By on February 8, 2014

ZF 9HP Transmission, Picture Courtesy of Land RoverIn a week we will post our first full review of the all-new and all-controversial 2014 Jeep Cherokee. The new Jeep isn’t just raising eyebrows for the love-it or hate-it styling. Or the resurrection of the Cherokee badge. Or the constant delays in production. Or the transverse mounted engine. Or the lack of solid axles. None of that laundry list seems to cause as much discussion around the automotive water cooler than ZF’s 9HP 9-speed transmission. Click past the jump for a deep dive into the tranny with more speeds than my bicycle. If you don’t want to explore transmissions in detail, don’t click. You have been warned.

When Derek drove the Cherokee at a launch event he complained about the transmission. When I drove a pre-production model for a very brief hour and a half I was more perplexed than anything. I chalked it up to pre-production programming issues and the fact that the transmission has 50% more speeds than a 6-speed, so I expected 50% more shifting. A month later I was able to sample a different Cherokee with newer software and some of my shifting complaints had been solved but something still felt “wrong.” Now three months later a full production Cherokee landed in my hands and while the shift logic (when and why the transmission would shift up or down) was finally where I thought it should be, the shifts themselves felt different from what I am used to. The reason is all down to clutches, but let’s start at the beginning.

In general terms an engine is most efficient in a somewhat narrow band of RPMs. That exact band varies from engine to engine based on what the designers intended at the time. The longer you can keep the engine in this range of RPMs, the more efficient the car will be. Secondary to this is a desire for improved off-the-line performance, this necessitates ever-lower first gear ratios. The distance between the lowest and the tallest gear in a transmission is called the ratio spread. (You get it by dividing the lowest ratio by the tallest and that gives you a number that represents the delta between first and last.) GM’s venerable 4-speed 4L80 has a spread of 3.3 while their new 6-speed 6L80 has a spread of 6. The deeper first gear and taller 6th allow the 6L80 to deliver better performance and better fuel economy. The reason ZF’s 8-speed 8HP doesn’t have the same delta in performance over the average 6-speed as the 6-speed had over the 4-speed, is easy to explain. The 8HP’s ratio spread is 7, just 1 higher than a 6 speed while the 6-speeds had a 3 point advantage over the 4-speeds. Aisin’s new 8-speed transaxle in Volvo and Lexus models goes a small step further with a 7.59 spread. These can all be seen as progressive improvements. The 9HP is different. With a 4.7:1 first gear and a 0.48:1 ninth gear the overall spread is a whopping 9.8.

928TE

On closer inspection you’ll notice something interesting about the 9HP’s ratios. Fifth is the 1:1 ratio where the output shaft of the transmission is spinning at the same rate as the engine meaning there four overdrive ratios. In contrast both ZF and Aisin’s 8-speed transmissions have just two overdrive ratios with 6th gear being the direct-drive (1:1) ratio. As a result the 9HP’s lower gears are farther apart, especially first and second gear. When you look deeper at the numbers you’ll also notice that the 9HP is geared much taller at the top end with 7th gear being approximately equal to 8th in the Aisin or ZF 8-speed units. Many reviewers of the Cherokee noted they never experienced 9th gear during their test drive and I now know why. At 0.48:1 with the 3.2L V6 (3.251 final drive) you have to be going faster than 80 MPH to engage 9th because at 80 your engine loafs around at 1,460 RPM. (The 2.4L four-cylinder in the Cherokee Trailhawk would be going about 1,810 RPM at 80.) According to ZF this results in an impressive 12-16% improvement in fuel economy versus the same final drive ratio and their own 6-speed automatic and 11-15% when compared to their 8-speed.

OK, so the 9HP has plenty of gears, but why does it shift the way that it does? It’s all down to the clutches. While a traditional automatic uses friction clutches in the form of either band clutches or multi-plate friction clutches, the 9HP blends friction clutches and dog clutches in the same transmission case. Dog clutches are “interference” clutches more commonly found in manual transmissions and transfer cases. Friction clutches work by pressing two plates together. The friction between them allows the transfer of energy and it allows one plate to spin faster than the other or “slip.” Think of slipping the clutch in a manual car, it is the same action. Automatic transmissions use this clutch type to their advantage because changing gear doesn’t always require engine power to drop, the transmission simply disconnects one clutch as it engages another, they slip and engage and you’re in another gear. Dog clutches however are different. If you look at the illustration below you can see a dog clutch on the right. Power is transmitted by the tooth of one side pressing on the tooth of the other. This type of clutch cannot slip so it is either engaged or disengaged. This is the type of clutch used inside manual transmissions. When you move the shifter to a different gear, you are physically disengaging and engaging dog clutches. This style of clutch is used because it suffers little parasitic loss and it is simple and compact. The use of a dog clutch in an “automatic” transmission isn’t new, dual clutch robotized manuals use this style of clutch internally as well, but it is the key to understanding why the 9HP shifts the way it does.

21erqc3

Because dog clutches can’t slip, their engagement must be controlled and precise. Going back to the manual transmission example, this is why modern manual transmissions have “synchros” or synchromesh. A Synchro is a mechanism that aligns the dog teeth prior to engagement. Without them you get that distinct gear grinding noise. Synchros work well in a manual transmission because when you are changing gear you are disconnecting the engine with the clutch (a friction clutch), then engaging a dog clutch for your gear selection. Because one end of the transmission is “free” the synchro synchronizes the two sides and then allows the toothed gear to engage. There is a “pause” in power when a shift occurs. If you look at an acceleration chart of a car with a good manual driver and an automatic you will see pauses in acceleration in the manual while most autos just have “reductions” in acceleration. That’s down to the pause required to engage a dog clutch vs a friction clutch that slips and engages without much reduction in power.

Let’s digress for a moment and talk about the DSG. The reason dual clutch gearboxes exist is because of the dog clutch. As I said engaging a dog clutch takes time and precision. This is part of the reason single-clutch robotic manuals like the one in the Smart ForTwo and the RAM ProMaster (and other Euro sedans) have such exaggerated shifts. Double clutch gearboxes get around this by having two gears engaged at all times. DSG style gearboxes are really two manual transmissions in the same case. 1st gear is engaged via the first transmission and 2nd is engaged but not active on the second. Changing gears simply involves swapping (via a friction clutch) from transmission A to transmission B. Once that is accomplished, the transmission A disengages and engages the dog clutches to select the next gear. Going from 2nd to 3rd involves swapping back from transmission B to the already shifted transmission A.

