Race Tire Heat Cycling
Published: October 15, 2018
To me, tire heat cycling has always been a mysterious topic. In this essay I will offer my current thoughts on heat cycling as well as explain some of the reasoning behind it as well as trade-offs.
What Is Heat Cycling?
Every time a tire reaches its operating tempearture and cools back down to ambient temperature, the tire is said to have gone through a heat cycle. In other words, any time a car drives more than a few minutes its tires get heat cycled.
When people talk about "heat cycling" new tires, they mean a very specific way in which the tires are brought up to temperature. If the tires are heat cycled by driving on them, this means the car is driven in a very specific way. Another way of heat cycling tires is to do it on a special machine, similarly to how tires are mounted and balanced on specifc kinds of machines. In this case the machine heats the tire up in a particular fashion.
You might be wondering, what is a non-specific way of heat cycling a tire? The two ends of the spectrum are street driving and a race session.
In street driving, the tires get heated up due to friction. However, compared to track driving, the tires achieve much lower temperatures. A race tire driven on the street, especially around town, is likely to never reach its optimal operating temperature range. I don't know if this type of a driving session counts as a heat cycle in terms of what it does to the tire (which will be covered in a moment).
The opposite is a full race session, where a brand new tire would be installed on a race car, and the car would immediately go into 100% attack mode. The tire gets heated up in the quickest manner possible - the driver may in fact overdrive the car slightly to put heat into the tire quicker when it is cold - and is then maintained at peak performance for as long as the race goes on.
Before we talk about what the proper "heat cycling" process looks like, it is necessary to cover what happens during the first heat cycle a tire experiences.
A Chemistry Lesson
When a tire gets heated up for the first time, rubber bonds in the tread and carcass go through a chemical transformation. The details of this transformation usually are not specified by the tire manufacturers, but for practical purposes we can assume that the friction coefficient and wear resistance of the tire are two parameters that may change.
The important points to remember are:
- There is a minimum temperature that must be attained by the tire in order for the transformation to begin.
- There is a maximum temperature that a tire is designed for, beyond which its rubber bonds break and do not self-repair.
- The transformation must happen through the entire depth of the tread and going into the carcass of the tire, not just on the surface.
Does Heat Cycling Do Anything?
First Competitive Session
While there are pros and cons to heat cycling, there is one undeniable aspect of heat cycling that always applies:
Heat cycling a tire foregoes using its first heat cycle in a competitive session.
Some tires, for example the various Hoosier compounds, as well as I imagine other full race tires, offer their highest performance in their first heat cycle. Hoosier tires, in particular, have the most grip on a specific lap (usually second or third lap) of the first heat cycle. By heat cycling a tire in a non-competitive environment (either by running an easy track session or by using a heat cycling servire provided by a tire vendor), you lose the ability to use this first heat cycle on track in a time trial, qualifying or a race session.
Tire manufacturers like Hoosier say that heat cycling can "extend competitive life of a tire" and make a tire "more consistent" over its entire life. What these claims mean is that performance of the tire later in its life will be higher if it was heat cycled in a controlled process compared to getting, say, an overly aggressive first heat cycle. In both cases the tire's performance later in its life is worse than its performance in its first heat cycle, though whether heat cycling makes the tire perform better in its heat cycle is a tricky question to answer.
Later Competitive Sessions
OK, now that we have established that heat cycling a tire definitely makes it perform worse in its first competitive session, does a heat cycled tire in fact perform better in later sessions?
Unfortunately there is no easy answer here. The challenge comes from the fact that on some tracks new tires are faster, and on some tracks the same tires are faster when they are older and harder. On some tracks street tires are faster than R compound tires. The only way to know for sure what is faster is to test each tire at each track at various points in its life. If you suspect this costs massive amounts of money, you are correct - and this is why only racers go through the exercise, and not all of them have the means to do so.
There can be so much variability in performance of the same exact tire as it wears at a given track, especially given the wide variety of possible driving styles (specifically with respect to how much the car is slid laterally in corners and how much the tires are slid longitudinally under braking and throttle), that the effects of the first heat cycle may well be lost by the time the tire accumulates 5, 10 or 20 heat cycles and becomes faster at the tracks which favor old tires.
But wait, there's more!
First / Rest Split
At a tire seminar I attended, a Hoosier daeler said the following: A tire has a certain total amount of grip in it. However much grip the first heat cycle uses up, the remaining heat cycles have the total amount minus that to work with. The catch, of course, is the amount of grip expended in the first heat cycle is much higher than 1/(number of total heat cycles). In other words, a very aggressive first heat cycle markedly decreases the tire's performance through the rest of its life.
