Two Guys Rally

Why WRC Cars Use Thin Snow Tires
[ September 29th, 2008 ] By: Charles Smith Posted in » Tips and Tricks

Watch this and look at how thin the tires/wheels are. Link for RSS.

In most cases the wider the tire the more traction is achievable. However in really snowy enviroments (ie Rally Sweden) thin tires can get more traction than wide ones. How? This works because the thin tire is putting a much higher pressure (PSI) onto the snow surface compared to the wide tire. This makes the snow compress and the tire/wheel sink down into the snow. Wouldn’t this just slow you down? Going straight…sure, but the problem in snow is stopping and turning, not going fast in a straight line. Snow builds up around the outside of the wheel and infront of it, so when the car needs to corner/stop it pushes against the snow. This increases the possible traction. This is the same concept as using ruts in motocross or with your car in mud. You can apply much greater cornering forces without losing traction.

There is yet another reason they use thin snow tires. The snowy rallies in the WRC are in areas that also develop a nice layer of ice under the snow. They use this ice to increase traction, mostly for straight line acceleration, and when I use acceleration that means in both directions (hint braking and throttle+). They use the ice by putting metal studs into the tire. The studs work just like cleats for grass sports. They dig into the ice (again because there is a ton of pressure applied to them thanks to the mass of the car) and allow force to be transmitted through them. But in order for the studs to get close to the ice the tire has to dig through the snow, and so that’s yet another reason thin snow tires are used in the WRC.