Let’s put it all together now. To save space and increase efficiency, the 9HP uses two multi-plate clutch elements, two friction brakes and two electronically synchronized dog clutches. (The 8HP uses two brakes and three multi-plate clutches.) The way the gearsets are arranged inside the case, shifts from 1-2, 2-3, and 3-4 involve only the traditional friction brake and clutch elements. As you would expect, aside from 1st being fairly low and somewhat distant from 2nd, these shifts feel perfectly “normal.” Under hard acceleration there is a momentary reduction in engine torque (courtesy of the computer to reduce clutch wear) and the shift occurs quickly and smoothly. The shift from 4-5 however is different. The transmission has to disengage dog clutch “A” in addition to engaging a friction clutch. This shift takes slightly longer than the 3-4 shift and the car’s computer makes a drastic reduction in torque to prevent wear of the dog teeth. Shifts 5-6 and 6-7 again happen with the only the friction elements at which point we need to disconnect the final dog clutch for gears 8 and 9 so we get the same kind of torque reduction in those shifts. The result is a transmission that has two distinct “feels” to its shifts, one that has only a slight torque reduction (1-2, 2-3, 3-4, 5-6, 6-7, 8-9) and one that has a more “manual transmission” feel where torque is cut severely (4-5 and 7-8).

2014 Jeep Cherokee Instrument Cluster

Because of the positioning of the two dog clutches in the shift pattern, the torque reduction isn’t objectionable in upshifts. Hard acceleration from a stop didn’t involve 5th gear even in the 1/4 mile. However, once you let off the gas the transmission will shift upwards rapidly for fuel economy settling in 6th or 7th in the 60-65 MPH range and 8th in the 70-75 MPH range.

Downshifts are where the 9HP truly feels different. Because of the design, if you’re in 8th gear and want to pass, the transmission will often need to drop 4 or 5 gears to get to a suitable ratio. (Remember that 4th gear is the first ratio going back down the scale that is lower than 1:1.) To do this the transmission has to accomplish the harder task of engaging two dog clutches. To do this the transmission doesn’t use cone synchros like a manual (too bulky) it uses software. Engaging dog clutches requires a longer and yet more severe reduction in torque than the disengagement because the transmission has to align the clutch and then engage it. In most automatics when you floor the car you get an instant feeling of acceleration that improves as the transmission downshifts. Although there would be moments of power reduction (depending on the programming) during this time, the engine is always providing some force forward. The 9HP’s software on the other hand responds by cutting power initially, then diving as far down the gear-ladder as it can, engaging the dog clutches and then reinstating your throttle command. The result is a somewhat odd delay between the pedal on the floor and the car taking off like a bat out of hell. According to Volvo’s powertrain guys, this shift behavior is one of the main reasons they chose the Aisin 8-speed (shared with the Lexus RX F-Sport) over the ZF 9-speed used by Land Rover and Chrysler.

All of a sudden the “odd” shift feel made perfect sense. In the march toward ever-improving fuel economy the automotive public will continually be introduced to cars that feel different from the “good old days.” Electric power steering numbs the wheel-feel but steer-by-wire promises to artificiality resurrect it. Dual clutch robotized manuals have a particular feel that was accepted by performance enthusiasts but has been a source of complaint for Focus and Fiesta shoppers. For me, understanding why the transmission is doing what it is doing is key to my like or dislike of a car’s road manners. Once I understood what the Cherokee’s automatic was up to, I was able to focus on the rest of the car. What about you? Are you willing to “sacrifice” shift quality at the altar of fuel economy? Be sure to let me know.

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132 Comments on “ZF’s 9-Speed 9HP Transmission Puts Dog Clutches On The Leash...”


  • avatar
    doctorv8

    Wow Alex. Fantastic explanation. Thank you!

    • 0 avatar
      rentonben

      Agreed! Thanks Alex for explaining that mysterious blob of incomprehensible gears.

    • 0 avatar
      gmichaelj

      But a poor summation –

      “Once I understood what the Cherokee’s automatic was up to, I was able to focus on the rest of the car….”

      Come on. Does it work like you want, or expect it to, or not? I don’t want to have to “understand” a quirky transmission when I go to pass someone.

      Sounds like a transmission that needs improvement to me. Or worse, a poor concept from the beginning. A no-go from the get-go.

      • 0 avatar
        redav

        I disagree. Understanding how a thing works enables you to make it do what you want, while not understanding it leaves you wallowing in the fear of strangeness and uncertainty.

        It’s a lot like those who claim they can’t drive a car with under 200 hp or say that a low-torque but high-revving engine sucks. Neither is usually the case; instead it is just a case of someone not knowing/understanding how to drive the car.

        Now that Alex understands the transmission, I have no doubt that he can drive in a way to make it do what he wants. He made it clear (to me) that different transmissions are different, but maybe not necessarily better/worse.

        • 0 avatar
          gmichaelj

          I’ll agree that one may know that it doesn’t need to be repaired when it is slow on the downshift (“oh yeah, that’s what it does”).

          However, I’d prefer/expect/demand better performance for a new car purchase. Based on his description, I’d avoid this transmission.

  • avatar
    hreardon

    Great explanation of a very complicated system, Alex. Thank you.

    I do wonder, however, at the long-term reliability of the ZF9.

  • avatar
    raph

    Man, when I hear dog clutch I think of Jerico transmissions and Detroit Locker rear ends both use dog clutches or a variation of to do their jobs. The former so a driver can dispense with using the clutch altogether and keep the throttle floored and the latter as a positive locking mechanism to couple both rear axles.

  • avatar
    Vance Torino

    Dang!

    However, I am now happy to know that the “Hybrid Synergy Drive” in my Prius is actually a paragon of engineering simplicity!

    So are the days of “rebuilding” transmission basically over?

