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Unread 10-08-2002, 12:03
Andrew Andrew is offline
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Join Date: May 2002
Location: Little Rock, AR
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I've got to stick my $0.02 into this very interesting discussion!

As has already been mentioned, F = mu * N is a convenient mathematical fiction to capture some of the relevant behavior seen when two materials rub against each other.

It should really say, Fmax = mu * N, though. The "real force" of contact with the ground is that force which is required to keep the point of contact of the wheel moving at the same speed as the ground. This was referred to earlier as rolling friction.

When the required force exceeds Fmax, then the wheel slips.

Whenever you see an equation like y = a * x in physics, you know that someone is trying to capture the dominant behavior under most circumstances. In this case, Coulomb said, "The maximum force that two materials can exert depends on the normal force. Let's try a linear fit."

When we design our drive system, we don't worry about coefficient of friction. We design around the rolling contact force. If we need more traction, we can always put more material in contact with the ground (or cut treads into wheels...). If we need less traction, we can take some off (or put a more slippery material on the wheel). We treat the issues of desired drive system force and traction as separate design issues.

Andrew
Team 356