believe).

      The sport differential is also separate from the quattro system and started being used in the B8 S4 and is now more widely used across front-engine longitudinal vehicles.

      • 0 avatar
        corntrollio

        I left “Starting with the RS4″ dangling there, but meant to take it out — that was the first car, along with the Q7, to get the Torsen Type 3. I can’t edit my comment any more to fix it. However, I would also add that the early quattro systems had manually lockable diffs — center and rear originally, and then rear only when the center diff became Torsen. Now the front and rear diffs are only electronically lockable, but the sport diff also allows torque vectoring.

      • 0 avatar
        Power6

        I like how the Audi R8 has the same AWD type as my friend’s Westy Syncro…

        • 0 avatar
          corntrollio

          Well, it’s the same idea, but the system is likely rather different in operation and more advanced, obviously.

          The Lambo Gallardo also uses a viscous coupling, and I believe the 993 and 996 C4s do too. The 964 used a different system — power ran to the center diff, and I believe it used a hollow shaft like a traditional longitudinal quattro system does.

          By the way, there’s an article on AudiWorld that gives a good executive summary overview of the Audi systems without getting too deep:

          http://www.audiworld.com/news/11/quattro-tech/

          • 0 avatar
            Power6

            I think it is pretty much the same no? RWD based, send power to front with viscous in between in the front diff case. Most power is sent rear, only when wheels slip does the viscous engage. All mechanical. Great to give a RWD vehicle some AWD traction. Obviously the part are not the same and not imply the R8 doesn’t have a good setup.

            I like this site too http://www.awdwiki.com

  • avatar
    DC Bruce

    The car that was quite good in the snow was the rear-engine VW Beetle. Back in the 1960s before 4wd/AWD “civilian” vehicles were common, the VW Beetle was one of the best snow cars out there.

    The AWD system in the Toyota Previa (which used a viscous clutch, I believe, to engage the front wheels) worked quite well and largely invisibly. In truly nasty situations (like more than a foot of fresh snow), you could spin one front and one rear wheel simultaneously. If only they had a locking differential on the rear, it would have been totally awesome.

    I don’t know whether the AWD system in the Honda Pilot is the same design as in the CR-V, but I have to say that the Pilot is my least favorite car to drive in the snow, better only than my 1987 Mustang GT with the Gatorback tires, which was simply impossible — even with cable chains on the rear. While I like the Pilot for other reasons, and it’s been very reliable, if I replaced it with another Pilot I would seriously consider just getting the FWD version to get a little better fuel economy.

  • avatar
    Synchromesh

    2 things:

    1. transaxle is ANY transmission+diff casing, not just necessarily front like it says in the article. Iirc, Porsche 928 has a rear transaxle for example.

    2. what about more exotic systems like on the latest Ferrari FF? It has 2 actual TRANSMISSIONS, one on each end!

  • avatar
    Power6

    Alex,

    Great article, I clicked, having researched AWD systems for years, trying to understand what a diff does, what is the best kind of diff etc. I wanted to tear this article apart but it’s pretty good. You don’t address everything out there but how could you, 4 wheel drive systems are so complicated, varied and hard to understand the physics.

    A couple points to address, not that you are revising the article but maybe keep in mind for the next time you write about this.

    Torque split: The Torsen is not the only diff that can split power unevenly. All bevel gear diffs are 50/50 normal. Planetary diffs can be designed for any torque split. Torsen is not just an ATB device but a complete diff replacement, I beleive only 50/50 until Audi/Torsen came out with the crown gear version which has a varied split.

    Viscous coupling vs. Viscous LSD: You didn’t clarify the difference, the viscous coupling is not a diff, but plenty of bevel gear diffs have viscous LSD incorporated, that is how Subaru (manual trans) systems work, you get 50/50 normal split with viscous LSD action.

