I've always heard that slick tyres are for more contact patch area, but why is that something desidered?
That’s a simplified standard friction formula that doesn’t apply in all cases. For those sports cars with wide tires there’s multiple reason, the first is that not all the time is on the ground at any one time, if it’s a sharp curve at high speed the tire can lift from one side, wider tires can minimize that by still having sufficient contact area with the ground. But perhaps more relevant to your question many racecar tires are extremely soft and actually sticky once they get hot. Stickiness is very similar to friction, but depending on the surfaces it will change based on area.
That’s simplified, but probably gets you on the right track. I’m n most cases area is irrelevant for friction like you said, but not always.
. The formula is a simplified model which is a reasonable approximation in some circumstances particularly in machinery design where hard materials rub together. Engineering schools should do a better job of explaining the limits of the various simplified formulas they use. Friction is a very complex and interesting topic, it really can't be reduced to simple linear formula except under near ideal conditions (like those often found in machine design)
I've always found it interesting that If you calculate the friction coefficient for dragster tires based on vehicle weights and acceleration you will find the coefficient is greater than 1
Flexible surfaces use a different mechanic of grip and not just simple friction forces. I can't remember what it is called but it has to do with the way the tire flexes under load (and slip) to mold to the the surface it is gripping on. This is why a tire needs to slip a little bit to generate the maximum amount of friction.
Mechanical keying is the word you’re looking for. The rubber flexes to conform to the road surface to give much better grip than from friction alone. A very smooth road actually gives less grip.
Tires also literally stick like glue to the road. You can see small rocks stuck to really sticky race tires.
Both of these characteristics are much stronger in high performance tires and both benefit from a larger contact patch.
Because tire grip is more than friction. Ever see a climber ascend a vertical wall? Obviously, they have no perpendicular force, they grip handholds on the wall. Tires deform to grip irregularities in the road. A wider tire is like giving the climber more hands.
Thanks, good example
There's even more than that going on. The surface of the rubber compounds form chemical bonds with the road surface too that provide part of the grip as well. When those bonds break they release heat into the tire which also increases the mechanical grip locally as well since rubber is one of the few materials that increases friction with temperature.
That's part of the reason why slicks are terrible when things are wet. Besides the potential for hydroplaning, wet slicks can't produce heat from chemical bonds breaking resulting in reduced mechanical grip, have less friction due to no chemical bonds forming with the road, and no way to evacuate water effectively that is trapped between the tire and road.
That’s only really true for simple examples with Indestructible materials.
Like, the problem with F1 cars is they put SO MUCH FORCE on those tires that sure maybe a smaller tire would still “grip” the ground, but it would also get torn apart by the forces going on. The rubber will grip the concrete, but the rubber will literally get ripped apart.
Remember that also those simple physics formulas are true in a controlled setting, but in the real world you have to account for things like “the tire will tear itself apart” or “the ground isn’t actually perfectly solid”
Because unlike most solid materials, rubber behaves really weirdly when it comes to friction.
The law you are referring to regarding the relationship between perpendicular force and friction is not a universal one, nor is it representative of how friction actually works. Rather it is an empirical law, meaning it is derived from experimental data, not from a fundamental understanding of the underlying mechanism. The law just happens to hold for a wide range of materials.
The actual mechanism that causes friction, and a comprehensive law describing it, is currently an unsolved problem.
A larger contact patch allows you to use softer rubber, which has a higher coefficient of friction.
The other comments are accurate but the primary factor in race car tire width is heat. A low horsepower car will generally have skinny tires in comparison to a high horsepower car because it doesn't have the ability to overheat the tire.
Race car tires operate in a pretty specific temperature band. Too cold and the grip is limited, just right and the tire is sticky, too hot and the tire material degrades and sloughs off the surface.
It does reduce the load on the tire, making the tire less likely to break down or deform (and lose grip) due to the lateral frictional forces.
To say that friction doesn't depend on area is a bit misleading. The friction does depend on area, but so does pressure, inversely. These effects cancel out. It's similar to the effect of different masses accelerating the same amount in the same gravitational field.
Also, the model used for friction is adequate for most cases, but it's more empirical than theoretically derived. The scientific community's understanding of friction is still pretty limited.
you need to look at dynamic friction vs static friction too.
Part of it is wider tires mean less pressure and thus less wear on the tire.
This lets them use a softer grippier rubber.
So wider tires give more friction, not due to the contact patch directly but because it allows different material.
So more contact patch is more grip is a good stand in for tires.
On top of some other issues like deformation and irregular surfaces.
