Tires are an area of a race car knowledge about which relatively little is understood. Racers know exactly what changing the rear width, a 1% of cross, or any other number of infinite adjustments will do to the handling of a car, and most of them know why those changes effect the handling. The same cannot be said about tires. Other than the effects of changing compound, few often know what makes a tire work.
I had the opportunity to hear Paul Haney, a respected Motorsports researcher, give a 45 minute presentation, and most of that time he discussed tires. So in this brief article, I will attempt to sum up what he said, and hopefully pass along some knowledge while I’m at it.
We all know that tires produce grip, and that without that grip, we would not be able to make a corner. Most of the total grip comes from the front of the contact patch. But was about tires give us that grip? The rubber that tires are made of is viscoelastic. This means that they are able to deform, and recover to their original shape. When a tire rolls over an irregularity in the track, or has a slip angle if it is cornering, it takes the tire about a ½ rotation to recover to its original shape. However, due to the fact that they are viscoelastic, this means that the hotter they get, the less viscous they get. So they will produce grip until the heat causes the tire to boil in a sense.
The tires produce grip 3 different ways. They are Adhesion, Deformation, and Tearing/Wear. Adhesion is the area most karters focus on when they are prepping their tires, even if they know it or not. This is just like it sounds; the tire’s ability to grab a hold of the surface and hold it.
The deformation of a tire is a substantial area of the tires total gripping ability, and very little is understood about this by most karters. When going through a corner, there is a microscopic sliding effect that is taking place. As the tire ‘slides’ over microscopic bumps that are on the track, the tire deforms to grip them. However, when it slides past that irregularity, there is a pocket in the surface of the tire, until the tire recovers to its original shape. Until it recovers or the pocket is exposed to the atmosphere, there is a pressure difference from one side of the pocket to the other, and this pressure difference is a main cause of grip. The larger the change in pressure, the more grip is produced. It’s hard to explain, but an analogy used is a suction effect. The lack of air sucks the tire to the surface.
The third effect is the tearing/wearing effect. Not only does a tire dissipate heat in this process, but the tires with able to tear at a microscopic level when going over an irregularity in the track surface, and a pressure difference is created once again. This effect is better seen on an asphalt track, with ‘sharp’ irregularities in the track (sand particles), but is also seen on dirt tires. Soft tires, such as those used on real heavy tracks, can actually tear to get more of a gripping surface on the track (better illustrated using treaded tires). This can explain why most dirt karters have gone to slicks on dirt.
The slip angle induced by a tire also an important aspect to look at when explaining tires. The slip angle is the angle between where the tire is truly pointed (based off the wheel) and where the car is going (the direction the tire is actually pointed at). A large slip angle means that more induced drag is being experienced at the tires surface, and the slower you will get through the corner. A way to control the slip angle is to make a stiffer sidewall. This can be accomplished by having less sidewall present (6” wheel as opposed to using a 5” wheel), or to angle the sidewall from the vertical. This happens when an 8” wide tire is mounted on a 10” rim. The sidewall is less vertical, and the sidewall acts stiffer than it really is.
Camber of the tires plays an important role in the amount of grip a tire receives. It was explained as getting “free grip.” The more camber that you have in the tire, the more grip it will have. The only drawback is that more grip means more heat, so excessive camber will be faster until the tires heats up and goes away. Lower air pressure is also a very efficient way to produce grip. You have to have enough air pressure so the tire will hold the kart up, but the least amount of pressure will give you the most grip. So if you are experiencing a 4 wheel slide, lower the air pressure to correct it. The grip produced by a tire is load dependent. The more load on the tire, the more grip it will be able to produce. For example, take a winged and a non-wing sprint car. The Winged car is faster because it has the wing on top producing more load downward on the tires, and this gives it a greater cornering speed. Karters can get this effect by spacing tires out from the lateral roll center of the car. The farther they are out, they longer the imaginary lever arm on them, and the more force they experience from the same static load.