    Subaru/Audi: The “only” difference is not putting the center diff in the trans case, actually with a Subaru the center diff is in an extension housing bolted to the trans tail. The key is the clever drive system with hollow shaft that drives the front diff through the trans output shaft, instead of needing a driveshaft outside and beside the trans to drive the front diff. I think the weight savings is over such a system, but as you point out there are lighter systems, though it was pretty genius in 1980.

    SH AWD: Driving the rear wheels faster is not RWD bias, it is yaw control for cornering, not much use straight ahead. The Acura RL was a bit more fancy, variable overdrive acceleration device. The Mitsu Evo pioneered this with AYC, the rear diff can overdrive each rear wheel independently.

    Anyways awesome synopsis you boiled down a whole lot of technical mumbo jumbo

    • 0 avatar
      Alex L. Dykes

      The claim by Acura is that because the rear wheels in SH-AWD (all current systems now since the RL is dead) is overdriven at a constant 1.7%, when the system fully locks the center coupling it has the effect of putting more than 50% of the power to the rear. The physics are a bit more complex, but even though theu are still “locked” or at the least “almost locked” having the rear wheels spinning faster and the fronts lagging a little does give a a more RWD feel than a simple 50/50 locked system where they all spin at the same rate. The claim being that since the fronts are actually dragging slightly that has the overall effect of reducing the portion of motivation they are providing.

      • 0 avatar
        Power6

        Maybe it is a distinction without a difference, looking back at the wiki page for SH AWD it seems it does have clutches on each rear wheel, so it can drive only one rear wheel which I am guessing is the outside rear wheel in a corner for yaw control, but there is a whole lot of complex stuff going on when you are driving wheels 1.7% faster through electromagnetic clutches on each rear axle. Its probably beyond me.

        The old RL system was more flexible, it had another device inline that varied the overdrive 0-5.7%. I assume that was to make it more useful in snow ice. I can’t imagine overdriving the rear wheels at a constant 1.7% would make a stable vehicle in a snowstorm to engage the rear wheels more than breifly, unless those electro-clutches can slip a lot.

        • 0 avatar
          Alex L. Dykes

          Yes, part of the SH-AWD reasoning is for yaw control so it can overdrive the outer rear wheel. But you’re right there’s lots of physics stuff going on when they are not 1:1 like most systems.

    • 0 avatar
      Scoutdude

      Prior to 1985 Subarus were 4wd with either a simple coupling engaged by a lever or a electronically engaged clutch, there was no center differential and 4wd should not be engaged on hard surfaces.

      In all the models w/ or w/o a center differential the drive to the front differential is below what otherwise looks like a conventional RWD trans and is driven off of the output shaft of the transmission section. In the case of the 4wd models it was just a simple pair of helical gears and on the back of the output shaft there was either a coupler or a electronic clutch to engage the rear axle. On those with a center differential it is a planetary differential whose case is driven by helical gears. There is no concentric shaft to direct power to the front differential.

      Here is a link to a diagram of a Subaru “AWD” transaxle, in this case with the DCCD. tinypic.com/anbgc1.jpg The DCCD is located below the rear output shaft at the rear of the case.

      • 0 avatar
        wmba

        Pretty good explanation, but the VTD system in Subarus, of which DCCD is a subset, is the one that uses a planetary center diff AND a planetary overdrive to the rear axle (as copied by Honda SH-AWD and Torsen 3) to give a static torque split of 35/65 in the STI and 45/55 as in my Legacy GT. This is actualized by different gear ratios in the planetary overdrive between models.

        When travelling in a straight line on a dry road, the whole assembly rotates as one, just as a simple open diff does on a front or rear 2 wheel drive. This system debuted in the 1991 Subaru SVX. By pure mechanical action, any slip at the rear will allow the rear wheels to spin faster than the fronts before center differential action can even occur to exacerbate the problem

        To offset this, an electronically-controlled multiplate clutch is attached between front and rear drive shafts to vary limited slip from zero to 100% locked. The STI has four user selectable lockup percentages of this clutch, one of which is auto. The auto mode is the only one on the old Legacy GT and any current 6 cylinder Subaru, only on automatic transmission models. It’s a very nice system, IMO.

