Tire contact patch physics question
 

Ok, I'm in a bit of a pissing match on a motorcycle forum with a guy (who is actually pretty smart and a good guy) about this subject. He swears that the amount of tread on the ground has nothing to do with the tire size, or in the case of motorcycles the profile of the tire. He claims that the only factors that effect how much tread is on the ground is the weight on the tire, and the tire pressure - regardless of how big or small the tires are.

I've used the Corvette as an example. I've asked him why GM would put a 325 tire on the Z06 vs the 285 on the base car. According to him, if the weight and tire pressures are the same then the contact patch area is the same for both tires.

I've contended that the physics theory that he subscribes to is taken out of context when making that argument. What do ya'll think?

Sorry, the other guy is correct, as far as thread contact area is concerned.

If he is right, we should still have bicycle tires on our cars.:D

I think you mean "tread"?

:D

That's exactly the point I've made.

So you are saying that the 325's on the Z06 really are just for show, and that all these wide-ass tires we are seeing on ever faster cars are a waste of money, and that manufacturers are willing to add more unsprung weight just for looks?

Here is an interesting read I admit to only skimming over it, but it would appear to support whichever theory you want it to, but leaning toward a conclusion that wider tires do put more rubber on the road.

http://performancesimulations.com/fa...on-tires-1.htm

325 Is a bigger tire surface to the ground causing better traction and I will tell you, the best advice is to let the guy believe what he believed, Gm did that because the car had more power and they were trying to get the power to the ground so the car can be more enjoyable, the car is also lighter than the base C6, so if weight was the factor, maybe the 285 would have done the job.
Motorcycle is a different factor unless its a straight line bike because a bike mostly uses the side curve of the tires because of the leaning process of riding.

Tire width vs friction vs heat vs stability
 

A wider, lower profile tire gives more stability, as it is lower and wider (duh on me). Try to roll/flip a manhole cover sidways versus a ladder. A lower profile tire also does not generate heat like a taller sidewall tire. Each time the tire rolls around, and touches the ground, it flexes the sidewall, doing this generates heat, the faster you go, the more heat is generated, as the sidewall flexes.

In statics, the normal force, times the friction coefficient, equals the total force to the road a tire can generate. There is nothing in the equation concerning area (contact patch). A wider tire distibutes the load differently than a narrower tire, in physics, pressure equals force(weight) divided by the area. The wider a tire the less pressure per square inch it forces onto the road. Although none of this really helps with tire dynamics, its too generic. I think contact patch size is mostly irrelevant to grip.

Grip has other factors, like total vector sum of all forces in the plane of tire contact with the road. If you roll into a corner, no power, no braking, the vector sum is only the load holding the car into the corner, F=ma. When you brake, or power on, then you add a small vector in the direction of the acceleration. If the new voctor sum is greater than the total grip the tire can withstand, you will slip, with now a lower kinetic friction coefficient. The new sum is the hypotenuese of the triangle, which is the greater of the 3 sides of the vector summation. That is why you never gas, or brake in the corner, but before.

Ole Tad Juechter (Corvette Chief Engineer) says that tires, wheels and shocks account for only 10% of a cars handling abiity anyway. Tire and steering technology is too in depth to ask simply"Why wider tires", see Hans B Pacejka, the magic formula; it is a very complex look into the physics of tires, slip angle, steering angle and tire load.

But.......big fat tires are the shiznizel!!!!!!!!!!!

This is not my area of expertise, it is aircraft structural integrity, but a physics weenie told me long ago,"Tire width has nothing to do with grip, it has to do with heat buildup", I have been curious ever since, so I like to chime into these threads about treads.

Doug Jones
Aerospace Engineer
BS/ME Physics/Mechanical Engineering

You both are right. If the two tires were mounted on the same size rim then I could see the two contact patches being the same. If the wider tire is mounted on a wider rim it will not curl up on the sides causing the contact patch to stay the same.

Another point is why do drag cars run skinnies up front? Less contact patch = less friction= less drag.

That being said wider is better to an extent. A 345 on a 12" rim on my Z06 works out but that same setup on say a prius would suck at life. Too much width can cause too much drag or friction. So what I am trying to say is a stretched out 315 on a 12" rim thats super sticky would be more effective than a 345 thats mildy sticky.

The physics of of a smaller and thinner rubber will not hold to that of a stronger more reliable size. As the saying the cup is only half full not half empty, your counter part will seem to endlessly indulge in a war against facts of this matter. The wider tire will eliminate more negatives due to more positive (345 vs 285) wider in the case does prove to be the better route. Physically provides the better "compound" for or equal to the power to weight ratio. All in all your friend seems to wanna win a never ending course of rebellion to facts, but lacks the knowledge to sustain the truth above all.

