The Falken 660 tires I’m running have a nominal as-molded tread depth of 8/32 inch. After the Moultrie Pro-Solo test event, where I got 60+ short runs and another 40 or so practice starts, both front and rear tires had lost 1/32″ in the center, 2/32″ on the outer edges and 3/32″ on the inner edges. This seems to me a reasonable wear rate on concrete. I was pleased that front and rear were very similar. The rear probably would have been worse except that those 40 extra practice starts were done in either wet or damp conditions which limited the wear rate even though I was having trouble controlling wheel-spin. Below are annotated pictures. Let me know what you think about the wear.
As I mentioned the inside of the front tires wore faster than the outside. In warmer temperatures more grip will be created, probably pushing wear toward the outside. At regular autocross courses there will be more long, high-G corners, also pushing wear toward the outside. However, as I will probably be adding air pressure and the Penske shocks will have more compression force the wear rate on the inside may very well stay a little higher. (Toe is not a factor here as it will always be set to zero or near-zero.) This wear pattern, inside faster than outside, is not necessarily an issue.
In a standard corner we theoretically only reach maximum lateral acceleration at one point- the apex, though we should be at the total limit of longitudinal plus lateral from start to finish of the corner. The apex is when wear on the outside edge will be the fastest, but it’s not for a very high percentage of the total time during a run. The apex is the point where we want to maximize the lateral grip from the tire and where our data system can most easily measure it.
Some might advise me to decrease the static negative camber in order to balance the tire wear. While that might work, I think it would be a mistake if you’re trying to win and not trying to maximize tire life. Reducing static negative camber would diminish the peak capability of the tire to produce lateral grip. In this Street-class car with relatively soft springs and rubber suspension bushings, which equates to too much roll and thus camber loss in the corners, we can never achieve enough static negative camber to keep the tire at its best angle (still slightly negative…not perfectly vertical) at the apex of the corner.
This is also why using a tire pyrometer in grid to measure the temperature across the tire and then adjusting camber to even the temperatures is not smart, no matter how many people tell you to do it. (I hope that includes all my competition!) We are interested with what happens mostly at those corner apex points out there on the course. The tire temps back in the pits are of little use to us. I really don’t care what the average heat generated across the tire width is across a run, which is what that tire pyrometer will tell you back in grid.
If you want to measure the surface temperatures in real time during a run with a set of thermal imagers so you can see what happens at those corner apex points, well, now you’re talking about something that could be useful. I think I know what it would tell me: the outside edge heats up really high at the apex because it’s lost too much camber and the edge is really highly loaded and wanting to roll under. The tire is not happy. The tire is screaming, “I need more camber to even out the loading and these temperatures so I can produce more lateral-G at the apex!” The rest of the time, the inside edge is warmer.
Using a tire pyrometer is no doubt useful for road-racing a real race car (with properly stiff springs, sway bars, and without soft rubber bushings) but it makes no sense to me for Street-class autocross where we are trying to achieve peak, momentary capability with tires that we already know are not at the correct angle to the pavement.
What I’m saying applies only to Street-class cars. Once up one preparation level to Street Touring, where you can install properly sized springs and bars, replace the soft rubber bushings with sphericals and even install camber kits then you need to worry about how much camber is too much. Even then a real-time look at the temps across the tire surface in the middle of a corner is preferred. With a 200′ skid-pad you could use a tire pyrometer to quickly check temperature differentials across the face before they cool too much.
The rear tires also wore more on the inside than the outside at the Pro-Solo test event, though rear camber is much less than in the front. Especially after this particular test event this makes sense. During launch the inside edges have more load (due to negative camber) so they wear more during wheel-spin. The only time the outside edge is highly loaded is on the outside tire in the middle of a corner when the car is rolled over and pulling maximum lateral acceleration. This is when the outside edge (of only the outside tire) wears fast. Given this it would be very strange if the outside edge had worn faster than the inside unless the course included a lot of high-G corners, which the ones last weekend did not. At the local test and tune in a few days there will be a lot more long sweepers and outside edge wear should be accelerated.
Autocrossers often use the triangles molded into the top of the sidewall to gauge how much the tire is rolling over. The triangles aren’t there for that purpose and they vary in location and size from brand to brand. They just make good reference points.
In Figure 5 you can see where the scuffed surface extends well down from the tip of the triangle. It looks to me like this is too far, meaning that the tire edge was rolling under too much when maximally loaded, even in the cold temperatures. I was only running about 31 psi in the fronts (28 psi in the rear) so almost certainly that will not be enough, especially in warmer weather on high-grip surfaces. These 275mm (10.8″) treadwidth tires are crammed onto only an 8.5″ wide rim, so there is, by definition, a lack of proper sidewall support. (According to the “standard” they need a minimum of 9″ of rim. This is autocross. We don’t need no stinkin’ standards.) More air pressure is probably needed. A friend confirmed this by informing me of the significantly higher pressure he runs for the same tire on a 9.5″ rim on a similar car.
The handling of the car was good, but not great. Overall the grip level was good, but it’s hard to make conclusions given the very cool temperatures. Also, I have Penske shocks on the way so I don’t want to go too crazy with conclusions or changes until they get installed.
I feel like turn-in could have been crisper. That, plus the indication of too much front tire roll-over, points toward needing more air pressure in the front tires to sharpen the response. The Penske shocks should also improve things because they will produce more compression forces on turn-in than the Konis do.
It was always possible to overload the front tires and create understeer by turning the steering wheel too far, too fast while trail-braking, but when I hit the right balance between speed, braking and steering angle the car turned really well. (I was finding the proper spiral entry path.) It took me until Sunday morning to finally get a feel for that point of balance. I’m not what you’d call a quick study. Thank God for all those runs.
The rear was quite stable. Maybe too stable. I had set the rear toe-in to 1/4″ total. You can always provoke oversteer with enough throttle coming out of a corner, of course (gotta love that LS3 and Z51 gearing!) but in general it did not exhibit any bad habits. No snap oversteer ever occurred. (That doesn’t mean I never got it sideways!) I will reduce the toe-in in the rear very slightly to assist rotation on turn-in and see how that feels.
Up Next: We have a local Test & Tune in three days on our local asphalt parking lot. Next post will probably be after that.
3 thoughts on “An Autocross Season-Part 8: Moultrie Tire Wear and Handling Impressions”
Thank you for post. Just the way you explained it clarified wear patterns that I noticed on my car and couldn’t understand. Never thought about the rear camber causing inside wear on straight line starting and stopping.
… and the toe-in usually necessary in the rear magnifies it.
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