Piston Slap: A Primer on Wheel Offset and Backspacing
Long-time TTAC Commentator 86er writes:
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.
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 firstname.lastname@example.org. 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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(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.
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)!