    • 0 avatar
      indi500fan

      Rebuildability is a function of what fails, how much do the replacement parts cost, and how many hours labor. This one would certainly take longer to do than a Powerglide 2-speed, but it’s still just a (complex) stack of parts.
      Of course the goal should be to have the trans life and engine life meet the vehicle “life”. I wonder how many vehicles these days get a premature trip to the scrap yard because of a trans failure? I really have no idea.

      As far as heat generation, the dog clutches have no slip, they’re either “in” or “out” so this design likely generates less heat during a typical high throttle upshift cycle.

    • 0 avatar
      lurlene

      From a mechanical engineering perspective it might be a paragon of engineering simplicity.

      From a software and electrical engineering standpoint it most certainly is not.

  • avatar
    AdamVIP

    Great read.

    My wife really likes the new Cherokee and it may be the new babymobile for us. I was able to drive one around town but not on the highway yet. It seemed great when I drove it but but I guess I can hold out till your full review. Ill make sure to throw it on a highway test drive prior to bidding it out.

    Although I have to wonder with 9th gear being so high. Whats the purpose? If Im going 90 Im not worrying about fuel economy. Seems that would have been better placed as a non overdrive gear.

  • avatar

    The auto enthusiast in me is greatly saddened that manual transmissions are going away.
    The engineer in me is really fascinated by how modern automatic transmissions work.
    Thank you for this highly detailed explanation.

    • 0 avatar
      dantes_inferno

      The auto enthusiast in me plans to keep my manual transmission vehicle for a long time.

      The engineer in me concurs with the auto enthusiast in me.

    • 0 avatar
      Rasputin

      The only reason I would ever own an automatic is if I found myself with a long daily commute involving lots of stop & go traffic. In the early ’80′s I found myself commuting half of Long Island into Bay Ridge, Brooklyn, driving the Southern State & Belt Parkways. After three weeks my ’76 Bavaria was garaged for an automatic Dodge Aries.

      Fortunately my daily commuting days are over, as are my days in an automatic.

    • 0 avatar
      Sam P

      I prefer a manual gearbox, but one of our cars has a 6 speed automatic transmission. It’s much easier to live with than the awful old 4 speed automatics of yore.

      As long as one changes the fluid every 2 years or 30k miles, a modern automatic should last a long time if the design isn’t fundamentally flawed.

  • avatar
    Roader

    “Because of the design, if you’re in 8th gear and want to pass, the transmission will often need to drop 4 or 5 gears to get to a suitable ratio.”

    I get around this on 7-speed bicycle by downshifting the 3-speed front gearset. Would a dual automatic, say a 5-speed box coupled with a 2-speed box, cost too much? The computer wouldn’t have any problems controlling them.

    • 0 avatar
      JimC2

      I think you not only answered your own question, but the way the 9HP works, if it drops down to 4th then it is basically doing just that- dropping down to the highest gear selection within the “low” range. Also, try to think of the 8-4 shift as 8-4, not as 8, 7, 6, 5, 4, and the 8-5 shift as straight there, 8-3, etc… just like you might shift a manual from 5-3 or 5-2 if you wish to pass with strong acceleration.

      … or something like that.

  • avatar
    NormSV650

    Does this change the tow rated? Does cooling capacity increase with the addiditon of dog gears engagement?

    • 0 avatar
      tedward

      It seems like it might but there are Aisin 8-speed vehicles out there with very high tow ratings and 400+ lb/ft torque ratings. Maybe it might influence cost of transmission fluid changes if one design needs less than another (I’m just speculating, too lazy to look it up.)

  • avatar
    sportyaccordy

    Holy crap!!!!! Awesome explanation Alex, keep em coming!

    Bizarre that they went through all the trouble for 9 speeds when top gear is essentially unusable though. Also wondering if they really gained any significant fuel economy advances by not using friction clutches in the transmission. Time will tell

    • 0 avatar
      Crabspirits

      You said it.

      These lifted mommymobiles have no business topping 80mph anyway. They should make it so fuel mileage is so bad and engine so obnoxious that you don’t want to go there, not the other way around.

      The bog shift doesn’t sound like much of a problem, unless it can happen in an emergency scenario, like pulling into traffic or crossing an intersection.

    • 0 avatar
      LeeK

      Maybe because ZF provides transmissions to European manufacturers where everyday commuters can and do exceed 80 MPH? It does seem like a waste here in the US (Jersey Turnpike excepted). In Germany, maybe not so.

      • 0 avatar
        JimC2

        Everyday commuters exceeding 80mph?!? Blasphemy! I was raised to believe that going the slightest bit over 55mph was dangerous! Think of the children! Harumph!

      • 0 avatar
        billfrombuckhead

        Cherokees will be sold all over Europe and around the world.

        Funny hearing a Honda fanboi talking about transmissions, I guess that is a an alternate reality.

        • 0 avatar
          CJinSD

          Ultradrive for the loss. A518 to pile on. Chrysler made the best automatics in the world. Then they didn’t.

        • 0 avatar
          AlternateReality

          Let’s see if you still think I’m funny in six months or so, billy boy.

          Lots of people have bought Cherokees with this gearbox. Flaming morons, every last one of them… but all that really means is that there will also be lots of very angry customers for Fiasler to deal with once things start breaking.

          • 0 avatar
            billfrombuckhead

            Honda is going to used these same ZF transmissions. ROFL.

            http://www.hondatalk.net/2014-honda-acura-will-adopt-9-speed-automatic-gearbox-of-zf-by-2014/

            http://wot.motortrend.com/we-hear-honda-acura-getting-zf-9-speed-automatic-by-2014-182755.html

            http://www.automobilemag.com/features/news/deep-dive-honda-acura-could-adopt-zf-9-speed-automatic-by-2014-118443/

          • 0 avatar
            AlternateReality

            Well, f***. I may have just bought my last HondAcura, then.

            If the rumors are true, at least the Japanese are wise to allow Fiasler customers be the guinea pigs for the 9HP before slapping it into its products on blind/desperate faith.

    • 0 avatar
      redav

      This thing’s 9 is similar to the new Vette’s 7 (in the manual).

      Plenty of people around here drive well above the 80 mph mark, regardless of posted speed. Also, while I’ve only been in Europe a couple times, I highly doubt there are more drivers there than here who would benefit from such a tall gear.

