Weighing in on Traction: The Importance of Friction and Car Response

In this installment we are going to look at one of the best ways that we can get the car to do what we want it to do. First off though, it is important to understand that the only way we can get a car to do anything at all, such as braking, accelerating or turning, is through the use of friction. Of course, the friction we are referring to is that which is developed between the road and the tires.

The area where each tire on a car actually meets the road is a section that we call the contact patch. On a typical street tire sitting in a static or even state, that contact patch is only about the size of a piece of standard notebook paper. Basi­cally, that contact patch is a pretty small area, considering that a typical car weighs over 3,000 lbs without any additional G-load acting on it. That means there is a lot of work for four notebook size pieces of tire to do when we are driving hard.

Probably the single most effective mod you can do to a car to make it accelerate, brake and handle better is to put newer, wider, lower profile, stickier tires on it. But, once those tires are on the car you are pretty much stuck with the stan­dard static contact patch that those tires can offer, right? Well, no ... that's not really true. Look at the Figure to see what I'm talking about.

If we transfer weight or load while we are driv­ing, we can actually affect the way that the tire rubber meets the road. When you step back and think about it, you realize that all a tire is, is a balloon with a really thick skin. And, just like a balloon placed on a flat surface, if we put weight or load on that balloon, we can get it to spread out so that there is a larger contact patch between the balloon and the flat surface. In the figure we can see that there are several differences between the two illustrations of a suspension, wheel and tire assembly. First off, we see a difference between the compression of the shock and spring area. We can also see a slight angle or camber difference between the two. The most obvious difference between the two though, is the contact patch area. The assembly that has more load acting on it has produced a larger tire contact patch.

"Hold on now," is a common response I get from students. "Are you telling me that if I have a heavi­er car it'll handle better than if I had a lighter car?" Actually, more weight, once we put it in motion, becomes inertia, or workload that the tires now need to compete against. It is weight or load transfer that we are talking about, not total weight of the vehicle. There is a big difference. It is also important to understand that during weight trans­fer, when we move load to one part of the vehicle and increase that contact patch, we are effectively taking load and grip from another area of the car. This is shown clearly in figure. In this illustra­tion we see that we have weight being transferred forward. The contact patch in the front is large and will provide lots of grip, while the contact patch in the rear has been decreased and won't be able to handle as much workload. An easy way to get that weight to the front of the car is to use the brakes. But there is a far more subtle way to accomplish that forward weight transfer. Try a simple lift off the throttle. A lot of drivers don't realize that taking your foot off a pedal can have just as much of an effect on vehicle balance as pressing it.

To shift the weight to the rear a driver can add pressure to the throttle, which is the most obvi­ous way to transfer weight to the back, or they can aggressively reduce brake pressure. In fact, in a relatively softly sprung car, a quick reduction in brake pressure can transfer more weight rear­ward than any other input. The rear bias of weight transfer allows a rear wheel drive car to acceler­ate well because of the traction that is given to the drive wheels. However, that rearward weight bias will act against the car turning well. This is an important point, because we see many drivers aggressively reduce brake pressure right at the point of turn in.

It's not just when we decide to transfer weight in a vehicle that is important, it is also how we transfer that weight. Are we quick and rough with our inputs or are we smooth, precise and easy with those inputs? The difference in technique is the difference between feeling fast and being fast.