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"How quickly a racing tire comes up to a good operating temperature, at least 180 degrees F, depends on the load, car speed, ambient temperature and the track surface temperature."


Message to Valtteri Bottas: Swerving Doesn’t Heat the Tires

by Paul Haney


Tire Webinar3While lapping behind a safety car at the Chinese Grand Prix, Bottas managed to lose control of his Williams F1 racecar while swerving back and forth across the track thinking that would help him keep his tires warm. He acknowledged “aggressive” swerving and an “amateur” error but, actually, he shouldn’t have been swerving at all. After a few laps at speed on a track tires get hot and it seems obvious that’s caused by frictional heating. But that’s not how tires get hot.

I’ll get to the details but there are two reasons why swerving doesn’t heat the tires any more than driving at speed in a straight-line. At any one time only a few percent of the tire’s tread surface, some portion of the contact patch, is generating frictional heat. At that same time all the rest of the tread surface is cooling. And, since rubber is a poor conductor of heat, very little of the frictional heating in the contact patch gets into the bulk of the tire.

Tires Do Heat Up

Mainly, tires gain heat due to hysteresis, energy absorbed in the rubber when it is compressed, stretched or deformed in any way. Hysteresis is a characteristic of a viscoelastic material, which certainly describes rubber. Visco says it’s viscous, elastic meaning 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. We need hysteresis in rubber because it gives us grip but that’s another story.

Air Supports the Load

We can see the bulge in the sidewalls at the contact patch but we don’t notice the straightening of the sidewalls at the top of the tire 180 degrees from the contact patch. These two deflections rotate around the tire at the speed of the car. Due to hysteresis some of the energy causing these deflections is absorbed by the rubber in the belts and plies, generating heat in the tire. So just rolling down the road under load heats a tire. How quickly a racing tire comes up to a good operating temperature, at least 180 degrees F, depends on the load, car speed, ambient temperature and the track surface temperature. With lightly loaded race cars it might take a few quick laps to heat up the tires. Drivers who can go fast on cold tires have an advantage early in a race.

Tread Temperature Variation

Bottas graphThis graphic shows that only a very small portion of the tread surface is heating at any one time. The black line traces the temperature of a spot (maybe only a few molecules thick) on the tread surface of a rolling race tire near maximum grip in a corner. There are no values shown because we can’t measure tread surface temperature in the contact patch. We can, however, place an infrared sensor looking at the tread surface as it rotates out of the contact patch and record temperature as high as 400 degrees F.

The black line shows how surface temperature changes as the tire rotates. Our spot on the tread is in the contact patch during the time between the two dotted lines. We can see the rubber is air cooling as it comes into the contact patch and is cooled further by the road surface at the front of the contact patch where the tread is stuck to the road. Further back in the contact patch where sliding occurs, frictional heating quickly spikes the surface temperature up as high 400 deg F. But when the rubber comes out of the contact patch it quickly cools, first by air rushing over the tread and then by the road surface as our tortured little rubber molecules once more enter the contact patch.

How Much of the Tread Is Generating Heat?

A tire 26 inches in diameter is about 82 inches in circumference. Inflated to 20 psi and loaded at 500 pounds this gives us a contact patch area of 25 square inches. You can find race tires with tread widths between 6 and 16 inches, so the loaded contact patch length is between 4 inches and 1 inch. That tells us our rubber molecule is in the contact patch for 2% to 4% of the time during one revolution. And, of course, it means that 96% to 98% of the tread surface is constantly cooling.

Rubber Is an Insulator

Rubber is a good insulator, about the same as polystyrene foam used in cold drink coolers. The insulating characteristics of rubber is whyF1 Drivers Swerving 360px hysteresis heating in the plies has a tough time getting out of the tire and frictional heating at the tread surface can’t get in.

Let’s say you’ve got a cooler full of ice and beer. Place a fan so it’s blowing air down one side of the cooler. Then put a heat lamp shining on that same side with a timer that repeatedly turns the heat on for one second and off for 20 seconds. Do you think much heat is going to get into that cooler?

Is There Any Benefit from Swerving?

One of the people I spoke with on this subject was Arnaud Dufournier, President of Dufournier Technologies, a French company with some specialized tire testing hardware and software. Here’s what he had to say:

“We worked a few years ago on that subject for Formula 1, and the study we did showed that swerving increases tread temperature a small amount but really less than people think. Braking has a very low impact on tire heating because it doesn’t last long enough, one or two seconds. The best way to heat-up tires is to spin them, but it works only for rear tires and it’s possible to damage the tires.

“Swerving with quick movements of short duration do very little heating. Longer movements over a wider portion of the track work better but still don’t generate very much tire heating and the driver risks picking up dirt and rubber from the track thereby degrading tire performance.”

Bottom Line: Swerving Maybe Cleans the Tire, But Doesn’t Heat

In the end the only benefit to swerving might be some cleaning of debris off the tread surface. A hot tire can quickly pick up any loose rubber bits, dust and gravel that happen to be on the track. Swerving can help roll this stuff off the tire but the tire can also pick up more debris in the process.

The guys working track support for brake and tire suppliers will tell you that drivers should drive straight while alternately braking and accelerating. That warms the tires and the brakes.

If swerving gives a driver something to do and helps that driver gather up the competitive juices, go for it.

But it’s not heating the tires!


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