        Which is why I bought one. As a mechanical engineer, I appreciate well thought out mechanical hardware that doesn’t cost a fortune.

        Since 1988, I have had only AWD cars, starting with an ’87 Audi 4000S quattro with driver selectable center and rear diff locks; ’90 Eagle Talon with viscous limited slip around the center diff like all cheaper manual Subarus (not STI); ’94 Audi 90 quattro with Mark 1 Torsen center diff and usual Torsen lockup/shudder when at full lock; ’99 Impreza with MPT (clever cheap AWD found in all but 6 cylinder Subarus today); ’08 Legacy GT as described above.

        I found the article a little underwhelming, truth be told, because it glosses over too much of the detail for the uninitiated, and might cause more confusion than clarification. No mention is made of the Subaru MPT system, the most common one, which nobody has copied, because, judging by the translated from Japanese SAE papers, its electronic control is too sophisticated to copy without a full blown R&D project – other manufacturers just cheap out with on/off systems, viscous or clutch, like Haldex, in their cheaper offerings and try to snow the customer by talking only about reaction times to engage, rather than percentage of engagement dependent on friction coefficient tire to road, the basis of Subaru control strategy, inferred from wheel slip sensors, yaw angle sensors, steering wheel angle and so on.

        Mercedes 4matic (prior to this new CLA’s Haldex) and BMW X drive are also proper AWD systems like Subaru VTD, and BMW and Audi on higher end models have an optional variable overdrive to the outer rear wheel on cornering, unlike the fixed Honda SH-AWD.

        Jeep’s highest line 4WD (of three types offered) 10 years ago was proper AWD as well, might gave been due to Mercedes owning Chrysler at the time. Haven’t a clue what they do these days since I’ll never buy one. Same for GM and Ford, although I assume Cadillac AWD is a proper one.

        BTW, Torsen is 100% owned by Toyota.

        • 0 avatar
          Power6

          Subaru MPT is not in its own class, it is similar to the Haldex and other FWD add-ons. These diff-less systems are not “off and on” most of them are progressive lockup systems like the Subaru MPT. The secret sauce to the Subaru has been pro-active lockup but it seems Haldex and the others have addressed that.

          Planetary diff and SH-AWD are not the same. A planetary diff allows uneven torque split, quite a neat function. The Honda/Acura system overdrives the wheels through slightly altered gearing, the planetary diff can’t do that, not in the same way. There is a reason Mitsu and Honda had to use specific hardware for yaw control rather than simply use a planetary diff.

  • avatar
    PonchoIndian

    No its not…now you see what it is so hard to understand! Think of it this way:

    RWD truck, single rear axle locked diff. Put right wheel in air left on ground, 50lb-ft applied, truck moves forward. Power split is 100-0 left-right. Now put left wheel in air right on ground. 50lb-ft applied truck moves forward again. No torque split is 0-100 left-right. You can’t apply 50lb-ft to thin air, it has to go to the wheel that moved the truck forward.

    You are caught up in rotational speed which is always 50-50 when the axle is locked. But torque split does not follow rotational speed. HArd to wrap the brain around it.

    Power6, that is easy to wrap my head around, but really not a very good way of describing it to people. If you want to get real technical, which you are trying to do, there must be some torque going to the wheel in the air to turn the wheel, overcome the friction of the bearings etc…
    Any “open” differential can be made to do the same thing with traction control, only the wheel in the air will not be spinning because the brake will stop it to force the power to the wheel with traction.

    • 0 avatar
      Power6

      Well you did understand me since your previous statement was a direct contradiction…so the purpose has been served. there really isn’t a better way to explain it that I have found. So many people cnfuse rotation speed with applied torque to infer a “torque split”. I did ignore the torque required to turn the wheel yes, not worth bringing it up to make the point.