The other guy is correct. For a given tire pressure, the contact patch AREA is the same. What changes when you go to a wider tire is the shape of the contact patch. A narrower tire will have a more square contact patch (fore/aft and side to side). Now if you go to a wider tire the fore/aft dimension is reduced but the side to side dimension increases. As you can envision, for lateral handling a contact patch that has a wider dimension will have better lateral adhesion.

regarding the OP: to a good approximation, the other guy is correct. f = sum(p*da) (assumes constant pressure)... assuming most of your force is caused by the air pressure in the tire, it will conform to have an area such that it produces enough force to hold the car up. so if you have the same pressure in the wider and narrower tire, then each will conform such that enough area is facing the ground to hold the car up... which is to say the contact area for each will be the same. in the real world, other things like sidewall stiffness probably also make a non-trivial difference. however... if the wider tires have a stiffer sidewall, this suggests that the narrower tires would actually put more rubber to the road.

it's an interesting concept; i haven't really considered it before.

this equation is only an approximation... for the most part it's a decent approximation, however in addition to what you mentioned, i believe there are also going to be significant contributions due to molecular forces which are also affected by the rubber's deflection under various forces and also due to macroscopic roughness, both of which will be affected by the tire's width, tread shape, and the way its pressure is distributed during various exercises. i think another benefit of wider tires is that they can put down a 'better' patch; among other things, the skinnier tires will probably down a more uneven pressure due to the tire resisting being flattened. also, i think the wider tires are better able to control their temperatures during spinning/sliding situations.

i think the OP has waded into a very, very complex subject :D

It's not so complex.

I used to believe this too (which is why I had the same questions as the OP long ago)... the problem is that the coefficient of friction is not a constant, it actually depends on pressure. The old high school/college physics approach of assuming that surface area and contact pressure do not affect static friction forces is false. The *lower* the contact pressure between two surfaces, the *higher* the coefficient of friction, thus the more "grip" you will get. Also, with racing tires, there is an adhesive, glue-like component to the friction force between the tire and the road, and adhesive strength is essentially directly proportional to contact surface area. Think of the tire's interaction with the track surface as being something like duct tape; even with zero normal force the tire would have to be 'pulled' away from the asphalt.

The pressure in the tire, and the weight (force) on the tire, are the only things that affect the size of the tire patch in contact with the road. If maximum grip is your goal, you want the tire pressure as low as possible, which has the side effect of causing the tire patch to be as large as possible. The wider the tire (and the lighter the car), the lower the pressure you can run in the tire (without deforming the sidewall and contact patch into something that looks like an oval footprint pointing in the direction the car is traveling). Lower pressure = good for static friction. Larger contact patch = also good for static friction, assuming there is an adhesive component going on.

To sum up: a larger tire allows you to run a lower pressure without deforming your tire in a bad way. Running at lower pressures increases the effective coefficient of friction, increasing grip.

Other upsides to larger tires (perhaps even more important than the grip advantage) include the ability to wear longer and handle heat better (more thermal mass).

The big disadvantage of larger tires is the additional weight, which alone can be a killer for lap times. The big disadvantage of using the lower pressure available to you with larger tires, especially super sticky tread compound tires, is increased rolling friction.

Interesting responses (let's leave it at that for now) and I appreciate them. But let's throw another factor/question in the mix. What about the thread pattern of the tire? Why then are pure racing tires slicks if they don't put any more rubber on the road?

I also go back to my earlier point. We all know that larger/heavier tires & wheels are not good in that they add unsprung weight and the weight of the rotating mass hurts performance and economy (this is a known fact). So again, why would manufacturers be putting such wide tires on vehicles?

I get that on some vehicles it truly might be a marketing decision, But on many high performance cars I seriously doubt they are going wider if there is not some real benefit.

Me thinks you are right. I may regret this one :leaving:

KISS;

Same rubber compound.
Solid rubber to eliminate a pressure question.
Same O.A. diameter.
Correct rim width for tire.

Tell me how a 26" bicycle tire has the same size contact patch that a 26" 285/35/19 has.

a 26" bicycle tire at 30 PSIG would deform until the car was supported by the rim, removing pressure from the model. for a bicycle tire to support the car by air pressure, the pressure would need to be very high... and therefore the contact patch would be small.

Dear God,

Please forgive me for the sins I have committed since we last spoke. Please continue good health to mom, dad and the rest of my family. Please continue looking over me as you have in the past.

Oh, and one more thing, please forgive me for starting the thread on the Corvette forum about tire contact physics. You apparently were working with someone else when I started that thread. I forgive you.

OK, so you guys are saying a Corvette with 275 width rear tire at 30psi has the same contact patch as the same Corvette with a 325 width rear tire at 30psi? No friggin' way! :hide:

Way to go.:rofl: