Tire Wear Analysis

[Loren]

I've been noticing some of my tires look pretty thin from certain angles lately, and someone commented at the last autocross I went to (after I posted the 5th fastest street tire time of the day, within .3 of the fastest) that "man, those tires are DONE!"... so I thought it was about time I did something about it.

I'm in an awkward tire situation. I had a set of four new T1R's (215/45-15... about the same width as a "normal" 205) about 12 months ago, but one of them got mangled on the road (piece of a utility knife blade cut a 3" long gash across the tread) back in October. I wasn't thrilled with the T1R's, so I bought a pair of Star Specs (195/55-15) for the front. Sooooo...

What I have now are two Star Specs up front w/ 8 months (maybe 6k miles and 10 autocrosses) and two T1R's in the rear w/ 12 months (maybe 8k miles, a track event and 14 autocrosses).

Looking at the fronts, they are both clearly more worn on the inside from the camber, but also worn on the outside edge from hard cornering... and a little "thick" in the middle because I ran 2-3 autocrosses with them underinflated. (I was just too damned lazy to check them... they got down to about 32 psi when they needed to be at least 38) The inside of the RF is table-top smooth, obviously I've been spinning that inside front tire a little too much accelerating out of turns. Shame on me, I should know better! But, looking at the left tire, I think if I'd kept them properly inflated, and had a matching set of 4 to rotate properly front/rear and side/side... the Star Specs would have easily lasted a year and at least 16 autocrosses. (bear in mind that our autocrosses in this area are typically 6-8 runs, not the 3-4 runs that are the norm for some parts of the country)

The rears tell an interesting story, too. They're ragged on the outside from hard cornering, but hardly worn on the inside at all. A VERY clear indication that I could use more negative camber in the rear. The left rear is slightly more worn than the right. No good reason for that other than maybe more autox courses that have hard rights and/or I corner harder to the right than to the left.

What's the plan? Well, I still have that third T1-R that's almost full tread (it only saw a few months of use on the rear of the car), and I can't bear to just pitch it. So I bought a single T1-R to go with it, and that pair will go in the front... the new one on the RF since it sees the most abuse.

On the rear, I'm going to have the other two T1-R's reverse-mounted to put the usable tread on the outside where it is needed. This setup should get me by for at least another 8-10 months.

I'm also going to address the rear camber issue with some shims. I figure about a half degree more negative should help even out that tire wear. BUT, it's going to have the side effect of giving the rear more grip (which is essentially taking grip AWAY from the front, which is bad). So, to counteract that, I'm going to experiment with raising my rear spring rates from 4kg to 7kg. I tried the 7's when I first put the K-Sport kit on the car last April, but at the time, they felt (freakin' great, but) like "too much" because I wasn't used to the car, and I was about to head on a cross-country trip driving on a bunch of race tracks that I'd never been on before... scary! So, I went with a set of 4kg springs instead. Now, I'm much more comfortable with the car, and I'm pretty confident that I have enough adustability with the shock damping and rear tire pressures to tame the rear if necessary.

Hopefully, these changes will all balance with the "not as good as the Star Spec" nature of the T1R's that I'm putting in the front and give me something that I can tolerate on an autocross course.

I'm breaking one of the fundamental rules of suspension tuning by changing more than one thing at a time... tires, rear alignment and rear spring rate. But, I've been doing this long enough that I think I can handle it. (and fine-tuning of this major round of changes WILL be done more methodically) Don't try this at home, folks!

Photos of tires attached. Not for the squeamish. Tires WERE harmed in the making of this post.