      You’ve taken off on another tangent though, with the brake analogy, since the brake is resisting the torque applied, it is causing the same torque to be applied to the other side of the open diff, assuming it is a a typical bevel gear diff. The torque split is always 50/50 (forgive me for saying “power” before I am talking just torque), so the input of 50lb-ft from my example would actually be 25lb-ft to each axle, one input resisted by the brake, the other 25lb-ft to the other axle, hopefully enough to move the truck forward. So you see it is not the same thing. Diffs are really complicated things, we are only talking about fully locked and fully open diffs. It gets very complicated when discussing the various type of limited slip devices.

      • 0 avatar
        PonchoIndian

        Belive me, I understand.

        I guess my point is the only real important thing is that you can get the wheel with the most traction to actually spin. Not always something an open dif can do, but something a locked one and some kind of limited slip one will do. You could easily write a whole article 4 times as long as this one to explain all of the differences and comparisons out there. Most people don’t care and won’t understand. 99% of the FWD based all wheel drive systems are fine for snow driving but worthless for real four wheel drive needs. They get you up your driveway but don’t offer any dynamic improvements that enthusiasts would want.

        • 0 avatar
          Power6

          You make the best point of all. For most people none of it matters. Even a 3 open diff AWD system with good tires and brake traction control is going to get you going in most any on road situation, snow ice sleet whatever.

          A co-worker in one of my co’s offices…he has a lifted Jeep, big tires, muddy all the time etc. he goes wheeling a lot. I asked him about his diffs, (he doesn’t have the Rubicon) he left them open stock. He said “anywhere you get stuck you need a locked diff to get out you’ve gone too far anyways” he just has a winch to help out of tough situations. Probably helps there are plenty of trees in VA.

          • 0 avatar
            Hummer

            Muddin’ =/= offroading

            You haven’t gone far enough in any well setup offroad machine if you haven’t engaged a locker.
            Crossing a ditch/deep stream/traversing rocks, when you have the majority of weight on 2 wheels, your gonna be stuck

          • 0 avatar
            Power6

            Sounds good fair to me I wouldn’t know I don’t ‘froad!!

          • 0 avatar
            PonchoIndian

            I’ve only owned one AWD car. It was a transverse V6 full time setup 60/40 split, open diffs but lockable transfer case. It was the only one I’ve ever driven that seemed completely transparent in operation. Unstopable in all weather and completely predictable…and it was GM car to boot..

          • 0 avatar
            Power6

            STE AWD?

            The center diff is key, like many a Subaru and Audi. If you start off at something like 50/50 the operation is more seamless vs a 100/0 type system, in my experience.

    • 0 avatar
      Vulpine

      If you ask me, Traction Control is absolutely useless on any vehicle with a limited-slip differential. It tends to not only put brake energy on the spinning wheel but also reduce power at a time when more power may be needed. For such vehicles, traction control should be eliminated or offer a means of shutting it down permanently in all modes, not just 4WD mode.

  • avatar
    Jacob

    Where does Lada Niva fall? 4WD I assume?

  • avatar
    Biscuitbob

    Does anyone here understand the system used by nissan in the Rogue? There’s a button to lock something up at very low speeds but the vehicle supposedly locks itself into 50/50 during initial acceleration? So when would the lock be utilized? If anyone is familiar with this setup and has the info, seeing it here would be appreciated

    • 0 avatar
      Power6

      The Rogue has a very typical FWD based AWD system as described in the article, there is no center diff, it has a computer controlled multiplate clutch. When accelerating the computer may very well use some clutch lockup to prevent wheelspin. The “lock” button is a sort of override for slow speeds, instead of letting the computer control lockup (rear wheel engagement) you are commanding the clutch to be lccked up in anticipation of driving on a slippery surface.

  • avatar
    LuciferV8

    The Armada 4WD has four electronically controlled modes, 2WD, 4WD hi, 4WD lo and Auto. Of these four, the Auto mode is the closest thing to pavement-ready AWD.