I have no way of verifying this claim, but a practical conclusion of it is that, if you are going to heat cycle tires on a car, it really pays to ensure the tires are not overdriven at all in this first heat cycle. The intentional heat cycle must be very disciplined, otherwise it's just a waste of time.
Up to now I have been looking at performance, but what about longevity? Let's look at two aspects of longevity: cycle life (how many heat cycles a tire goes through before its grip falls below a certain percentage of new tire grip) and tread life (how many hours of driving a tire yields before the tread is worn off).
For tread life, I don't think heat cycling makes a difference. How hard the tire is driven throughout its life makes way more difference than how it was driven in its first heat cycle. If a heat cycled tire lasts longer, I would say that is because it hardened quicker and hence was slower early in its life, thus wore slower during that time.
For cycle life, whether heat cycling improves it is an open question. Again how the tire is used throughout its life is more of a factor, but perhaps a gentle first heat cycle slows down the grip loss in the subsequent heat cycles. I don't have data to suggest this happens or does not happen.
What I do believe though is it is possible to overheat a tire at any point in its life, and doing so makes the tire lose some performance from that point onward. This is more a conjecture than conclusion based on hard data, with the primary evidence being blue shoulders and poor performance of tires on cars with insufficient camber (rules limited). The earlier a tire is overheated, the more performance it loses (both in the amount of drop and how many hours of life the drop affects). Hence it pays to not overheat tires, specifically to set up cars to keep tires wearing roughly evenly which avoids overloading shoulders.
My Heat Cycling Procedure
If I decided to heat cycle a set of tires, this is what I do. To recap, the goals of heat cycling are:
- Heat the tires up progressively to their operating temperature;
- Heat the entire depth of tread and carcass evenly;
- Avoid overdriving the tire - slides, lock-ups and spins.
The process then is:
- Run one lap at 7/10ths pace.
- Run two laps at 7.5/10ths pace.
- Run the rest of the session at 8/10ths pace, but a minimum of 5 laps.
- Take the tires off the car and leave them to rest for a minimum of 3 days.
Some points to note:
- Since the tire is not being overdriven, it doesn't really matter if the session is 15 or 20 minutes long (10 minutes probably won't be sufficient given the first few laps are below 8/10ths pace).
- Full throttle, if it leads to wheelspin at high RPM/in low gears, is counterproductive. Similarly redlining the car is unnecessary as most of the energy is put into the driven wheels at lower speeds when the car is accelerating harder.
- As a result of the previous point, the attained lap time is likely to be significantly below the peak lap time the tires are capable of. This is perfectly fine.
- The rest period is important - tires must be heat cycled in a separate weekend from when they are used next. If this is not possible, I would do a scrubbing session rather than a heat cycling session.
- As a result of the previous point, heat cycling tires requires buying them in advance and carrying them to the track in advance of their normal use, as well as expending a track session to heat cycle them.
Scrubbing Vs Heat Cycling
Tire manufacturers often recommend scrubbing in tires if they are not advising a heat cycle. The difference between scrubbing and heat cycling is that scrubbing applies to the surface of the tire whereas heat cycling applies to the entire depth of the tire. A scrubbing session is much shorter than a heat cycling session (4-5 laps should suffice for scrubbing). Similarly the rest period after a scrubbing session can be as short as 30 minutes, rather than the 3 day minimum after a heat cycling session.
When I Heat Cycle
I run both race and DE tires until they cord. My "time trial" tire sets which I only use for 1-2 sessions in a weekend to set the fastest time cycle out before they cord; most of the other sets of tires are usable to zero tread depth at which point they start to fall off enough to not be enjoyable. As such to me there are no drawbacks to heat cycling tires, and there may be benefits. Therefore I try to heat cycle my tires.
I heat cycle tires on the car rather than pay a tire vendor to do so. It is often possible to allocate a session late in the day to perform the heat cycling, when the temperatures are hotter and the car isn't as fast as it was in the morning anyway. This saves me money and I am still able to work on, for example, vision while driving in these sessions.
If I do not have lead time to cycle the tires (because I only have a single set and it's brand new, for example) I scrub them in the first session, then use them normally in the next session in the weekend.
I found that scrubbing RE-71r makes a difference, i.e., the tires are significantly slower in the very first session compared to the second session. I would scrub other street tires as well.
I don't think heat cycling street tires does anything, i.e. there is no difference between a scrubbing session and a full heat cycle session for today's high performance street tires.