Care and feeding of your dirt racing tire

One of the most important parts of a race car, besides the driver, are the things that connects the car to the track. Tires are meant to be nurtured and loved. Tires get  caressed and fussed over more than a grandmother fusses over her grand child. Well, maybe not that far. But, you get the point, take care of your tires. Below I’m posting an excerpt from a little dirt racing manual I am working on; it is from the tire section.

I must preface this section by saying that tire engineering, as I understand it is more an art than a science. There is no perfect answer. Tire companies are continually changing rubber compound, and tire construction to stay ahead of the competition; and competition between tire companies is fierce. A Formula One engineer told me a story one time about a race where two cars were in a tight battle for first. The two cars had on different manufactures tires. In the final laps one of the cars rubbed the other and left black marks all over the side of the car. Immediately after the race tire engineers were all over the marked up car. They had the crew removed the side pod, before anyone could touch it, and wrapped it in plastic to send back to the lab. They needed to analyze the tire compound samples the opposing tire companies tires left on the side of the car. Something like this would never happen on the world of dirt track racing, but competition is fierce none the less. Let’s look at how a tire works and pushes the car around the track.



Slip angle

In order for a tire to give maximum traction when turning, accelerating or braking it must run at its maximum designed slip angle, or slip ratio. So what exactly is slip ratio and slip angle?  The slip angle of a tire is the difference in the angle of the steering spindle verses the angle of the actual turning of the tire. Picture it this way. As a driver starts to enter the corner he begins to turn the steering wheel. The tire wants to continue to track in a straight ahead direction. Because of the stretch in the side wall of the tire the car will not turn at the exact angle the steering spindle is turned to. The steering spindle must turn more than the intended arc. Think of the tire as a rubber band. As the tire rotates into the contact patch it is stretched to the side. As it leaves the back of the contact patch the tire snaps back into place, aligned with the spindle, and the energy of the tire returning to its original state is what pushes the spindle to turn the car. The slip angle of the tire is built into the tire configuration and is different for every type of tire. Modified tires have a very small slip angle and are not as forgiving as say a sprint car right rear tire that has a very large slip angle. There are many factors that go into the construction of the tire that effect the amount of slip angle the tire has, but what I use as a gauge of how much slip angle a tire has is the side wall. A tire with a large bowed out side wall similar to a sprint car or a late model tire will usually run at a greater slip angle than a modified or street stock tire. The more the slip angle the tire has the more forgiving the tire is, but the mushier the car is going to feel.

On track tire management

The energy pulled into the stretching the tire also creates heat in the tire. When you stretch and twist a tire it will actually heat from the inside out, bringing the tire up to temperature gradually. All tires have a temperature range that they operate the best. The art of good on track dirt racing tire management is to get the tire up to temperature quickly and keep it there without overheating the tire. Spinning the tires is not always the best way to get heat into the tire because it heats the tire surface very quickly while the inside of the tire rubber is still cold and the tire has a good chance of sealing up. If you are racing at a track that has some abrasion to it, and the tire has heat in it already, you may be able to spin the tire to help keep the tire at operating temperature, but be careful. Once a tire seals over you are pretty much done.

The above section on tires is not totally polished, but it is a sample of content that I have been working on lately. I pulled some paragraphs from different parts of the tire section.Till next time, happy grooving.

Kevin