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Friction Misunderstanding?

  1. May 12, 2009 #1
    I saw this post on a forum and thought it wasn't right, it's about why the force of frction on rotors is independent of the surface area of the calipers in contact with the rotors. I thought this was wrong because friction is a non conservative force, but am seeking further validation

    the post:
    Not really. Friction between the pad and the rotor is what stops you. And friction force is only dependent on the normal force and the coefficient of friction. The normal force is the force that is perpendicular to the direction of travel, which, in this case, is the force the pads apply on the rotor. So, since friction only depends on that force and the coefficient of friction (which depends on the material of the pad and rotor), the surface area of the contact patch has no effect on the braking performance. So, you may ask, why don't we use the smallest pads possible? Well, the more force we try and apply to a small area, the more likely that small area has of failing under the stress. So if we get more of a contact patch, that force is distributed more throughout the pad and rotor. But, braking performance and contact patch has no relation.
  2. jcsd
  3. May 12, 2009 #2


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    Staff: Mentor

    Surface area is part of the coefficient of friction.
  4. May 13, 2009 #3
    Ok. Let's reduce the size of the pads by a factor of 10. They might be a little less expensive to replace. I would then have them replaced every time I have the oil changed, and maybe have the rotors turned also. No thank you.
  5. May 13, 2009 #4
    This claim is simple enough to test.

    Get two objects of different dimensions and the same weight... say, a box covered with sandpaper.

    Let it slide down an inclined plane.

    Measure the critical angle and the times required for various angles.

    The hypothesis: more surface area means greater critical angle and longer times for sliding down.

    The null hypothesis: there is no difference, or the inverse of the hypothesis.

    Air resistance is going to be negligible if you use fairly rough surfaces.
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