In last month's article, we explained the physics behind weight transfer. That is, we explained why braking shifts weight to the front of the car, accelerating shifts weight to the rear, and cornering shifts weight to the outside of a curve. Weight transfer is a side-effect of the tyres keeping the car from flipping over during manoeuvres. We found out that a one G braking manoeuvre in our 3200 pound example car causes 640 pounds to transfer from the rear tyres to the front tyres. The explanations were given directly in terms of Newton's fundamental laws of Nature.

This month, we investigate what causes tyres to stay stuck and what causes them to break away and slide. We will find out that you can make a tyre slide either by pushing too hard on it or by causing weight to transfer off the tyre by your control inputs of throttle, brakes, and steering. Conversely, you can cause a sliding tyre to stick again by pushing less hard on it or by transferring weight to it. The rest of this article explains all this in term of (you guessed it) physics.

This knowledge, coupled with a good "instinct" for weight transfer, can help a driver predict the consequences of all his or her actions and develop good instincts for staying out of trouble, getting out of trouble when it comes, and driving consistently at ten tenths. It is said of Tazio Nuvolari, one of the greatest racing drivers ever, that he knew at all times while driving the weight on each of the four tyres to within a few pounds. He could think, while driving, how the loads would change if he lifted off the throttle or turned the wheel a little more, for example. His knowledge of the physics of racing enabled him to make tiny, accurate adjustments to suit every circumstance, and perhaps to make these adjustments better than his competitors. Of course, he had a very fast brain and phenomenal reflexes, too.

I am going to ask you to do a few physics "lab" experiments with me to investigate tyre adhesion. You can actually do them, or you can just follow along in your imagination. First, get a tyre and wheel off your car. If you are a serious autocrosser, you probably have a few loose sets in your garage. You can do the experiments with a heavy box or some object that is easier to handle than a tyre, but the numbers you get won't apply directly to tyres, although the principles we investigate will apply.