Tire Temperatures

One of the things that drove me crazy for a while was that if you ask the tire companies what the ideal temperature is for their tire, you get a value that is nearly 80 degrees F hotter than the temperature at which the tire noticeably gets greasy and "goes off". Furthermore, as tire technology progressed, the temperature at which they "came in" got colder, and the temperature at which they "went off" got colder, even though the recommended operating temperature remained pretty much constant.

As it turns out, there is a reason for this:

In a road racing environment, you have cars running for extended lengths of time at high speeds and high loads, but the total time spent at the cornering limit of the tire, as a percentage of lap time, is fairly small. The primary source of heat into the tire under these conditions is the constant flexing of the carcass as the tire rotates through the contact patch, the same way a coat hanger heats up when you bend it back and forth.

In an autocross environment, you have cars running for a very short period of time with a very large portion of the lap spent with the tires at the cornering limit. The primary source of heat into the tire in this environment is friction/flex in the tread at the contact patch.

Put simply, road race tires heat up from the inside out, where autocross tires hat up from the outside in.

This means that if you ever manage to get an autocross tire, in an autocross context, to a carcass temperature consistent with a road race tire in a road race context, that you have cooked the tire - the heating mechanisms are completely different.

This also makes tuning autocross tires by temperature, common amongst road racers, much more difficult, as the carcass temperature you measure with your pyrometer probe is just a "shadow" of the real temperature at the tread face.

Pointing infrared tire temperature sensors at the tires and recording temps on the datalogger proved very illuminating, as you find out that the tread surface heats up - and cools down - very quickly. In a typical slalom, the temperature at the tread face actually drops in between G peaks!

Some day, the price of colour infrared cameras is going to drop enough to where it is affordable for mere mortals to point a camera at each tire and record the temperature gradient in real time, and the first guy to do that is going to learn a TON and get a lot faster. But for now, the best you can probably do is a triple bank of spot sensors on each tire (inside, centre, outside).

If you can't afford that, however, I still think recording carcass temps in grid is worthwhile. I wouldn't hang my entire setup on tire temps (the difference between the temps on course and in grid is just way too large) but persistent hot spots or persistent cold spots are indicators of something being wonky. A persistent cold stripe around the outside edge of my rear tires was a clue that there was too much static negative camber in my setup.

One thing that makes collecting tire temps a whole lot easier is the memory pyrometer Yes, it is expensive, but it also makes collecting post-run tire temperature data so much easier and faster that it actually becomes doable. It stores each temp in memory and automatically advances the index to the next cell, so instead of probe tire, call out number (or scrawl it down), probe tire, drop pen, kick tire in angry frustration etc the process is just probe the tire, wait for the beep, probe the tire, wait for the beep etc - then read the temps later. It's just a much more civilized way to collect data when real-time IR data is not available.

I'm also a believer in the passive tire insulator. Made from metalized bubble wrap and available at any Home Depot or Rona, blankets fashioned from this stuff can trap a surprising amount of heat and retain it in the tire. It is possible to go too far (I cooked a set of tires to the greasy point at a Peru Tour in 100 degree ambient temperatures) but in most other cases, they're a win.

Tire Insulators at Work

If they were allowed, I'd use heated tire blankets too. If you look closely at the rules, they actually ban electric blankets, so I actually designed a set of fuel-fired tire warmers that circulated hot water around tires via a manual pump... never got around to building it....

A similar effect can be had for much less effort by pulling wheels off and leaving them out in the sun for an hour before gridding. And some hard braking before showing up in grid (to heat up the brake rotors) can also be used to generate some heat.