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Breaking down the excessive rear, inner tire wear

ngng

Active Member
Jul 23, 2018
1,271
687
Bay Area
I had a bunch of warranty work done at the dealer, some of which included replacement parts for the shudder. The entire front end of the car was apart. After I picked the car up I noticed that it was pulling right and immediately suspected an improper alignment (or, in this case...no alignment). Figured this would be a good time to post my thoughts on the topic of tire wear since everybody seems concerns, but nobody really seems to understand what's going on.

I am not an alignment expert by any means, but I routinely align my race cars within 1/16" so I have a basic understanding of what's going on.

If you look at the before measurements there are a handful of callouts: camber, caster, toe. You can Google all these for more info, but the high level:
- camber is the wheel's angle looking at it from the front/back of car, camber is generally negative (leaning in towards the centerline)
- caster involves only the front, steering end of the car. I generally think of it as the angle between the centerline of the upper and lower ball joints. It is responsible for the "self-centering" effect when you are driving: in a high perf car, if you're sliding the rear and let go of the wheel and it recenters; or if you're making a turn out of a parking lot and let go of the wheel and it recenters/unwinds. You want both sides to have similar, positive caster
- toe is the angle, viewed top down that the tire is rotated inwards (negative) or outwards (positive)

These values are all related. First you set camber, then caster, and finally toe.


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Let's ignore the before measurements since they are absolutely terrible.

Camber: everything looks reasonable here. I've driven plenty of cars with similar, or more aggressive negative camber. While it does wear the inside of the tire faster, unless the camber is very aggressive it's not typically the culprit.

Caster: Close-ish on both sides. This looks good. The car will self-center.

Toe: Front toe looks great. I typically align my cars with 1/16" total front toe out (negative) and 1/16" total rear toe in (positive), sometimes zero. You'll have to convert back and forth between degrees and inches/mm if you care. The rear has waaay too much toe out. To put things in perspective, if you convert the 0.42 degrees of total rear toe you are looking at nearly 1/4" of toe in. That doesn't sound like much, but 1/8" of total toe results in the inside of the tire being dragged sideways almost 100 feet per mile of driving. THIS is why our rear tires are wearing so quickly.

Now that I have a baseline, I will track the inside tire wear and report back. The tires are brand new, and I now have a baseline alignment. If the wear is unacceptable, I may consider a set of toe links, but I doubt I will spend the money for the aftermarket ride height adjustor and camber arms.
 

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