      I get the feeling that the 9 (instead of 8) is more about marketing so those who buy cars based on stat comparison have a line item to highlight.

    • 0 avatar
      danio3834

      The ZF design was basically pulled off of ZF’s shelf. The original design intent had the Autobahn in mind where 160mph is common.

    • 0 avatar
      carve

      It depends where you live. Here in NM the speed limit is 75 and you’ll often be passing all kinds of mommy mobiles going 80-85 in the right lane. I took my 95 Cherokee skiing last weekend and spent plenty of time going 80-85. It tachs about 2300 rpm and gets about 20 mpg doing it. So, for me a 9 speed would be great for my weekend road trips, and I DO care about fuel economy at 80+ mph. On dry days I’ll take my 335i and go about 90-95 on the freeway. Same for road trips to Vegas or Arizona. Lots of straight, smooth, open road and few people are doing less than 85 during good conditions.

  • avatar
    RogerB34

    High risk complex tranny dependent on a computer program. Overhaul would be very expensive.
    Recall the CVT intro problems?

    • 0 avatar
      Crabspirits

      I doubt overhaul would be more expensive than something like a GM 4L80. There are more gears at work here, but less wear items like clutches.

      You do have a design where your clutches have an increased duty cycle though, so longevity can be a concern.

      • 0 avatar
        28-Cars-Later

        Good info, so its very possible the CVT will enable cars to be more disposable than many of them already have become. I say this because as it is when an auto transmission blows depending on the age/condition of the car they are often junked.

    • 0 avatar
      AlternateReality

      This promises to be even worse than a CVT. We’re talking about many, many more moving parts and an infinitely higher level of complexity, stuffed tightly into a compact case that must fit in the cramped confines of a small engine bay, with the attendant cooling problems…

      …All bolted into a first-year Fiasler CUV, which history (Dart) shows is likely no paradigm of assembly quality or long-term durability in the first place.

      I suppose it’s possible these will prove to be fairly dependable, but I sure as hell wouldn’t put my own money up for that bet.

      • 0 avatar
        Wheeljack

        It’ll probably be more reliable than a Hacura (Honda/Acura) automatic.

        • 0 avatar
          AlternateReality

          Keep dreaming – Or better yet, put your money up. I’ll be laughing at your broken down Cherokee as I pass you by in my HondAcura.

          • 0 avatar
            tubacity

            As a matter of fact, I did drive my older Chrysler minivan past the shop where my Honda was getting its transmission replaced. Did not laugh as I had to pay for the replacement.

          • 0 avatar
            Deaks2

            Umm, what???

            http://wheels.blogs.nytimes.com/2011/01/08/honda-transmission-problems-seem-to-persist/

          • 0 avatar
            AlternateReality

            Umm, yep. For one, I’m not trying to put V6 torque through my lowly 5-speed, which has been shown to vastly improve its reliability. (Plus you’re citing a three year-old article to boot.)

            And tubacity, I’d trust even an Ultradrive-equipped, genuine Chrysler vehicle before this transmission – which has already proven to be troubled enough to delay a vital product launch – bolted into a Fiat-derived half-breed.

          • 0 avatar
            danio3834

            Ironically, Acura will be using the same 9 speed transmission in the upcoming TLX.

          • 0 avatar
            carve

            Are you kidding? Automatics are the one thing Honda makes that are notoriously unreliable, particularly when coupled to more powerful engines.

          • 0 avatar
            jd95

            And I’ll be laughing at your broken down Honda as I pass you by in my 1997 Cherokee. Haha

        • 0 avatar
          sgeffe

          Isn’t the Honda automatic entirely “dog-based?” I thought that the entire thing is essentially a hydraulically-shifted manual.

          • 0 avatar
            kmoney

            It is essentially a hydraulically shifted manual, but the gears are constantly meshed and there is a traditional wet clutch pack for each gear. Some have a sliding reverse gear, but all forward speeds are constant mesh.

          • 0 avatar
            sgeffe

            Thank you..I thought that explains the firm shifts of a Honda slusher, and may also explain some of the problems they have on occasion.

            That’s one area where GM’s stuff has always shined — IMHO, THMs are darn near bulletproof!

    • 0 avatar
      kmoney

      The thing I would be scared of on this is not only the computer timing of the dog gear shifts, but the associated valve body components. Shift timing is, as on all modern AT’s, a combination of electric solenoids over a hydraulic circuit and minimal wear of the valve body can result in shift timing problems. This is much less of a problem on units that utilize friction and sprag clutches; however with this design it has to be spot on all the time.

      To the 4L80 example. In terms of repair costs, clutches aren’t really a big expense in rebuilding a transmission — an entire “friction kit” is under $20 at cost for most common units. The expense is in factory-only valve bodies, $200 EPC and shift solenoids, compound planetary gears, pump assemblies and other things like this.

      I don’t know if the aftermarket manufacturers (Sonnax, Transtar, et al.) have parts out for these yet, but if they don’t the units will likely be very expensive to repair at first. It isn’t until aftermarket parts come out and shops build up a decent supply of used units from which they can pull hard parts from that the price begins to drop. This is essentially the same pattern that follows on the release of any new transmission.

  • avatar
    PrincipalDan

    “The 9HP is different. With a 4.7:1 first gear and a 0.48:1 ninth gear the overall spread is a whopping 9.8.”

    Holy cow. Does anyone think that there is a theoretical limit to the spread? 10.0? 12? Does the Trailhawk version hold the transmission in a low gear when going off road?

    “At 0.48:1 with the 3.2L V6 (3.251 final drive) you have to be going faster than 80 MPH to engage 9th because at 80 your engine loafs around at 1,460 RPM.”

    So you see officer to use all the gears I paid for I need to drive 90 mph.

    • 0 avatar
      Dan

      Theroetical limit? With a torque managed 1st gear that redlines before 35 mph and a 9th speed you effectively can’t use at all we’re already there and beyond.

      • 0 avatar
        JimC2

        Regarding overall ratio spreads, it took me some searching to find the ratio spread on my long-ago-sold Civic Hybrid (second generation), which was a then-impressive 6:1 (2.53 low and .42 high). Two significant notes, this was a belt CVT and it did not have a torque converter (torque converters approximately double torque during off-the-line acceleration and at very low speeds). Obviously the 9HP is entirely different.

