Physics Def - tire grip

[Dominic]

Just a little thought...

Those of you who have taken a mild physics class should remember that tire/traction is governed by the coefficienct of static friction (Fs) - because when they begin to be governed by the lower coefficient of kinetic friction (Fk) they SLIDE :D .

Say I have a line of rubber 1"x12" on the road and I want to apply maximum force to this rubber without sliding (like taking off at a stoplight).
Imagine there are 100lbs of force/weight on this piece of rubber and (as looked up in a table) the coefficienct of rubber to asphalt is 0.6.

Now, 100lbs of downward force on this (massless for simplicity) rubber strip, regardless of its size/width will allow me to but 0.6*100lbs of horizontal force (or 60lbs) before it slides.

Based upon this concept, a wider tire doesn't help with off-the-line traction, but more weight ON the tire does... Positraction of course helps on a car because you are adding a "weighted" tire to the car which can take more launching force...

Now, going into a turn, wider tires do grip better - or seem to, but I need to do a little work and find out why.

Anyone want to discuss this?

[Dominic]

Darn, I need to do some research on rolling friction

...to be continued

[Backfire]

I actually had a similar discussion with a Prof. of mine at school. We were designing a device to climb stairs using a rubber tracks(think tank tracks) and were trying to figure out the width needed. In an ideal situation, the area does not matter. As you stated, the friction force is a product of the normal force and the coefficient of friction. The area of contact does not effect the forces.

The reason the automotive situations don't really follow this is due to all the other variables that come into play. The surfaces we drive on are not perfectly clean with no debris or variation in surfaces. The larger tires help to minimize the impact of the variations. If you drive over a patch of sand with a 215 tire, I believe the effective coef. of friction will change. The same patch of sand with a 315 tire will still cause a reduction in COF but not as significant. This is really simplified but seems to make sense. There is more to it, but I don't remember a lot of that conversation, I had a project to finish :D

I am still fuzzy on why tread patterns make a difference. I can see how they would change grip in bad weather situations, but why in dry conditions. Now most performance tires have a stickier rubber to increase the COF, but why is pattern signifigant?

As far as rolling friction, I don't think that is what translates to traction while rolling. At the point of contact, if you are not spinning, it is essentially a problem of static friction. If you are doing a burnout, sliding in a turn, etc, then you are concerned with kinetic friction. Rolling friction relates to the energy losses due to the rolling of an object over a surface, I think For example, if you rolled a tire on a perfectly flat surface, static friction would prevent the contact area from sliding and would cause the rolling on the surface after you push the tire. But even on a flat surface the tire will stop rolling. That is due to rolling resistance.

[ March 02, 2002: Message edited by: Backfire ]</p>

[DominusEtDeusNatus]

275 will have more contact patch then 315. So I heard. Pattern matter cause it determines the % of the thread in contact with the road. A 275 rain tire will have less rubber touching the road then 245 nitto.

[Backfire]

So, the only impact of the tread pattern is the size of the contact patch? It seems alot of emphasise is placed on patterns in marketing, is that all it is, marketing hype? I am looking at dry conditions, as poor weather would toss some other variables into the mix and complicate things.

I hadn't heard that information concerning the 275 vs 315. Is this for a specific tire/tread pattern? Or just those sizes in general?

[Dominic]

The significant factor in dry pavement traction is % of contact patch touching the road.

Most good rain tires only have a 60-70%
Whereas most dry performance tires are above 90%

Simply put if you increase the # of grooves and width of grooves, you get better wet performance because the tread can *sink* through the water to touch the road better.

Decrease the grooves and end up with more rubber on the road and you end up with better dry performance.

This is all very basic of course, there are other factors such as groove angle and depth which factor in for speed variations and road conditions... Thanks for the input, I thinky ou're right about the rolling resistance thing, perhaps I was correct at first in saying so long that the tire isnt sliding its merely a factor of static friction [img]smile.gif[/img] .

[HAZ-Matt]

The actual reason is that wider tires are also lower profile, and since they are lower profile they can be made of softer compound, which changes the coefficient of friction. Also there is more to traction than mere static friction. There is also shearing effects from the rough road on the rubber... etc I don't know all about the "advanced model" of traction, but the compound is really what matters the most.

[DominusEtDeusNatus]

<blockquote>quote:</font><hr>Originally posted by HAZ-Matt:
The actual reason is that wider tires are also lower profile, and since they are lower profile they can be made of softer compound, which changes the coefficient of friction. Also there is more to traction than mere static friction. There is also shearing effects from the rough road on the rubber... etc I don't know all about the "advanced model" of traction, but the compound is really what matters the most.<hr></blockquote>

Lower profiles are always made of harder compound cause there is less rubber to damp the impact so it has to be stiffer.

[V6Bob]

The fundamental thing here is that the physics in the original post only applies to ideal _smooth_ surfaces. Rubber tire against rough road - the tire presses into the irregularities in the road. It's a little like gears meshing. That's why size matters :D .

[Patrick Javert]

Width matters because when you apply shearing force(step on the gas) It takes more force to spin the tire to the point of breaking loose. Tire compound does increase/decrease friction as well as pavement type and quality. Since it is late, I will end with this: Wider+Taller=Better.

[HAZ-Matt]

<blockquote>quote:</font><hr>Originally posted by Dom:
Lower profiles are always made of harder compound cause there is less rubber to damp the impact so it has to be stiffer.<hr></blockquote>

Are you sure about that one? Since the side walls are (relatively) shorter, they will not flex as much. Since they are shorter, with the same force you get less torque on the sidewall. So you can use a softer compound than you could with taller side walls. That would be for sideways forces... for damping forces parallel to the sidewall, the pressure inside the tire should determine the ability to damp the force... cause the pressure of the air wants to limit how much the tire is compressed. I believe this is why you have the psi on your door that you are supposed to use depending on the weight. The pressure of the air determines the damping ability. Plus, lower profiles are known to sacrifice ride quality for grip... so the damping will not be as good. You will get overdamping cuase there is more force per unit height of the sidewall.

I'm still not claiming to be the ultimate authority. However, I am relating the info that I have read, which seems to make sense.

[ March 03, 2002: Message edited by: HAZ-Matt ]</p>

[AZ3.8Camaro]

Dudes physics is theoretical crap. If you really want to know something, go try it out. (Read some Petrarch). Slide the tip of your finger across a table. Good. Now try your whole hand. There you go. As for Tread Designs, its because roads are not smooth like physicists like to use. The tread helps grab bumps etc. If the roads were super smooth, we'd all have drag slicks and more contact areas. Treads are like shoe bottoms, the more nooks and crannys the more grip.

[AZ3.8Camaro]

Also, lower profile tires are harder compounds, at least on the sidewall. Go to the tire shop and try to mount up at 225/40ZR17. Cant do it. A 275/35/15. Easy. The sidewalls are way stiffer.