I’ve been wondering why sport and race cars use very wide tires for improved grip. Friction is as far as I’m aware a function of a coefficient and pressure. So, with wider tires that pressure should reduce no? Why do fast cars then use wide tires?
In: Physics
Wider tires are less likely to loose contact with the ground
The actual contact area doesn’t matter so much as much as having contact at all. So if you drive over a piece of gravel it doesn’t pop your wheel off the road.
Also there’s a balance between tire wear and softness and how fast the tires heat up when deforming which changes that coefficient of friction of the rubber.
The friction model you have been taught in school is the Columb model which is a fairly good approximation of how friction works for most cases. And in this model the friction does not depend on contact area. However there are some assumptions in the basis for this model which does not always stay true. For example in motor racing it is easy to saturate a small contact patch if you apply too much force to it. The materials will then bend and deform from the huge force alone. In fact this is often the case when people drive on the edge of friction and this can cause rubber and pebbles to be thrown free from the tyre and track. Increasing the contact patch means that there is less forces going through each section of the tyre and track meaning it can takes more forces before it gets damaged.
Tyres do various jobs and their design is a compromise of those things. As a general principle, you want enough surface area to be able to perform that function properly, but not too much or you have excess / unnecessary friction.
When you accelerate, you want your driving wheels to transfer as much rotation as possible from the wheels to the road. More surface area = more grip = more rotational force transferring to the road = more forward movement. The more power you have, the wider your tyres are (up to a point). If you don’t have enough grip, you break traction and your wheels spin without actually pushing you forward.
For your steering wheels you wants wider wheels for more grip so you turn more accurately. If your steering wheels lose grip, they can turn relative to the car but they won’t move the car relative to the direction of the road. Similarly for braking, wider tyres offer more grip which means better braking.
Tyres also have tread that pushes water out of the way. The more tread lines you have, the less rubber you have in contact with the road. Your grip is reduced (for acceleration and turning and braking) but you can push more water away which improves grip. Most high-level racing uses slick tyres because they have the most grip for the width of the tyre, but they are terrible in wet weather, so there are other tyres for those conditions. As I said, it is a compromise.
The limit here is that you don’t want your tyres to be too wide or you have “too much grip” and you end up wasting / losing too much speed due to friction. This would make it inefficient / pointless.
The current setup is a carefully calculated compromise to harness as much of the vehicle’s capacity without wasting it through having to overcome the additional friction.
It adds surface area. So with a wider tire, more tire is coming in contact with the ground which adds friction/grip which increases the speed and control for the vehicle.
It’s kind of like athletes wearing cleats. It increases how much they can grip the ground which let’s them run faster.
This is actually a REALLY good question. It was actually a challenge problem in one of my undergrad physics classes. You are correct, friction force is only dependent on CoF and Mass.
If you just look at the math, it seems impossible to increase the grip of tire. However; in the real world, these frictional forces are dynamic, not static.
First, many people assume that increasing tire width increases the surface in contact with the road. This is actually NOT true. What increasing the width of the tire does is increase the width of the contact surface perpendicular to the direction of travel, but it becomes narrower in the direction of travel (You are distributing the forces across a wider area and decreasing the pressure). Think of it as a 3×5 rectangle vs a 5×3 rectangle. Because of these dynamic forces and the road surface not being completely smooth, this wider contact path perpendicular to the direction of travel increases the ability of the tire to actually reach the maximum CoF.
Because the contact area isn’t really constant. As the car turns, the tires don’t have an even pressure to the ground across it’s width. A wider tire allows for better contact management overall. A lot of racing is “won” during the braking and turning phases of the track – so better traction in corners allows for much better track times.
The tires are also not totally dependent on “friction” as they are somewhat sticky when at operating temperature – the simple equation used for friction is not applicable. In general, the larger the contact patch, the better.
Rubber(and other materials) has its limit in friction until the surface just tears appart from the surface under and you get a burnout. (Shear constraint)
Increasing the surface helps spread the force that would tear away the rubber and thus makes the tire take more torque before making a burnout
ELI5: an eraser. You can put all you strength to drag it and it will make big chip or tear in chunka instead of gripping what is under if it is stronger.tape 2 and 3 erasers together and you will have to use more force to get the same result.
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