    The Armada achieves AWD by way of a set of clutch packs rather than a center differential. I’ve heard that a similar system was also used in the Skyline GT-R so as to give the vehicle a rear-wheel drive bias (and associated oversteer handling characteristics).

  • avatar
    Vulpine

    When did the Jeep Wrangler get full-time 4WD? I believe there’s an error in that particular listing. The Wrangler–at least the JK up to now, still uses the Part Time Longitudinal system as described for trucks.

    • 0 avatar
      AJ

      Correct, the transfer case in Wranglers is still part-time.

      I’ve got a ’04 Jeep Liberty in my garage with the SelecTrac transfer case, that does have full-time and part-time. It’s an awesome transfer case. I love having the full-time for mixed road conditions, and the part-time when more is needed. I actually wish my TJ Wrangler had SelecTrac. There was a guy I read that did install one in a TJ. The SelectTrac transfer case is just a little longer then the CommandTrac.

      • 0 avatar
        DeadWeight

        Brother-in-law had the last gen ’03 Liberty 4×4 (not the new, worse generation).

        Aside from the thirsty 3.7 liter, that was one hell of a vehicle, IMO, and I don’t understand the lack of love it received from the Jeep heads.

        I drove it for a few months when he was overseas, and it had a rock solid chassis, great steering (especially for that class of vehicle), a stout torque curve, and was way more civilized on road in terms of road and wind noise than any Wrangler by a massive factor, while still being very capable off road.

        It was also extremely reliable. I do not believe he had a single problem with it over the course of the 100k+ plus miles he owned it for.

  • avatar
    Andy D

    Yah yah yah. You completely omitted the AMC FUll time Quadratrac system on the SJ platforms. They pre-dated the Eagle by 7 0r 6 yrs. They even made one with dual wheels When AMC went with the 727 Torque flight , a new process 219 was used These had a low range. Also the 229 which was used later It featured AWD, 2 wheel, and 4 low They used a viscous coupling, the earlier Borg Warner 1339 used cone clutches. They are pretty darn truck like and the Grand Wags cruised at 70 mph, in air conditioned, leather upholstered luxury.

    My Ford Ranger has part time 4 wheel drive .Around town I run around in 4 wheel with the front hubs unlocked. The electric shift on these is problematic, so I replaced it with a pair of needle nose vise grips. It is a PITA, but is a positive shift now.

  • avatar
    scottcjordan

    I’d like to see the legal definitions addressed as well.

    I was once advised by a Chrysler dealer that the “AWD” minivans they offered at the time (circa 2000) were NOT considered 4-wheel-drive by the California Highway Patrol for the purposes of chain control. IIRC, their design offered only about a 15% maximum power split to the rear wheels. If you went skiing in an area where chains were required for non-4WD vehicles, the salesman advised that the Chrysler AWD minivan was required to wear chains, and not just carry them.

    Way to kill a sale. Since my mission in life is to avoid putting chains on my car, that eliminated those vehicles from consideration, to the disappointment of my mechanic. For what it’s worth, the CHP website currently suggests all “all-wheel-drive/4WD” systems are the same from a chain-control standpoint: http://www.dot.ca.gov/hq/traffops/trucks/ops-guide/chains.html#allwheel …this would seem authoritative for California, but I’d be interested in an analysis of the issue as it may apply elsewhere.

    • 0 avatar
      Alex L. Dykes

      There is no legal definition in the state of California other than all four wheels be powered. There is no differentiation between systems that lock or not. The problem is that the CHP officer may try to make you put chains on if they are not well informed. What is your option at that point? Essentially nothing. So far I haven’t had a problem with the CHP in all manner of AWD cars, wagons and crossovers.

  • avatar
    pattech2000

    I drive a Lexus RX with AWD. I don’t like the handling when accelerating or decelerating at speed in a turn on dry pavement. I have tried various tires and the handling seems to be the same. In my opinion I think that car is dangerous and I wouldn’t buy another one with AWD.