        The car could and would smoothly loaf along in the CVT’s top ratio (moreso on slight downgrades or if I was drafting, tsk tsk, but also sometimes on level roads) even as low as 1100-1200rpm, and there were fuel savings to be had from that. Not sure why ZF and Chrysler apparently decided to lock out 9th gear below 80mph, but I’m sure they put substantial thought and effort into that decision.

        Anyhoo, at nearly 10:1 we’ve come a long way from the Powerglides and Torqueflites.

        • 0 avatar
          PrincipalDan

          I only ask because when I sampled the 3.6 VVT DI GM V6 with FWD 6 speed trans the car loafed at less than 2000 rpm at 85 mph. What more do you need? That engine was at as low an RPM at 85 mph as the old SBC 4 speed auto combos were at 65 mph.

          ZF is now building a 9 speed that doesn’t even use 9th gear except in extreme situations. What is the point? I know that there has been discussions of 10 speed units. When would 10th gear be used? 100 mph?

          • 0 avatar
            JimC2

            10th gear would be used at ludicrous speed! :)

            “Sir, hadn’t you better buckle up?”

          • 0 avatar
            mike978

            Thanks for asking the question I wanted to know, where would this end. It sounds like 8 gears is pretty much the top of the useable range. I can see why there was such an improvement going from 4 to 6 speed transmissions (and hence the complaints of those few cars that still use a four speed transmission), but going from 6 to 8 isn`t such a great improvement.

          • 0 avatar
            bumpy ii

            I would suspect that ZF had in mind countries with higher speed limits than the US when they designed the 9-speed.

            As to why 9th is locked out below 80 mph, maybe the Cherokee is aero-limited below 80 mph in 9th gear?

          • 0 avatar
            JimC2

            “As to why 9th is locked out below 80 mph, maybe the Cherokee is aero-limited below 80 mph in 9th gear?”

            Right- although like more like a combination of very slow acceleration in 9th gear plus the fact that the 8-9 shift, just as the 4-5, requires an awkward moment to complete equals too many customer complaints to the dealers.

          • 0 avatar
            CJinSD

            Is there any country other than Germany where traveling over 81 mph is legal? Traffic enforcement in Europe is far more effective than in the US due to the governed having no voice against speed cameras and the like. I’ve watched a few Brits come to the US prepared to mock our highway speeds only to find them quite a bit quicker than their own, at least between LA and San Diego.

          • 0 avatar
            mcs

            >> Is there any country other than Germany where traveling over 81 mph is legal?

            85 mph in Texas. I wonder if 9th kicks in on some downhills?

          • 0 avatar
            dantes_inferno

            > I know that there has been discussions of 10 speed units. When would 10th gear be used? 100 mph?

            I guess the manufacturers are trying to achieve an 800 rpm engine speed at 100 mph.

          • 0 avatar
            Brian P

            IIRC Poland’s motorway speed limit is 140 km/h.

          • 0 avatar
            Pch101

            Bulgaria is also at 140.

            Italy theoretically allows 150, although I’m not sure whether the higher limit is actually in effect.

            The Northern Territory of Australia has begun conducting a trial with no posted limit.

          • 0 avatar
            mike978

            “I’ve watched a few Brits come to the US prepared to mock our highway speeds only to find them quite a bit quicker than their own, at least between LA and San Diego.”

            US roads can allow fast travel but the risk of having a police car see you is quite high. At least in the UK you know where the speed cameras are since they are painted in a bright yellow and there is a website telling you where they are located ahead of time (http://www.speedcameramap.co.uk/). I have seen very few (<5) police cars patrolling motorways in my 15 years of driving in the UK.
            I am still amazed at the sheer number of police forces here in the US – for example just in my local area we have Orange County Sheriff, Hillsborough town police, Chapel Hill town police, UNC police, Duke University police and the State Highway Patrol. Grossly inefficient to have so many overlapping police forces. Does a university really need it owns police force – in other countries the local area police would cover it.

    • 0 avatar
      redav

      Number of gears and the spread are two different things. Like a bicycle, you can add gears to either increase the spread or to permit better fine tuning of engine speed. It’s about working with the engine & weight & aerodynamic drag of the car. If you have a good engine with a broad rpm range of flat torque, fine-tuning the engine speed is not critical. As for spread, both top and bottom end should be considered. Over the last couple decades, speed limits have gone up, sometimes significantly. I have a 13 yr old car that honestly is not well suited for >65 mph. That just doesn’t cut it in today’s market. Cars need to be able to drive well & efficiently at 80+.

      Why wait to engage that top gear? Others mentioned the rougher shift, and that’s likely part of it. I think there may be an issue of at that speed, the engine just doesn’t have enough reserve power, so any incline, passing, etc., would cause it to downshift anyway. The higher top gear would thus be available for the rare cases it can be of benefit, but wouldn’t go into it when it would end up shifting right back out, anyway.

      The other side is making a lower 1 gear to help acceleration. Heavier cars would benefit from that. And as noted, traditional automatics have the torque multiplier, which is sort of like an extra low gear, anyway.

    • 0 avatar
      cobra7

      The Cherokee limits the available gears dependent upon the drive mode. For example, if using “Snow” mode, the maximum gear is limited to 6th. There are similar limitations for other drive modes and also for when using 4WD low range.

      In addition, the driver can select the maximum gear by pulling the shift lever to the left and then pushing it forward or backward to set the maximum gear that the transmission can use.

      So, there are several controls in place, both computer-controlled and driver-controlled, for various terrain and driving conditions.

      These features aren’t specific to the Trailhawk, they are present in any Cherokee that has the ActiveDrive II or ActiveDrive II-Lock 4WD system. The former is available on the Limited trim level, while the latter is standard and exclusive to the Trailhawk.

      I only know this stuff because I happen to own a Trailhawk.

