This week’s feature author, Paul Haney, knows more about those round black things on our cars that our driving life depends on than anyone this side of the secret boffins hidden in the basements of a few tire manufacturers. In fact, Paul has written the book on them – The Racing & High-Performance Tire.
I first learned of Paul when he co-wrote Inside Racing Technology with Jeff Braun, and then I got to know him more when I presented a paper at the SAE Motorsport conference a few years ago. He’s smart and a good guy (hey, what’s he doing in racing?! :)).
I don’t know of anyone better than Paul to talk about tires.
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At the start of a race or track session we see drivers weaving back and forth across the track. Ask a driver why all the swerving and you’ll hear, “I’m getting the tires up to temperature.” Well, I don’t think that’s what’s happening. Most of the tread surface is cooling, not heating. And, since rubber is a poor conductor of heat, very little of the frictional heating on the tread surface gets into the bulk of the tire.
Rubber is a very weird material. And a four-wheel vehicle with rubber tires, shock absorbers and a human driver is an extremely complex system. When it comes to tires it’s easy to grab on to a seemingly obvious explanation for what we see a tire doing only to find out it doesn’t really work that way. What we see is actually happening, of course, but the explanation we assign might not be correct. The sun, moon and stars seem to be rotating around our planet, but reality turns out to be more complicated. It took the invention of the telescope to reveal reality.
Tires Do Heat Up
Mainly, tires gain heat due to hysteresis-energy absorbed in the rubber when it is compressed, stretched or deflected in any way. Hysteresis is a characteristic of any viscoelastic material which certainly describes rubber. Visco says it’s viscous, elastic means it will stretch. Being viscoelastic, rubber stretches and bounces back, but not all the way back. Energy in not-all-the-way-back is lost in the rubber, but you never lose energy – in this case it becomes heat.
The thumbnail test (digging your thumbnail into the surface)…
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