  • avatar
    Kiwi_ME

    Regarding the list of Full time systems with a longitudinal layout, you can add the 2005-current Suzuki Grand Vitara. It has an unusual pawl and cam type center differential which provides some mechanical LSD function. The front and rear axle (open) diffs are of a similar size and somewhat smaller than what you expect for the rear of a RWD-only vehicle. Although there are lessor variations of the Grand Vitara sold in some markets including 2WD only, the full spec 4WD models have a 2:1 low range in center-locked only mode. Models from 2009 also have electronic traction control using the brakes.

  • avatar
    EricTheOracle

    Ford’s auto-track system in their F150s is wonderful. I drove 200 miles in an ice storm in Minnesota t 70 mph passing basically everything and when the truck stepped out, electronic nannies stepped in the square things up. in 2WD or 4WD, the truck tracked very, very well, and this on the stock Goodyear Wranglers with 20,000 plus miles on them.

    I did wish i had the new two speed electronic transfer case that came out in 2012 on my 2011 F150 as then I could have left the truck in AWD, rather than manually switching from 2WD to 4WD as conditions warranted.

  • avatar
    tenmiler

    Really informative and the comments are great as usual for this low-tech guy.

    I’ll add to the pile of anecdotal proof here, but I’m surprised Haldex doesn’t get a bit more detailing here, just purely from a technical standpoint (and in the list of cars not one Volvo?).

    I’ve owned 2 Volvo XCs, one Outback, and currently the best AWD system (for me, where I live, in 300+” of snow Summit County, Colorado) is my 6speed Volvo V50, with beefy Hakk snow tires for winter only (best dump of cash I’ve ever spent).

    My Outback was not great in the snow. It was serviceable (and is the wagon of choice up here), but the AWD system on my 07 XT was unpredictable as was the noisy 5 speed that was one of the most unpleasant drives I’ve had on dry pavement. I feel like they’ve done a masterful job owning the segment from a marketing standpoint and it sounds like they do have a very unique AWD design, but having driven them for years, it is nothing superior in the world of ice and snow up here in the Rockies, other than it’s ride height.

    Anecdotal, but I’d love to understand better how Haldex improved (I’m guessing software) over the years between my older XC Wagons and my current little V50 (which I can power up a dirt road with 8″ fresh altho I have no fun clearing snow out of the spokes). I was told it’s Haldex 7, but to me that sounds like marketing.

    No more Subarus for me for a variety of reasons, when Outback owning neighbors drive my wagon, they remark how much more stable it is. Not sure if it’s something other than the AWD, but I personally think Subaru trades more in marketing.

    Here’s a very long youtube roller test, and if you go to about 6:15, you’ll see Volvo XC 07, then a Subaru Forester (looks like a 2011-12) fail, then an XC90 pass. https://www.youtube.com/watch?v=tpp5tW71qYI&feature=youtu.be&noredirect=1

  • avatar
    Snowrocket

    Well, this shows that anyone can write an article, but not an accurate one.

    Your description of the AMC Eagle’s transfer of power is wrong. It took its Quadratrac transfer case directly from the Jeep Cherokee and J-series tricks, going back to 1973. The 1980-81 Eagles sent power from the transmission to the transfer case, which contained the center differential. The center diff had a viscous limited-slip unit attached to it. Then the power went from the transfer case to the front and rear diffs. The rear diff had a standard clutch-style limited slip unit. The front-to-rear power split was 50/50%, the same as the Cherokees/J trucks with Quadratrac. If EITHER the front or rear axle would spin (on ice, let’s say), the viscous unit would transfer some power to the axle with MORE traction. This was adequately explained in every technical write-up in all the magazines that reviewed these cars in the 1980 model year.

    Later versions (1982 on up) offered Selec-Trac, where the driver could choose RWD or AWD. I RWD, the center diff would lock, and part of the front drive would disconnect for fuel savings.

    • 0 avatar
      Jack Baruth

      Appreciate the clarification!

