[Tyre]Why does bigger Force reduces coeff. of friction?

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Caito
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Joined: 16 Jun 2009, 05:30
Location: Switzerland

[Tyre]Why does bigger Force reduces coeff. of friction?

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I can successfully understand several parameters of tires, but one which I don't is the coefficient of friction vs force curve.

Which is as follows:
Image


The interpretation is "simple" coefficient of friction decreases as load increases.

I can't find any acceptable reason that explains why such thing happens.

Any explanation, or recommended book is much appreciated.


Bye bye!

QSQ.


EDIT: The curve is not real, it's from a game. Rather than the values, it is useful for the shape.
Come back 747, we miss you!!

Mystery Steve
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Joined: 25 Sep 2009, 07:04
Location: Cincinnati, OH, USA
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Re: [Tyre]Why does bigger Force reduces coeff. of friction?

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I haven't specifically studied tribology of rubber on pavement/concrete. Although, I do know of tests performed by Montgomery (I think there is a free PDF somewhere on the internet, didn't find it quickly though...) that show the coefficient of friction for steel-on-steel sliding is a function of the product of pressure and sliding velocity, Pv. The coefficient of friction for steel decays exponentially with respect to Pv.

If you zoom in on a side view of the surface of even a "smooth" material, it actually has many jagged points called asperities. As materials slide against each other, these asperities interact with each other. The actual frictional force can result from a variety of mechanisms such as: adhesion, abrasion, and hysteresis. As the load increases these asperities "mesh" together and the resulting energy dissipation due to the previously mentioned mechanisms increases, which causes the frictional force to increase. At some point the asperities are "fully meshed" (for lack of a better description) so the frictional force more or less peaks.

I think the confusion results from the definition of coefficient of friction. It is the ratio of the frictional force to the normal load. While it decreases with increasing load, this doesn't mean that the frictional force is decreasing. It continues to increase, but the rate at which it increases with respect to normal load decreases.

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mep
29
Joined: 11 Oct 2003, 15:48
Location: Germany

Re: [Tyre]Why does bigger Force reduces coeff. of friction?

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I try to explain it with a simple comparison:

Imagine something like a chewing-gum sticking to the road.
It is very lightweight but you already need a quite huge force to remove it from there (adhesion is dominating, friction coeff. is bigger than 1).

So now imagine you have a complete tire standing on the road. The adhesion force is still the same as on the chewing-gum but your tire and the car mounted to it is now much bigger. Adhesion force is not dominating anymore so your friction coefficient drops under 1.

The adhesion force is something you get "for free" you don’t need to put any weight on your rubber. So a light car will perform better because the relative amount of adhesion force is bigger even when the force itself is the same.
One thing you can do to increase adhesion force is to make contact area bigger.
If you zoom in on a side view of the surface of even a "smooth" material, it actually has many jagged points called asperities. As materials slide against each other, these asperities interact with each other. The actual frictional force can result from a variety of mechanisms such as: adhesion, abrasion, and hysteresis. As the load increases these asperities "mesh" together and the resulting energy dissipation due to the previously mentioned mechanisms increases, which causes the frictional force to increase. At some point the asperities are "fully meshed" (for lack of a better description) so the frictional force more or less peaks.
That’s an explanation of a physical point of view which we learn at school. The chemical force is not taken into account that’s why it can’t explain why a wider tire is better. It’s only true for rigid bodies but it is ok when you have a high load.

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Tim.Wright
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Joined: 13 Feb 2009, 06:29

Re: [Tyre]Why does bigger Force reduces coeff. of friction?

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Thats a damn good explanation.

Mep are you a tyre engineer?

Tim
Not the engineer at Force India

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mep
29
Joined: 11 Oct 2003, 15:48
Location: Germany

Re: [Tyre]Why does bigger Force reduces coeff. of friction?

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Thanks for compliment but I am not.
When you know me a job I would take it. :wink:

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