  • avatar
    Aleister Crowley

    Excellent explanation. It appears that Volvo went with the Aisin 8 speed automatic for the driver experience, and Sergio went with the ZF 9 speed for the fuel economy and marketability. A suburban mom might say, “My transmission has more gears than your transmission.” As Alfred Hitchcock once said, “Never underestimate the intelligence of the American public.”
    Still waiting for your review of the new Subaru Forester. Your reviews are always thorough and informative.

    • 0 avatar
      Wheeljack

      It’s the same phenomenon that has driven many point & shoot digital cameras into the 20+ megapixel range, which is totally unecessary for most people. All it does is result in larger and larger file sizes and the need for larger and larger external hard drives.

      • 0 avatar
        schmitt trigger

        And not only that, the smaller photosites resulting from the higher pixel count, give a higher noise levels for an equivalent ISO setting….but I digress.

        Anyways, fascinating article, explained in a simple way.

  • avatar
    Dan

    “According to ZF this results in an impressive 12-16% improvement in fuel economy versus the same final drive ratio and their own 6-speed automatic and 11-15% when compared to their 8-speed.”

    Read the fine print in ZF’s press kit and you’ll find that the final drive they chose for this comparison was correct for an 0.48 OD. Which is, of course, so short that the compared 6/8 speed (0.69/0.67 OD) are turning 3000/3100 rpm at 80. Strawman much?

    Taken away from the liars in marketing and put it in the real world with a reasonable axle that 12-16% will turn into 1-3%.

    Worth it? Figure out what 2% of a cute ute’s lifetime fuel bill is and when you stop laughing you can tell me.

    • 0 avatar
      mikeg216

      Why do I smell do I smell crap in all of these comments? I love how you are comparing this state of the Art transmission to a cvt which is almost as old as the automobile itself and nobody has gotten right, though Nissan is close. Zf makes great transmissions and this is 2014 and design and engineering tolerances are so tight that I would not worry at all. Any issues if there are any will be early in the cycle,not later. You have no idea how hard they worked to make it perfect before letting it out in the wild, I do. There were 18k of these in toledo that had to be flashed and put through their paces before they would release any

      • 0 avatar
        mike978

        But isn`t it far to ask that if the final drive in the 9 speed is rarely used in normal usage for these sort of vehicles (we are not talking a Corvette after all) then 8th in this transmission vs 8th in an 8 speed transmission would give similar fuel economy. It does raise the question why have 9 speeds? Seems like Volvo and Lexus made the better decision.

  • avatar
    Jakesmobility

    Can we take this info one step further and link transmission choice with differential choice? I imagine the wrong diff could offset some of the benefits of such a wide gearing.

  • avatar
    RHD

    When I was in high school, a friend showed me what a “neutral drop” was in his step-dad’s Gran Torino.
    I’d hate to see the repair bill for a 9HP if a teenager tried that on his family’s Jeep!

    Very well written article! Much appreciated.

  • avatar
    JD321

    Excellent Alex! Thank You!

    I enjoy your work…You are the smartest man in auto journalism.

  • avatar
    suspekt

    Great write up Alex.

    Question:
    Do you have any thoughts impressions on how the Acura RL SH-AWD 7 speed DCT works? If I understand correctly, it is not a typical “DCT”.

    • 0 avatar
      sgeffe

      There’s a regular torque converter someplace in the mix. (I might be full of it, however!)

      • 0 avatar
        Alex L. Dykes

        There is no torque converter in the Acura 7-speed DCT. The confusion is because the transmission employs a dual clutch transmission like VW’s DSG box but it also adds a 2-speed planetary gearset. 1st is obtained by setting the planetary to low and the 6-speed DCT to 5th. Then the planetary resets to high and the DCT progresses 1-6. (Which become 2-7)

  • avatar
    alsorl

    Looks like a lot of new technology for Jeep to handle. Will probably be a good SUV. But, I still can’t stand looking at the front.

  • avatar
    Lightspeed

    Great analysis, made it understandable to a non-engineering type like me. Especially interesting about the purpose for this transmission and why the ratios are such as they are. My Lexus GS400 (2000) has a 5-speed auto and I find it turns way more rpm above 80mph than it really needs to. But when considering Lexus had in mind to out-gun the BMW 5-series, then gas-mileage took a back seat.

    • 0 avatar
      Roader

      X2. I-15 for long stretches in Utah has an 80mph speed limit; cops don’t bother with anything under 85mph. The 0.67 top gear on my Accord has the engine buzzing at that speed. A 0.6 or 0.55 top gear would be nice.

  • avatar
    ihatetrees

    Great write up.

    This transmission is no doubt an engineering marvel. That said, I’m skeptical that a long term cost/benefit analysis of new complex 9-speed vrs improved fuel economy will benefit consumers.

    I’m fairly certain no such prediction modesty bedevils Washington’s CAFE Legislators and Regulatory Geniuses. They, like Soviet 5-year Planners, have absolute confidence in their forecasts.

  • avatar
    05lgt

    Thanks for the informative write up Alex. Personally I hate the “who? me…!!” downshift far to much to accept it just for the incremental fuel economy gain. Aisin please.

  • avatar
    Big Al from Oz

    A synchromesh transmission does have clutches. These are conical on the shaft. This synchronises the shaft speeds to allow for the mesh of the gears.

    You can experiment, try this out by sitting at a vehicle stationary at idle. Then apply pressure by forcing the gear lever forward trying to engage 1st gear. On a level surface if you apply the correct pressure you will feel the vehicle move forward. This is caused by the clutches in your gearbox try to sychronise the rotating ‘drive’ shaft from the engine to the idle shaft to drive the axles. A castlelated that reflects the gearing does the rest.

    But make sure you have a synchro first gear, or it can make a lot of grinding ;)

  • avatar
    Big Al from Oz

    A castlelated collar.

  • avatar
    Carzzi

    Moar tech articles like this please. Very well explained. As someone who shifts his MT82 clutchlessly (gearbox replacements are cheaper than knee replacements), the article made perfect sense.

    • 0 avatar
      Big Al from Oz

      The taller the ratio the greater the load on the gears. I remember when the 5 speed gearboxes came out many had problems with the overdrive ratio with failures.

      I wonder how these taller gears will affect the reliability of the gearbox due to the additional torque required on the much taller ratios.

      Time will tell, especially when a Cherokee is used loaded and/or towing a caravan.

