On Mon, Aug 6, 2012 at 5:10 PM, John Holladay <docholladay0820@g...> wrote:
>  I can buy
> the idea of same weight with different surface contact area = more weight
> per square inch.  Especially in the thought of the idea of static vs.
> kinetic friction.... in terms of "Kinetic" friction,
> I would argue that there is more that comes into play and I will offer some
> practical examples below that seem to be contradictory to the idea that the
> surface contact area is not relevant.

> One area that you don't want to reduce friction
> is in the brakes (One other part of the car where greater friction is
> desired would be the tires). The obvious rule of thumb is, in order to
> improve braking performance, you have to increase friction.  One of the
> ways to improve braking performance is by increasing the surface contact
> area of whatever braking system is used....More surface area, without making
any
> real change in the overall weight of the vehicle still means more friction
> for better braking.
Have to stop you here John.

The increased surface area generating friction is in relationship to
the the caliper and the rotor, not the car to the road. The
relationship of better brakes to more friction between the car and the
road relies on tires. The capacity to handle the additional friction
must be within the design constraints of the tires. If you put
high-performance brakes on the car and give it wooden tires, it'll
just lock up and the total friction between the car and the road will
not be properly affected. The potential to handle the friction was in
the tires all along, but the ability of the brakes to apply that force
to the tire wasn't.

In your example you'd need to compare the amount of force upon the
calipers & rotors and the surface area. A caliper of X area and Y
force would create the same friction of a caliper of X/2 area and 2Y
force. Of course, other factors such as heat (mentioned below) make a
larger surface area desirable, but in terms of potential friction the
formula is accurate.

Or you can compare the force of the car to the road. Weight is the
wrong term when you get to discussing things such as cars. Forces,
usually measured in N/m or foot-pounds, are really relevant. As an
extreme example, Formula 1 cars weigh less than an average coupe but
brake better. Why? Because they have much larger & stickier tires and
because they generate more aerodynamic down-force than the car
actually weighs. If you have more mass acting laterally against the
car (fore, aft, port or starboard) than you have down-force and tire
capacity, it will slide. The actual mass of the car is one part of the
puzzle and not always the most important.

> ...braking systems also perform better (more
> friction) when they are at a cooler temperature.  They degrade (less
> friction) as the temperature rises.  The friction, in this case, is
> definitely influenced by the temperature of the contacting surfaces.
Correct, but they have an operating range. Too cold and they also
don't perform correctly. Luckily, using brakes warms them fairly
quickly and usually isn't noticed much. When too hot, the pad's
material begins breaking its bonds and the surface material is
removed, acting temporarily as a surface lubricant because it's loose.

Of course, I could be completely wrong, as someone stole my degree in physics ;)
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