      —Jack Baruth, hopeless AMC Eagle fan

      • 0 avatar
        Snowrocket

        Jack, you’re quite welcome. A few worthwhile points (in my view) about the AMC Eagle:

        * It was “parts-bin engineering” at its most economical and finest of that era. It REALLY put floundering AMC back on the map for a time. It was effectively a 1970 AMC Hornet with a 1973 Quadratrac transfer case, a little more ground clearance, and some fender flares. But, it was UNIQUE and WORKED well.
        * Subaru lies. They once claimed their Outback to be “the world’s first sport utility wagon”. Paul Harvey, a well known radio personality coined the term “SUV” …in the late ’80s/early ’90s. The Eagle wagon was first, about 10 years ahead of the term “SUV”, and 15 years ahead of the Outback.
        * Even though I’ve never driven or owned one, from all accounts, the AMC Eagle was the best American designed snow car ever built. It had LEGENDARY go anywhere capability.
        * One of the dumbest Chrysler Motors moves of the ’80s: Not redesigning the Eagle, and expanding that lineup as the “AWD division” of Chrysler. A truly missed opportunity. Audi and Subaru forged ahead while Chrysler looked the other way.

  • avatar
    Snowrocket

    The short version that answers the question of “what is/can be considered to be AWD?” is this: If there’s a mode available on the vehicle that lets it drive in AWD/full-time 4WD on dry pavement with no limitation on speed, you can call it AWD. What’s interesting about this article and it’s attendant discussions is the dizzying array of ways to power front and rear axles. Additionally interesting is the real/perceived effectiveness of any given system.

    I’d love to get AWDs from every manufacturer, and split them into various groupings, IE: cars, SUVs, and trucks. Then put each group onto whatever tire make and model could be found to fit each vehicle within the group. Then do extensive testing and seat-of-the-pants drives on and off road, then rank them. When you eliminate tires as a variable, the true value of the AWD hardware will be more readily apparent.

  • avatar
    Texmiguel

    I understand that if you have a 4 x 4 and get a flat , u need to change all four tires !! That is if the flat cannot be repaired . . . ride height has to be the same for all or risk damage to AWD system.

    • 0 avatar
      Snowrocket

      Texmiguel: The answer is “yes and no”. Ideally, you use a full-circumference spare, be it a regular tire or a space-saver spare. The AWD won’t know the difference and all will be fine. I had a situation with my Subaru Impreza where I bought it knowing that all four tires were a larger than stock circumference/diameter (195/70/15). I put two spares (185/70-14; correct diameter for the car, but smaller than ON the car) in the car before a long trip. If I got a flat, I’d change one front and one rear tire. The front and rear diffs are open, so no issues across each axle. The average rolling circumference/diameter of EACH AXLE would be correct as far as the viscous limited slip center diff was concerned. Therefore it would be no problem. You can’t do this though if you have limited slip diffs at either or both ends. They’ll “sense slippage”, and try to “resolve” that problem, ruining the limited slip hardware. On a part-time 4WD with open front and rear diffs, just throw on your spare (any size) and run in 2WD until you get your tire fixed.

  • avatar
    Texmiguel

    THX Snowrocket for the info. Complicated comes to mind. Would like to throw on some cheap snow chains(2) on a 2wd front wheel drive regular all season car. Can you get more than one or two yrs with that setup? And is it worth the trouble – top speed limit and hassle guessing when to put them on and doing it. 4wd and snow tires seems so free-spirit but expensive. Understand u r from Ohio.

    • 0 avatar
      Snowrocket

      Texmiguel: AWD tire diameters are not complicated; keep all the tires within about 5% circumference of each other all the way around and everything will be fine. Snowchains on a FWD: NAAAH! Too much noise, vibration, and inconvenience! Go with four narrow snow tires instead. Go to TireRack.com or DiscountTireDirect.com and see which snow tires are highest consumer rated for your FWD car. Then buy them and put them on four junkyard rims. The difference it makes is huge. It’ll go, stop and steer SOOO much better than summer and all-season tires. Tires are the singular biggest improvement you can make to the average car.


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