      • 0 avatar
        Brian P

        Looking at the cross-section of the transmission and comparing it to other traditional designs, it seems to me that the torque converter is relatively small, and the lock-up clutch is relatively big.

        The real economy-booster with these transmissions isn’t necessarily the taller (apparently near-useless, in this case) overdrive gearing, but rather the ability to lock up the torque converter almost immediately after starting off, thus almost eliminating the losses associated with torque converter slippage. They can do this, because with gear ratios closely spaced, the RPM drop with each shift isn’t big enough to make the engine drop out of the powerband and bog down. Old automatic transmissions relied on slippage in the torque converter to not bog the engine after the shift – that’s the only reason 3 speed automatics survived as long as they did.

        In the early days of lock-up torque converters, they would only lock up in top gear at light engine load above a certain speed. With plenty of 4-speed automatics, locking up the torque converter is the last thing that happens (after shifting to overdrive) and the first thing that happens when climbing a hill or accelerating while already moving is to unlock it.

        I suspect that with this one, the lock-up clutch is designed strong enough for full engine torque. A locked-up torque converter means less heat and that should be better in the long term.

        • 0 avatar
          Big Al from Oz

          @Brian P
          Locking up the torque converters early will improve FE.

          I wonder if the manufacturers will go back to a manual transmission?

          But I also think it’s coming down to engines as well not just the gearboxes.

          Stationary, constant speed engines have been around for a while with plant and equipment. They have very narrow power bands and are efficient much more efficient and cheaper to design.

          Diesels customarily have narrow powerbands, hence the number of gears to move a laden truck.

          Well, as we move into the future engines will become less flexible. They have too, to give better FE.

    • 0 avatar
      Big Al from Oz

      @Carzzi
      Do you drive a Mustang? If you do it has a different shifter setup to the Ford Ranger/BT50 and Land Rovers that a using the MT82?

      It’s odd I have a MT82 in my BT50 ;)

      I had a hydraulic master cylinder fail on me once in the middle of nowhere when I had my 86 Nissan Navara. Had to use the gearbox as a clutch to get me going. All you have to do is have your shaft speeds match.

    • 0 avatar
      Big Al from Oz

      @Carzzi
      The MT82 turned out to be a problem for Ford, didn’t it? Supposed to give a quick shift?

      The shifter setup in the Ranger/BT50 has reduced the shift to a very average shift. I would have thought Ford engineers could had designed a better shifter. The problem lies in the geometry of the shifter linkages for the Ranger and BT50. Hopefully Ford will redesign a new shifter.

      So, what do you think of the Colorado mid size? Ford invested heavily into aluminium, no midsize Ford pickup for the US then I suppose ;)

      Oh, if your knee is in poor condition, why didn’t you buy an auto?

      A very odd statement you made and thanks for correcting my spelling.

      New to this site? or you just don’t blog often.

  • avatar
    Beerboy12

    Interesting stuff indeed! I can’t help wondering if there isn’t a simpler solution, like a lever that comes out the top of a gear box into the cabin and a foot operated lever that can engage and disengage a simple pressure plate so that the driver, who can hear the engine, observe the rev counter can choose gears based on speed and the road ahead… Oh! Wait!

    On a more serious note, does this 9 speed auto still use a torque converter?

    • 0 avatar
      Brian P

      Torque converter is visible in the cross section – but as noted in a post somewhere up there, it’s smaller than normal and the lock-up clutch is bigger than normal, and it likely only uses the torque converter for starting off from a stop and perhaps for smoothing out the gear changes in the lower gears.

  • avatar
    Beerboy12

    I hope the electronics are well insulated because if you go off road and something gets wet enough to go phzzt… That will be a very expensive box of cogs and dog clutches that’s not going to get you home…

  • avatar
    SCE to AUX

    Excellent article – very helpful. As a mechanical engineer and someone who has typically worked on his own cars, I found this story both interesting and scary.

    The new 2015 Chrysler 200 will be using this 9-speed tranny. I expect to at least test-drive one, but now your story gives me pause about actually owning one.

    That 9th gear sounds like it has marketing value, but little else. Too bad the public will never understand this. All they know is that ‘more is better’.

    This makes me appreciate the 1-speed reduction gear in my Leaf that much more.

    • 0 avatar
      rudiger

      Given Chrysler’s less-than-stellar track record with new-tech automatic transmissions, I’d be quite leery about taking a chance on this one, as well (particularly as the miles accumulate past the warranty period).

      • 0 avatar
        NMGOM

        rudiger – -

        I think you are referring to Chrysler’s own home-brewed automatics (and yes, I agree, but their NP division made great manuals). This is a German-sourced Zuffenhausen.

        —————-

        • 0 avatar
          wmba

          The ZF is being made by Chrysler at two plants.

          http://www.allpar.com/mopar/transmissions/ZF9.html

          ZF also have built a plant in the US themselves to make it for other than Chrysler. Look it up on Wikipedia. None are made in Germany.

          • 0 avatar
            billfrombuckhead

            For all the fools living in an alternate reality rather than the real world, HONDA/ACURA is strongly rumored to be using this new ZF nine speed

          • 0 avatar
            AlternateReality

            I just pray that HondAcura’s version isn’t assembled by Fiasler labor.

        • 0 avatar
          rudiger

          Unless Toyota was involved, I’d still be quite hesitant to buy anything sold by Fiatsler with a transmission of a brand-new, technically-sophisticated design.

          I’d go so far as to say that the German reputation for engineering superiority is rather overrated and a thing of the (distant) past.

          • 0 avatar
            George B

            The Mercedes Benz 722.6/5G-Tronic automatic transmission used with the Hemi V8 Dodge Charger/Challenger and Chrysler 300 is pretty rugged.

    • 0 avatar
      chaparral

      It makes 80-90 mph cruising on Michigan freeways quieter and more economical. That’s a use case that the engineers all see on their way to work. If you don’t live in Ohio or a similarly benighted state 85 is a common cruise speed

  • avatar
    Conslaw

    I wonder what the difference in parts count is between the 9-speed ZF transmission and the CVTs used in the Nissan Altima and Honda Accord. If the CVT has more than half the parts, I’d be surprised.