If grip depends only on the perpendicular force and the coefficient of friction and not on the contact patch area
Because this isn't actually true. It's just a simple model that's taught in high school to help you build intuition, and because it's a decent enough approximation for a lot of purposes. In reality, friction is ridiculously complicated, and it's even more ridiculously complicated when the materials involved can deform - like the rubber that tires are made of.
The simple explanation is that, in reality, for the levels of force relevant to tires, the effective coefficient of friction is higher at lower contact pressure. So bigger tires means a larger contact patch and less pressure (because the same weight is spread over a larger area) and therefore higher grip.
This is also why cars benefit from being lighter. Less mass to accelerate, with a higher coefficient of friction so proportionally more available grip force per kilogram.
This is also why sports cars have the lowest center of gravity possible. Lower cg means less side-to-side load transfer in a turn, which means more grip. If grip were just proportional to the load, then total grip wouldn't change when the weight of the car transfers to the outside in turns, because the total weight doesn't change. I'm reality, having all (most) of the load on the outside tires and none (little) on the inside results in less grip overall.
The width of the tyres does not change the size of the contact patch.
The size of rhe contacg patch is purely based on the tyre pressure, and the weight of the car.
A 2 tonne car with tyres pumped to 32psi will have the same sized contact patch if it has big wide tyres or tall skinny tyres.
(Simplifing for a few things like sidewall thickness, rubber stiffness, etc)
So, first, you have a mistaken assumption that a larger contact area reduces the load on the tire. Friction (as a first approximation) depends on the total load on the contact area, not the load per unit area. So, no matter what, it's the weight of the car times the coefficient of friction. A larger patch spreads it out more, but it's still the same total.
Second, the reason I put " as a first approximation" in there. If you have a bit of dirt or dust on the road that has a lower coefficient of friction than your tire, when you roll over it, you lose that grip force. And the amount you lose is a function of the area that loses contact with the road over the total area of the tire, so the smaller the contact patch, the more you lose.
Third, despite the name, "slick" tires aren't actually slick, or slippery. The outer layer is generally made with a softer rubber that has a higher coefficient of friction than treaded tires. But the softer rubber requires a larger surface area to support the weight of the car.
Even under the simplified assumption that friction is directly proportional only to normal force, there's still a big advantage to wider tires: more consistent grip.
E.g. if there's a patch of oil or piece of gravel or something on the road, creating a low-friction patch, then a wide tire driving over it will still have solid grip on the surrounding road, while a skinny tire may have to rely entirely on whatever traction it can get on the slippery patch.
And if it starts sliding on the slippery patch, then even after crossing it it will now be experiencing only kinetic (sliding) friction with the road - which is generally much lower than the static friction it was relying on moments before.
The idea that friction is independent of contact area is a useful simplification, but not actually true. It assumes that the two sliding surfaces are smooth, and don't deform at all. In reality, traction relies on both sliding friction, and the tread of the tire conforming to the rough surface of the road. The two surfaces act like the teeth on a set of gears. On a paved surface, traction is best achieved by getting as much contact with the road as possible by leaving the surface of the tread smooth. More surface contact equates to more surface irregularities to interlock with, and results in greater traction.
Look up slowmo videos of drag racing rear tyres. The deformation they go through is unreal.
To be a more precise, the wrinkling of drag tires serves to increase the contact patch and is not the "deformation" that is spoken to above. The tire conforms to the texture of the road surface and can mechanically interlock with it. This is a much small scale of deformation.
Aslo side note is that drag strips use a prepped surface that is literly sticky. They basicly apply glue to the surface for additional traction.
That's a different type of deformation. They also use slicks to get the most traction but drag race tires deform that way specifically because the tires themselves act as another set of gears.
At first the tires twist around the hub to help act as a spring to launch the car, then as the car continues to accelerate the centripital forces cause the tires to become thinner, but have a much larger diameter. This acts like another set of gears as the ratio between the axel spinning and the tire spinning changes.
The original comment is just talking about the tire conforming to the road to increase contact area.
Thank you, very clear. I feel stupid for not thinking about that
Amontons law of simple friction does not cover rubber friction very well. Here is Mike On Bikes a very qualified racer and engineer giving a breakdown on why wider tires generate more grip: Tire Grip Explained
With a flat tread you maximize contact with the road and maximize traction. But when water comes between the road and the tire, the water can't compress and has nowhere to go so the tire floats on the water and traction goes to zero.
With street tires that frequently drive on wet roads you have the grooves in the tread, it decreases traction overall but gives water a place to go on wet roads allowing the tire to maintain contact with the road.
With racing tires it's easier to just not race when the track is wet.
The flip side is that the greater the contact surface area, the less compressive force there is per unit area to force the tire to conform - so as you increase the surface area, you really need to also soften the tire in order to get the benefits. Which increases the wear rate, so it's not without costs.