Physics, Friction, Tires?

T-bird4vr

Registered User
Since surface area does not increase or decrease friction, what is the purpose of really wide tires? Dissipate heat quicker or something?
I have always wondered this.
Any explainations?
Julian
 
Julian...Remeber 99% of everything you learn is Physics holds true only in a vacum :O) Hey I;m sure someone will have time to post on this..Im goin to bed haha
 
You are right, the surface area will not increase friction force.

The problem with the tires is that you are creating more force than the rubber can withstand. So, the pavement starts to rip the surface of the tire apart and you get a nice cloud of smoke. With a wider tire the force per unit area is lower therefore you can use more of the friction force to move the car.

Softer compounds place more tire surface in contact with the road by allowing the rubber to conform to the uneven road surface. They will also be easier to rip apart, but the extra grip surface compensates for this until the force again excedes the rubber strength.

Aaron
 
Eh, excuse me?

If you look at it as friction per square inch, it makes a little more sense. The more square inches you have, the more power required to overcome that friction.

Use a simple analogy. Place a full glass of water on a peice of paper on a countertop. Try and slide the paper and glass sideways with 1 finger on the paper. Probably can't or it requires a lot of finger pressure. Now try the same thing with 2 fingers. I think you'll find it moves a lot easier even though you haven't gotten any stronger. Then if you put some honey on your finger (make it sticky) it also becomes easier.

That's also part of the reason lowering the tire pressure gets you slightly more traction - more surface to the track.

That's sure not a very "technical" explaination, but it illustrates lateral motion and friction.

Lee
 
Last edited:
You are excused:D

http://www.colchsfc.ac.uk/maths/friction.htm

You will see that surface area is not used in the calculation for friction force.

My poorly worded explination holds, the tire compound fails causing the loss of static friction - kinetic friction (much lower) changes everything once the tire begins to spin.

Aaron
 
Re: Eh, excuse me?

rivlee said:
If you look at it as friction per square inch, it makes a little more sense.
Lee

That is my whole point. Friction per square inch means absolutely nothing. Look it up. The experiment I saw a couple years ago was a 2X8 board pulled along the 2"X10' side and then the 8"X10' side. The professor used a spring scale to pull and it was the same force each tme. It is tough to buy, but it's true.

Aaron, good response. That makes sense, or I buy it at least.
Thanks,
Julian

Damon, only in a vaccum, pssttt. smart as$:D :D
 
Dang it Aaron you posted when I was. Thank you very much for the answer though. I have wondered this for 2 years now.
-JM
 
Gee, if the theory holds true....

you could run 1" tires with the same results as 10" tires. When the empirical evidence does not support the theory, I kind'a think the theory is missing something.

Your example with the board is more about overcoming mass than about the friction. The coefficient of friction plays a part, certainly, but the mass in that example is a greater factor.

You've got mass/inertia, friction/gravity, vector/angle of attack and power/torque.

It's not just a function of friction.

Lee
 
Ok, here's one for you. Instead of thinking width, why does a taller tire have more traction than a shorter tire? All other factors being the same.
 
Contact patch!

Aaron,
A taller tire ends up having more rubber in contact with the street. If a tires of similar stiffness are compared they will yeild similar flex at the point of contact with the road. Think of two very extreme examples. A fourteen(height) X one(width) foot tire will have far more rubber on the road than a one X one tire. If the tires maintained a perfect circular shape this would not be true. The road would theoretically be the tangent line. However, as we have all seen, they give a little at the point of contact. Once you see that the is more rubber on the road, subscribe to either one of the arguements above to find that more rubber=more stick.

Anthony
 
I am familiar with taller tire having more rubber and bigger patch but wanted to throw a wrench into the works for them. Physics in the conventional sense doesn't always make sense.
 
That one's simple.

For the same reason that underdrive pulleys use less power - torque vs resistance. It's like a lever and moving the pivot point in or out. You're either increasing or decreasing the power applied to the point of friction/resistance.

Lee

Oops, Anthony got in before me. I think you'll find that the small difference in patch area is less of a factor than the torque multiplier.
 
Last edited:
Well, truthfully I didn't know that. Well at least didn't think of it that way in regards to torque vs resistance. I always thought of it as amount of rubber. Taller tire has more of a sidewall to flex and can have a bigger patch. Now which do you think has a bigger influence? Say you have an 18" rim with same tire, meaning manufacturer and type, at 28" tall and one on a 15" rim. I would think the 15" would hook up a lot better.
 
I respectdully disagree.

Lee,
Your statement certainly holds true but lateral loads are not about a pivot point. The shear force applied to the tire is unaffected by the tires diameter. However most people highly overestimated the amount of rubber actually in contact with the street. For some monster like the Silverbird this may not be true, but for us simple folk the combined contact patch from all four corners could fit in our hands. That being true, any increase in patch size is significant.
Anthony

Edit/Aaron
Interesting thought but with too much give at the sidewall, the center of the tire will begin to pull away from the surface. I think there is an advantage in a straight line scenario to a point but in corners I would tend to disagree.
 
Last edited:
Guys, you are readking stuff from this post that I didn't mean. Here is the question, that I think Aaron already answered.
This is what we know to be true:
Wider tires, taller tires (sidewalls), softer, less air pressure, get more traction. Weither it be from friction, less force or torque applied, whatever.

Physics - Surface area has flat out nothing to do with the coeficient of friction. If you could successfully prove this wrong you would be one rich fellow and in many textbooks.
BUT
In the real world there is some explaination for why the above factors increase traction. (not sure if I can use friction here)

The material of the tire will change the coefficient. And the cars dynamics has a lot to do with it. (May account for height changes, air pressure).

I read something on the internet, maybe from MMFF, about a guy with a TT 5.0 mustang, that kind of proved/ explained the tire width thing, with launching G's and such.

Breath. Did I miss anything?

It was obviously a good question.
-Julian
 
Re: Gee, if the theory holds true....

rivlee said:
Your example with the board is more about overcoming mass than about the friction. The coefficient of friction plays a part, certainly, but the mass in that example is a greater factor.

It's not just a function of friction. (car's dymanics)

Lee

It is the same board, thus the same mass.

I know it doesn't hold true for a car. My question is why?
 
O.K. 1 last point about the circumference.

Sure the lever is not an exact comparison for the tire height issue, but the effect is the same. But it also works the same as diff gearing. Increasing the tire circumference has the same "effect" as going to lower gearing, which we all know has a great impact on launching.

There are one helava lot of factors involved. :p

Lee

BTW: In the "theory" you're only focusing on friction. Whatever happened to "An object in motion trends to remain in motion"? If a block of rubber of the same dimensions was used and a sensor was used to meassure the pressure to initially start the motion was graphed, I think the results would be a little more informative. The experiment is always effected by the experimentor.
 
Last edited:
Ok, my head is spinning now. huh?!?!?! Something I did think about though. I think what Lee has been saying has a lot more to do with why Top Fuel has limited the tire height of the cars. Imagine how fast they could get going if they allowed 40" tires. I would love to see someone do that just for show. Or even have a peel away outer layer on a tire for better traction so during the intial launch the outer layer is so sticky it peels off and sticks to ground and provides a perfect contact surface for the first rotation of the tire underneath. Those things make so much power that they can spin the tires at over 200 mph. Imagine how fast they can go if they had traction to handle the power.
 
Agreed.
Torque=r cross Force. The smaller the overall r=radius of tire/rim with the same amount of torque applied the more force there will be at the road.

edit (wrong formula order for cross product)
 
Last edited:
Back
Top