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Why we have wide tires

Tire grip is the interaction of the tread deforming to fit the pavement.
Drawing By The Author

Somewhere in your youth, you took a science course. And somewhere during that class, the teacher told you about friction -- that it is the resistance objects have when they rub against each other. Then, I'll bet, you were told to rub your hands and "feel the heat" of that resistance for yourself.

It is also probable that your teacher told you that the force of friction depends on only two things: the weight of the object and the coefficient of friction. That is, the force required to move the object divided by its weight. which varies according to the materials involved. Further, you were told that surface area had nothing to do with the amount of friction force.

On the other side of the coin, the first thing someone wanting to make his or her car handle better will do is put wider tires on it. Why? "To get more friction."

These two opposing answers can't both be true, can they? So which one is right?

Both are, each in their own way; here's why:

The first explanation -- that the force of friction only depends on the weight and coefficient of friction -- is correct most of the time;and almost always for two smooth, solid surfaces sliding past each other, like a hockey puck on ice or a coffee mug on a counter top. However, things are not so simple when it comes to tires.

First, tires are not entirely solid. The tread is flexible, bending to conform to the road surface by sinking in to the open spots between grains of asphalt. This creates an interlocking effect, not unlike gear teeth meshing, or cleats on a football or soccer shoe digging into the turf.

Second, the whole tire, including the sidewall, flexes under load, both when weight is put on it, and when it goes around a corner. All three of these figure into the amount of grip a tire generates and how width affects that grip. And tire flexing is the heart of the answer of why "Wide Tires Are Better."

Here's an interesting fact: If you put the same load (weight) on two tires of similar construction and profile height (the distance from the road to the edge of the wheel), they will have the same size contact patch. Surprisinginly, tire width doesn't affect this. Grabbing some numbers out of the air, us say you have a six-inch wide tire (152 mm) and one that is 12 inches wide (305 mm); both of them will have the same number of square inches on the road. The six inch wide tire will have a contact patch 12 inches long, or 72 square inches. The 12 inch wide tire's contact patch will have the same area, but it will be only six inches, front to back. This, in case you've ever noticed it, is why an economy car's tires look "half flat," while a wider set of tires on a similar car look "fully inflated."

So, if the contact patches are the same size, why are wide tires "better?" It's because of what happens when you go around a corner.

Try this experiment: Take a pencil eraser and rub it back and forth on a piece of paper. Notice how the leading edge (the side toward the direction you're erasing) "tucks under" and flexes away from the pencil. Then look at the trailing edge of the eraser and notice how it flexes, with the top (the part attached to the pencil) "ahead" of the bottom part, where it touches the paper.

Now look even more closely. Assuming you're trying this with a new eraser, you will also notice that the "leading edge" is in contact with the paper, while the "trailing edge" has lifted clean off the surface! Believe it or not, tires do exactly the same thing when you put a side load on them as you go around a corner, though you have to look closely at photographs or video to see it happen.

Next, repeat the eraser experiment with a big, wide eraser, the sort that is not attached to the pencil. You will notice the same kind of bending, but you will also notice that the trailing edge of the eraser does not lift as readily from the paper -- basically, more of the eraser stays in contact with the paper.

Here's where the difference between a narrow tread and a wide one becomes important. As you've noticed, both erasers flex, and by the same token, so do both narrow and wide tires; sidewalls. The difference is what happens at the tread.

As a narrow tire rolls under, the inside edge of the tread lifts, leaving the outside edge to do all the work of keeping the tire from sliding. Since the tread "interlocks" with the road, that puts a lot of strain on the rubber. Sooner or later, the strain becomes too great and the rubber tears off. At this point,  you have sliding tires and black marks on the pavement.

When you put the same side load on a wide tire, the sidewall will tuck under, but the tread will not lift as much on the inside edge. Thus, more of it stays in contact with the road and more of the rubber interlocks with the pavement irregularityes. That means that the load on each individual piece of rubber is less; thus, the whole tread can take more force before the rubber starts tearing. That's what makes wider tires hold the road better.

Racing tire designers know this and take advantage of it. Since wider tires "share the load" of sideways grip better than narrow ones, they can make the tread compound softer; sometimes so soft it gets sticky. This makes for still more grip before the tires let go. To enhance the effect still further, they make the tread "slick," that is to say, with no grooves or individual blocks of tread, at all. This converts the contact patch from a number of small erasers into one great big one, with each inch of the whole width of the tread supporting every other part, rather than being ground up individually.

The one drawback to this is that slick tires have no place for water to go. That's what the grooves in the tread are for -- to channel water away from the rubber and allow it to stay stuck to the road. If you didn't have them, the tire would aquaplane at very low speed in a light rain that a grooved tire would scarcely notice.