    • 0 avatar
      Brian P

      Don’t forget that Nissan’s CVTs still have a lock-up torque converter and a planetary gearset with several clutches inside it. The early ones just use the planetary set and clutches for forward and reverse, but the later ones have a two-speed gear selection in addition to the CVT – in order to get a bigger range between the lowest and highest available ratios … They’re not all that simple, and they are not rebuildable. If a Nissan CVT blows up, and it is not unheard of, you chuck the transmission out and put in a new one.

  • avatar
    Kinny

    Amazing article Alex!! Keep it up…. I love your video reviews as well.. no fake BS attitude.. no BS sound bites..just legit, well thought out material

  • avatar
    V6

    i came to a similar conclusion while having a Focus for the last few weeks. i knew it was a dual clutch and that explained the way it shifted, however for most people buying that type of car they know they want an automatic and that’s all they know/care about. the powershift dual clutch is just no where near refined enough to compete with a regular auto. if it was my own car and unaware of the transmission, i’d think there was something wrong with it and take it back

  • avatar
    tralls

    The only comment I will make is not about the article, but about my frustration with the motor media and their insistence that this is the resurrection of the Cherokee name. That is just not the case. The Jeep Liberty lived in Europe with the name Jeep Cherokee. The North American market is the only market that is getting a resurrected name.

  • avatar
    danio3834

    Nice write-up. It’s good to see authors that are willing to take the time to break things down into the gritty details. It’s not easy, we appreciate it.

    As far as the longevity of the transmission is concerned, time will tell. Less friction clutches mean that if all the control systems work correctly, the transmission should theoretically have a longer life.

    The “smooth” shifting in a traditional hydraulic automatic is achieved by a certain amount of clutch slip. The side effect is the clutches wear faster. In modern adaptive transmissions, when people complain of “harsh” shifting, it’s often the module logic protecting the clutches from the operator’s driving style. A harsh shifting auto is often a longer lasting one.

    • 0 avatar
      Sam P

      “A harsh shifting auto is often a longer lasting one.”

      Old school Mercedes 4 speed automatics as used in the V8 powered W126 did this and were just about impossible to kill. Kind of lazy to downshift unless you floored and hit the kick down switch.

  • avatar
    Andy D

    First poster X1. So a dual clutch transmission is mostly a well synchronized gear engagement with the usual manual clutch ? And this differs from a synchromesh exactly how? Its done by a computer. Oh goody! How much will this cost to fix? Oh, you should be into a 2016 by then. I’m starting to get it now.

    • 0 avatar
      Brian P

      The gears-and-synchros part of a dual clutch transmission is just like a manual transmission aside from one synchro pack handling 1-3 and the other 2-4 and being on a separate (but concentric) input shaft. The clutch part of a dual clutch transmission is very different from that of a manual transmission, though, and it’s more like the multiplate friction clutches in a normal automatic – and there are two of them, one for each concentric input shaft to the gears-and-synchros part of the transmission.

      Most automatic transmissions of ANY sort nowadays – barring ancient, hoary holdovers like the GM truck 4-speed autoboxes – have limited capacity to be rebuilt, and it doesn’t matter if it’s a CVT, dual-clutch, or regular automatic with a zillion speeds. Aside from dealing with the simplest problems (externally accessible switches, sensors, solenoid packs, etc), the local transmission shop is not going to be able to do it. If a Nissan CVT, or a ZF 9HP, or a VW DSG goes boom, you don’t fix them, you chuck it out and put in another one.

  • avatar
    carguy

    A great technical deep dive – thanks Alex!

    It sounds as if the design of the 9HP prioritized size, weight and efficiency over driver enjoyment. I hope that the new GM 8 speed and the 10 speed joint Ford and GM venture will avoid these pitfalls.

    • 0 avatar
      Alex L. Dykes

      The RWD transmissions under development right now don’t seem to have this problem for a simple reason: there isn’t the same sort of space premium going on in RWD vehicles.

  • avatar
    George B

    Thank you Alex for an excellent article. The behavior of the 9HP would annoy me if I was trying to pass a car on a 2 lane highway or accelerate into a lane opening on a crowded interstate. Would be nice to be able to force it to downshift from 8th to 7th while waiting for an opportunity to pass.

  • avatar
    xflowgolf

    It’s been said multiple times already, but I wanted to add to the chorus. GREAT article. Love reading deeper dive explanations like this into the workings of new automobiles. Keep it up!

  • avatar
    Flipper35

    OK, everyone is really missing the real reason for the tall 9th gear. It is so when the SRT version comes out it will be able to go 206mph before hitting the redline.

    I agree about the well written article. Pictures help as well.

  • avatar
    tedward

    More please. Awesome article.

    I wonder if the 9th gear has anything to do with the coast down portion of the EPA fuel economy formula. It doesn’t seem like it would as you would be forgoing effective engine braking, but as the EPA test isn’t really a real world measurement I thought it might be worth asking.

  • avatar
    charlie986532

    Great story Alex, the devil is always in the details. I purchased a 2014 Jeep Grand Cherokee Ecodiesel with the 8HP and have been very happy with the smooth shifting so far. On a recent business trip, I had the opportunity to compare SUV rentals, Toyota RAV4 and Jeep Cherokee. First was the RAV4 which didn’t impress me, had a check engine light so I exchanged it for the Cherokee. First drive impressions were good, big change from the RAV4. But after putting 900 miles on the Cherokee, the big issue was the intermittent transmission bumps. Launching the car from stop gave different results, sometimes smooth, sometimes harsh. Once driving at speed though, I liked the Cherokee over the RAV4. Styling would stop me from buying either.

  • avatar
    kurtamaxxguy

    Thanks for explanation! Seems like this new 9 speed has a designed-in liability (..”The result is a somewhat odd delay between the pedal on the floor and the car taking off like a bat out of hell”…)to get its 9 speeds. Just how long is this acceleration-induced (downshifting) hesitation?

    FYI I’ve noted that quite a few modern cars have delayed response to “stomp the pedal” acceleration .vs. smooth depression of the pedal. Perhaps because the ECU’s or emission control systems can’t respond quickly enough to a stomp?


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