# The principle behind "big tires=big grip"?

I understand the principle that bigger (wider) tires give better grip than smaller ones. It’s a fairly well-known phenomenon.

But I don’t understand the science of “why.” My understanding from HS physics is that you can take two substances (say, rubber compond+concrete) and calculate a coefficient of friction (actually two…one for static and one for motion). Since they are coefficients, they shouldn’t vary on account of weight…grip ought to be linear, and 1sq.in. supporting 10#/sq.in. ought to grip as well as 10sq.in. supporting 1 #/sq.in.

Further, why would wider tires have a bigger contact area? Assuming a car’s tires are at 32 psi, and the car weighs 3200#, there ought to be 100 sq.in. of contact area, regardless of tire width.

So, this really stumps me. There must be some real-world deviation from ideal physics (which there generally is), but I can’t quite figure out what it is!

Forget the physics for a moment. Big tires are great in loose snow and on sand and mud, since they actually grab more material to get traction. Dragsters have big bald rear tires so that they don’t completely burn off the rubber each time they accelerate. Your normal coefficient of friction theory applies here, unlike snow, mud and sand tires.

Formula 1 cars have larger tires too to get the best combination of life, traction foreward and side slipping ability. A Formula 1 car could race fast on skinny tires, but they would not last long.

Performance car tires have special rubber and read pattern that flexes and sticks to the road (like track shoes), so therefore good traction, but shorter tire life.

So your theory only applies to bald tires as used in racing and dragging. And there they make them big so they’ll last a while.

Hopes this helps.

I disagree with your well known phenom. The principal of wider tires giving better grip is only relative to race cars on dry pavement where all have the same tread design. Wider tires are generally worse in snow and mud (deeper mud and flotation an exception) and dependent more upon tread design. There are too many other factors for the typical sedan. A stiff side wall may affect the contact area more than width rendering your “wider having the same contact area” suspect. So, wider tires are not just wider w/o other factors to consider as well. Theoretically, radial ply tires keep the contact area more consistent regardless of inflation and load.
So theoretically a narrower radial may have better traction for cornering than a wider bias ply. The differences also exist within each type. I wouldn’t look for simple answers.

Agree; traction in mud, snow and loose dirt is mostly a function of TREAD DESIGN, not tire size. On farm tractors we only add dual wheels when flotation in deep mud is an issue.

So, putting big tires without good treads on your drive wheels in snow makes for WORSE traction. Standard size tires with a good snow/mud tread is best.

You’re right, for a given pressure the contact area doesn’t much change. What does change as tire get wider they also get lower profile (78->70->60->50->40->35, or even less!?!) means the tire is much less able to roll over, so the tread is held in better contact with the road, improving handling and shortening the reaction time from twisting the wheel to changing direction.

Tires are dynamic things, not static…They are constantly changing shape according to the load being applied…As they become more distorted from the “normal” shape, they become less able to maintain traction. Your theoretical contact patch is always changing as loads change.

Fat tires excel in magazine skid-pad tests. But few of us drive on skid-pads, and ultra low profile tires have drawbacks all their own…

“Since they are coefficients, they shouldn’t vary on account of weight”

With real tires the coefficient of friction does vary a little with weight, and many other factors.

Top fuel dragsters accelerate faster than the theoretical coefficient of friction of rubber to asphalt will allow. They use ammonia to “gum up” the tires and allow them to gear into the asphalt.

Wide tires on sand allow the vehicle to “float” over the surface and not dig into the soft sand.

The contact patch between tires and road as well as the side wall dynamics play major roles in overall performance.

Twotone

its just wider tire = wider contact patch. a rolling pin has wider contact patch then a pizza cutter. lower PSI makes the contact patch even bigger, allowing the tread to form around the terrain.

The problem with the “coefficient of friction” theory is that is applies to SMOOTH surfaces - and not surfaces with texture - like a road surface. Sure, on a micro level all surfaces have texture, but in the case of a road, you can actually see the texture.

Rubber, being flexible, penetrates that texture, and additional grip is generated. That is how cars are able to generate more than 1.0G cornering force - which is theoretically impossible under the coefficient of friction theory.

If you were to exceed the maximum grip of a tire on a road surface, the rubber is torn off. You can see this as bits of rubber that accumulate outside the groove on racetracks. These are called “marbles” - oh, and stay away from the marbles. They are called that for a reason!

BTW, the theory that you can calculate the load on a tire by measuring the contact patch and the inflation is incorrect. I cover that here:

http://www.barrystiretech.com/airortire.html

Interesting to see how surface rouhness increases grip in excess of theory. I had always thought the excess was due to (hot) rubber being a sticky, somewhat adhesive semi-liquid. Now I realize that’s not the whole story.

An additional benefit of a big contact patch (thinking more MC here) is that it’s less likely sand or other surface imperfection can totally compromize the contact patch.

PS Thinner tires are best in loose snow. More cutting and less plowing.

There are many variables, but one example of the error in that theory would be snow tyres. Generally (not always) a thinner tyre will have better traction in snow than wider tyres.

Skinny tires also are less likely to hydroplane in the rain. In fact, I doubt that you can go fast enough to make a 700X23 bicycle tire inflated to 120 psi hydroplane.