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Racing Cars and skidding

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  • Racing Cars and skidding

    Dear Readers

    I asked one of my colleagues who is a "bit of a car racing fanatic" why
    performance cars have wider tires, and his reply is appended below. He
    can be contacted at for more info!

    Regards, Brian Davis
    I agree that the frictional force is proportional to load, and that
    the higher the contact pressure, the greater the frictional force.
    However, tires are "a weird animal" that don't always follow the
    laws of physics.

    For rubber tires, the coefficient of friction decreases with contact
    forces. The net effect is that past a certain load, the frictional
    force needed to overcome inertia of a cornering automobile cannot
    keep up with the centrifugal force. This means that the heavier the
    car, the worse it corners on the same size tires. This makes sense
    from experience, but not necessarily from physics. Again, the reason
    behind it is that the coefficient of friction of tires drops with
    increasing load.

    This means that if you want to corner harder, you need more surface
    area to decrease the contact pressure, and hence get wider tires.

    Now, here's a twist. What about antiroll bars? Anti roll bars
    prevent the car flow leaning out of the corner, and generally improve
    handling. But they way they work is that they transfer weight from
    the inside wheel to the outside. This means that a car with antiroll
    bars has a higher proportion of the load on the outside tires. This
    should mean that these tires should corner _worse_ according to the
    principle just described. Well, they, do, but the loss of total
    frinctional force is greately offset by the increase in frictional
    force that can be generated because the tire is now perpendicular to
    the road because the car isn't leaning.

    Here's another twist. Why do cars that race on the ice remove the
    antiroll bars, and often remove the shock absorbers as well? This is
    so that they can corner better! Yes, but this goes against what I
    just said above. It does, but it supports what I said right at the
    beginning. Increasing the load _decreases_ cornering force. It
    turns out that on ice, rule #1 is more important than rule #2.

    This is why using rubber tires as an example of friction is such a
    bad idea....

    ------ Forwarded message ends here ------