i run de's on toyo R888's , keep hearing chatter about heat cycles, what limits the tires performance; tread remaining or heat cycles , I would assume a weekend DE would put two cycles on a set .

 

A heat cycle is any time a tire goes from hot to not hot. You need to count every session as a heat cycle. Most of my experience is from racing slicks in motorcycle road racing, where one too many heat cycles means a great deal more than on a DE track day, but the mechanics are the same. With each cycle the tire will wear, and also loose some of its traction properties. Too many, and it hets greasy.

I've always kept a race log on tire condition and number of cycles, noting the point where they start to go off. Then you can better judge how many cycles is optimum for your setup. It's a balance between performance and cost... In a race bike I run no more than 7 heat cycles on a tire. Two sets per weekend. That's usually two days, 3 practices per day, qualifying, and two races, but a faster guy may go through double the number of tires...

The important thing is that your car, pressures, setup, will mean you have different results than the next guy. Logging your experience is key for tire management.

 

I too have experience with heat cycling on motorcycle tires where it is a bit more important than your average DE.

One of the tale tale signs that a tire has gone through too many heat cycles is that they become slightly blue and may even have a bit of shine to them.

On the track you'll notice that they are "hard" and "greasy". They will also not give you as much feedback before they begin to slide.

For the most part I've never had to worry about it on a bike because the tires don't last very long anyway. I'd get 2-4 days on them depending on the track. That is 5-6 sessions per day so between 10 to a maximum of 24 sessions or heat cycles on a tire. On the track on a bike you'll go through front tires about twice as fast as rear tires. So really at most I'd be looking at 24 heat cycles on a rear and 1/2 that on a front tire before they were done.

On the car I'd just run them until they are shot or until you can really tell they aren't giving you the feedback you need to drive safely. It's not like they just go bad all of a sudden or anything.

 

Is time also a factor? For example, one 20 minute session (heat up fb cool down) vs a one hour session but still a heat up/cool down cycle? In other words, are 10 20 min sessions the same as 10 one hour sessions, as it relates only to heat cycles?

 

Nope. It's only one heat cycle either way unless you stop and the tires cool below operating temp. Of course longer sessions have a greater effect on tire wear.