Cal Club Autocross Forums

I purchased a set of Dunlop Direzza ZII Star Spec tires for my Ecoboost Mustang about a year ago.

They now have 240+ runs and 3500 street miles on them and although they have about 40%+ tread left I think they are slowly losing grip.
Even though there is lots of tread left could these tires be going downhill? Or should I be looking at a loss in engine HP.
Is there anything I can do to prolong the life of these tires or at least slow the degradation down?
These tires have to get me through the rest of the season. Sundays only?

I think I have managed these tires well:
Alignment by West End at the beginning of every season.
Pressures are reset after every run. 44 front and 37 rear. High on the front to prevent roll over. Reset to 30-32 lbs front and back for highway use.
Pyrometer readings across the tread are quite even when measured immediately after a skid pad run. Slightly higher on the outside edges as expected
I spray the tires when they are too hot to keep my hand on them.
Tire wear across the tread is monitored regularly with a gauge and is quite good.
They have recently been flipped and they are rotated front to back after every 3-4 runs.
Rubber has looked the same since I first ran them. No hard, discolored or shiny looking areas. Evenly abraded and no tread block separation.
Is there anything I could do to better manage these tires for longer life?

Any help would be appreciated. Thank you.

We usually get ~100-120 runs out of our Bridgestones before they're done. Their performance starts falling off around 70-80 runs.

The Dunlops should have a slightly longer life, but probably not 2x. Expecting street tires to last >200 runs probably isn't realistic, especially on a big heavy car that's running in Street class where you probably can't get adequate camber. It sounds like you're doing everything you could reasonably do to stretch their life.

As they get older they'll lose performance, and you should also keep in mind that generally having more tread seems to help on asphalt lots (Fontana), so yours having less tread is going to effect performance as well.

Jeff Stuart wrote:We usually get ~100-120 runs out of our Bridgestones before they're done. Their performance starts falling off around 70-80 runs.

The Dunlops should have a slightly longer life, but probably not 2x. Expecting street tires to last >200 runs probably isn't realistic, especially on a big heavy car that's running in Street class where you probably can't get adequate camber. It sounds like you're doing everything you could reasonably do to stretch their life.

As they get older they'll lose performance, and you should also keep in mind that generally having more tread seems to help on asphalt lots (Fontana), so yours having less tread is going to effect performance as well.

Jeff, thank you very much for your quick and considered response. Wish you had better news but I expected they might be done. I have to continue running these tires for the rest of the season so I will just have to grin and bare it. Things should get really exciting and slow by the 300 run mark!

Jeff, I meant to ask why tires start to go off like this? Is it a change in the rubber compound that occurs from constantly going through heat cycles? Or are there other things in play.

Jeffear wrote:Jeff, I meant to ask why tires start to go off like this? Is it a change in the rubber compound that occurs from constantly going through heat cycles? Or are there other things in play.

I'm by no means a tire expert, I just know what I've seen over the last 6 years of autocrossing.

For the Bridgestones that we run, I've heard (though don't actually know for sure) that the last 2mm of rubber is a harder compound, so as more of the tire wears down to that level you start losing grip pretty quickly and they fall off a cliff grip-wise. For tires that are the same compound all the way through, the rubber can just get heat cycled out and it starts getting harder, which means less grip.

Jeff Stuart wrote:

Jeffear wrote:Jeff, I meant to ask why tires start to go off like this? Is it a change in the rubber compound that occurs from constantly going through heat cycles? Or are there other things in play.

I'm by no means a tire expert, I just know what I've seen over the last 6 years of autocrossing.

For the Bridgestones that we run, I've heard (though don't actually know for sure) that the last 2mm of rubber is a harder compound, so as more of the tire wears down to that level you start losing grip pretty quickly and they fall off a cliff grip-wise. For tires that are the same compound all the way through, the rubber can just get heat cycled out and it starts getting harder, which means less grip.

Very interesting, that would explain why my tires don't seem to be wearing much at this point. So instead of a 200 wear rated tire they may now have the grip of a 300 or 400 tire and will just continue to get harder the more they are heat cycled. By definition is a heat cycle one run or multiple runs in a short period of time (no cooling) as in our 4 competition runs.

Two more questions.

1. Since my tires are now past their useful life and still have 40% tread remaining should I have had the tires shaved when I purchased them. I would expect that less tread depth might extend the useful life of the tire by reducing heat (tread squirm). Does less tread equal more grip for longer? You mentioned that more tread works better on an asphalt track (Fontana) so can I assume that less tread will work better on a concrete track (El Toro)? I thought road racers typically shave their tires for better performance on asphalt tracks.

2. Tirerack offers a heat cycling service on new autocross tires. Do you think that has any benefit and do you use this service for your new 71Rs.

Thank you.

Jeff,

Asphalt like Cal Speedway tend to like tires with tread depth. While the tires may not seem like they are gripping well at ACS, they will indeed grip very well at El Toro. Since we are heading back to El Toro next it would be a good thing to keep that set still as it may still work on the higher grip surface. (save money!)

Jonathan Lugod wrote:Jeff,

Asphalt like Cal Speedway tend to like tires with tread depth. While the tires may not seem like they are gripping well at ACS, they will indeed grip very well at El Toro. Since we are heading back to El Toro next it would be a good thing to keep that set still as it may still work on the higher grip surface. (save money!)

Thanks Jonathan, I will be trying these at the El Toro events. It will be great if they do grip well.