Piston Slap: A Primer on Wheel Offset and Backspacing

Sajeev Mehta
by Sajeev Mehta
piston slap a primer on wheel offset and backspacing

Long-time TTAC Commentator 86er writes:

Hi Sajeev,

Could Piston Slap furnish me with a be-all/end-all explanation about wheel offsets? The more I try to read up on it on the web, the more confused I get. I’m pretty clear that RWD (at least traditionally) went with the low-offset while the FWD revolution made high positive offsets the industry standard, at least in passenger cars.

A few years back, I had purchased a set of winter tires on rims for my trusty ol’ 92 Vic and later after research found out that the rims were medium-offset that went on a 4×4 Ranger of similar years. I’ve heard that putting on a different-offset wheel can hurt steering/suspension parts like ball joints, but I’ve never seen it in black-and-white, so to speak.

Sajeev answers:

Let’s cover the basics of both wheel offset and backspacing: offset is the location of the mounting hub in relation to the center of the wheel’s barrel. This mounting hub goes to flat surface where car’s suspension holds the wheel (i.e. the hub on the spindle).

A positive offset pushes the wheel’s hub away from center, closer to the outside of the car. Negative offset is the opposite: sucking the wheel’s hub closer to the inside of the car. Zero offset means it’s smack dab in the center.

I question if the traditional FWD/RWD offset difference still holds water. While FWD wheels often have a more positive offset than their RWD counterparts, all (most?) modern vehicles have flat faced wheels (for aerodynamics and countless suspension needs?) stemming from a more positive offset wheel. Need proof? Look at your own platform: peep the redesigned front clip and the mandated wheel redesign of the 2003+ Crown Vic.

Oh wait, the Crown Vic barely changed at all from 1979 to 2011. It was such an antiquated pile: must remember to toe the autojourno line, never speak of Panther Love! But I digress…

In theory you should keep a close-to-factory offset to optimize steering geometry and wheel bearing health. In practice, it might not matter: especially for a set of winter tires. You probably can’t drive aggressive/fast enough to care. Probably…

There’s also the matter of torque steer on FWD machines, mostly for those with unequal length half-shafts. But most modern vehicles use equal length shafts? (Have at that, B&B!)

You also need to consider backspacing. This ensures the width and offset of wheel you chose will clear your body or suspension, especially on cars with strut suspensions. Instead of my usual ramble, I think this video really nails it.

[Image: Shutterstock user 80’s Child]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

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11 of 23 comments
  • 86er 86er on Jan 19, 2015

    (OP) Some more details for clarification: The winter wheels in question are 5.5 inch, while stock on those years of Crown Vic are 6.5. This may or may not affect the scrub radius or what-have-you of the tire vis a vis the suspension. As of this writing I am not sure what the exact backspacing and offset is for the Ranger wheel I'm running currently and the OEM wheels I use in the dry months. When comparing offerings for the Ranger and Crown Vic, websites like Tire Rack are no help either because they offer aftermarket wheels in a variety of offsets and backspacing, especially for the Ranger. I've read different sources saying the stock wheels on a '92 Crown Vic are slightly positive (+13 if I recall) while offerings for aftermarket wheels on Tire Rack seem to all be -6. I've also read that too much offset is where the real problems begin, such as with the tuner crowd that like the wheels sticking way out, that way lies wheel bearing premature wear. I've assumed that the difference in offsets between the Ranger wheel and the Crown Vic wheel aren't enormous, but I stand to be corrected. I have run this setup for 4-5 years now and have yet to require wheel bearing work. In fact the car has needed ridiculously little steering/suspension work in the 7 years I've owned it. A tie-rod early on in late 2007, a few sway bar endlinks and most recently one idler arm, and that's it. The vehicle now has 277,000 km, 125,000 of which has been under my ownership.

    • See 1 previous
    • 86er 86er on Jan 19, 2015

      @Scoutdude I don't know what year the Ranger wheels are from but they're steel wheels like so: (http://www.wheelsandcaps.com/ford-ranger-1985_p-23395-steel-wheel-rim-15x55-1314.aspx) I know clearing the hub was no problem; these fit the hub just fine.

  • Slowtege Slowtege on Jan 19, 2015

    Great article. It can be confusing at times, which is why I'll dive into "Show Us Your (insert model of car here) Lowered/On Aftermarket Wheels" threads on forums to try and get a better reference for real world looks. I also came here to comment on THAT VIC. Sitting really pretty on those wheels--probably SN95-New Edge-era Mustang GT/Cobra variants--with that offset and drop. I like the deeper dished pre-'03 wheels, but naturally the '03+ upgrades to everything else. Gah, this is making me want one (again)!

    • See 6 previous
    • Scoutdude Scoutdude on Jan 21, 2015

      @Slowtege It was not a huge change going from the 17's to 18's but I was running 235/50-17 F and 255/50 R to initially 255/45-18 all the way around. I really like the look of both the Bullit and the Mach 1 on the CVs. For the 03 up Panthers the 05 up Mustangs are direct fit no need for spacers since they both use 45 or 50 mm offset. In fact the Marauder wheel was optional on V6 Mustangs in a couple of years. They are slightly different in the fact that they changed the depth of the machining for the center caps. That means the Marauder center cap sticks out about 3/16" on the Mustang version. AFS now makes an 18" Mach 1 style wheel with the 50mm offset so those are tied with the Marauder wheel for my choice for an 03 up Panther. Of course the 8" Marauder wheel could use widening and it wouldn't hurt to widen the 9" AFS wheels too so that you can use 295 or 305 on the rear. Once you widen them since it is done on the rear portion of the barrel spacers do become necessary.

  • Jeff S Some of us don't care either way we are not into this type of car. Most of these will be stored in garages waiting for their value to go up. As someone above noted this is an old body style which is retro 70s Challenger which after researching it came out in the 2008 MY which means a long run for a model that is in its 16th year. I have always liked these but if I bought one I would not spend this kind of money on one probably get the V-6 version and use it as a family car but then I am not into drag racing or muscle cars. For the type of car it is it has a decent rear seat and not too bad of a trunk. Most of us are not going to spend 100k for any vehicle at least currently so its not something most of us will buy and stick in a garage waiting for its value to increase. I am glad that these editions came out for those who can afford them and it keeps a little more color into what has become a very dull vehicle market but then with age I pick the dull appliance like reliable vehicle because that's what I need. Impressive car but not for me.
  • Jonathan The Germans. So organized they can appear disorganized. I agree with some others, classic names like Thunderbird, Imperial, Grand Prix, Ambassador etc. just have more appeal.
  • Bobbysirhan A friend had one when they first came out. He was CFO of some green California company and could charge the Volt at work. At home, the PHEV gave him an excuse to make his wife park her nicer car outdoors while the Volt get their condo's one-car garage. He liked the Volt, and he spent very little on energy during the 'first one's free!' era of EV ownership. Of course, the green company went bust soon after, and he wound up with a job that involved far more driving and ultimately the need for a more substantial car. I drove the Volt once after his wife had made a return trip to Los Angeles, depleting the battery. I don't know what a first gen Volt drives like with a charged battery, but it was really gutless with two adults, a yellow lab, and a dead battery. My other memory of it was that it had a really cramped back seat for a car that was about as large as a Civic. My friend who bought it liked it though, and that's not always been the case for GM vehicles.
  • MrIcky I think the Shakedown is more my speed of the last call editions- but this is impressive.
  • Dukeisduke I tried watching the live reveal last night, but after 15 minutes of jawing by MT+ personalities (and yes, I like Chris Jacobs and Alex Taylor), I turned it off.