Wider Tires and Contact Patch Shape

[79-TA]

Putting wider tires on a car is often referred to as "putting more rubber on the road." For the sake of technicality, the contact patch (given the same tire pressures) must retain the same area, albeit in a different wider shape. (this neglects the slightly greater weight that the wider wheel/tire package itself would add to the load at the contact patch)

I've tried to explain to myself why the wider tire patch is better, but have yet to come up with any good technical explanations. I could generically say that the shape is better for lateral grip, but that's really not an explanation. I've tried to illustrate it with a piece of scotch tape, noting how it is more difficult to pull it lengthwise than peel it off from the side. Also, I've tried to think about how tires deform under different loading conditions, but that's not something I know much about.


Any better explanations would be appreciated. :)

[silver69camaro]

The simplist explaination on a molecular level is a series of interlocking "fingers" that grip each other, like a gear mesh.

Imagine two gears that are meshed, one atop another. Now, try to pull those gears apart horizontally. The gear teeth prevent the movement, right? Well, at some point there will be some slippage. Now imagine those gears twice as wide as before. It would take more horizontal force to break them apart. This is what a tire does on a micro level, the "teeth" on the tire's surface interlocks with the "teeth" on the road surface. More teeth, more grip.

You can also attain more "grip" by placing more weight on the tire ("normal force" in friction calculations). However, it's a gain of diminishing returns as double the weight will not result in doubling the grip, more like 1.5.

[Turbo67camaro]

Wider and/or taller tires = larger contact patch area = greater grip on the road for acceleration or braking.

As acceleration or braking forces increase, the larger contact area will take longer to lose grip on the road, because it is capable of higher surface adhesion. I disagree that the wider tire will retain the same contact patch area size.

Example: If you stick a small and a large piece of tape to a wall, it will take more energy to pull the large piece of tape from the wall.

Downside of that better grip? - More wind and road friction drag, which is why economy cars tend to have small, narrow tires, which isn't very good for safety.

[Procharmo]

Further more, if a car has excess power to weight ratio and excess braking ability it can utilize the extra contact patch and over come the increased rolling resistance. Compound and condition (heat cycles) of tyres have a big roll to play on performance when compared to just diameter and width. (This is on tarmac/concrete).

[Boesch]

Here's a simple way to prove to yourself that a large contact patch provides more grip. Take a block of wood about 4 inches square and try to slide it across the floor. It will slide pretty easy. Now take a 4x8 sheet of plywood and try and slide that across the floor. It'll slide a heck of a lot harder than that small piece of wood. 4x8 sheet of plywood has a lot larger contact patch, therefore more grip.

Think about when you're moving furniture around your house that's pretty heavy. The first thing you do it pick up one end a bit so that it becomes easier to slide. The reason it's easier to slide once you've lifted on end slightly is because you've decreased on the contact patch with the ground.

[79-TA]

Thank you for all the responses. I do already understand why a larger contact patch is better, but I still appreciate the thoughtfulness.

My problem is that for the wheel to sit static, the sum of the forces must be 0. The force (from the weight on the tire) must be equaled by the tire in terms of pressure x area. If the tire pressure is held constant, the width of the tire can only change the shape of the contact patch. Given this alone, the contact patch area should be constant for a given tire pressure and load.

However, I think the biggest thing I haven't taken into account is that the force on the ground from the tire is not distributed equally (more pressure in the middle of the footprint.) How exactly that occurs when the pressure inside the tire is uniform is now puzzling me a bit.

And Matt J, while I thought I was familiar enough with coefficients of friction and normal force, I had never really considered that the relationship would not always be linear. Thanks.

[Boesch]

My problem is that for the wheel to sit static, the sum of the forces must be 0. The force (from the weight on the tire) must be equaled by the tire in terms of pressure x area. If the tire pressure is held constant, the width of the tire can only change the shape of the contact patch. Given this alone, the contact patch area should be constant for a given tire pressure and load.

However, I think the biggest thing I haven't taken into account is that the force on the ground from the tire is not distributed equally (more pressure in the middle of the footprint.) How exactly that occurs when the pressure inside the tire is uniform is now puzzling me a bit.



When you say that force is equal to the pressure x area, that is the pressure (or you can think of it as force) between the tire and the ground. This force must be equal to the load or weight coming from the axle. That's not the same pressure as the pressure of the air inside the tire. The pressure inside the tire is not constant either. As a tire is deformed the pressure inside goes up as the volume of the tire decreases.

The pressure patch on a tire is no where near equally distributed. When I was doing my master's degree I had a plot of the pressure distribution done for someone else's thesis. In the tire that they had tested (a light truck tire), the pressure actually was the highest in a ring around the outside. This is largely due to the deformation that you get when you deform a cylinder or sphere. If you ever get hit with a racquetball you'll know that the resulting welt is actually a ring on your skin, illustrating the the highest pressure is around the outside, not in the center as people commonly think.

[79-TA]

I realize that volume (along with the rest of the pv=zmrt equation) affects pressure. However, the deformation a tire experiences does not drastically change the volume and therefore the pressure change is small as a result. For instance, my tire pressure varies less than 1 psi when I measure it with the wheel off the ground and with the wheel supporting its share of the car's weight.

Anyway, that's really interesting how the most pressure was on the outer ring in that test.