Contact Area and its Relation to Friction?

[114Klutz]

Originally Posted by 114ManualLabor

This is an interesting thread.

Short disclaimer: I have yet to take physics, so anything I write is a result of observation.

Anywhoo. I think I understand what people here are talking about, yet I'm still confused. If everyone says that the contact area doesn't matter, then can someone explain how this makes sense: I built two robots. Both were with the kitbot chassis. Both were driven off of Banebots transmissions. Both had two driven wheels, and two casters. One bot had two andymark kit wheels per axle for driving, and the other had only one per axle. I added extra weight on each in order to make the weight exactly equal. I wired both motors to a single battery and switch. Then I put the two drive bases head to head, set so they would drive directly into each other, on an area of FIRST carpet. When turned on, the base with 2 wheels per axle could overcome and outpush the other base every time. We did this 12 times, each time changing to a new, fresh battery.

Anyone care to help me out here? I guess that physics and math and stuff say this shouldn't work, but it did. So I'm confused.

I can explain. The physics supports this.

Lets take bot A and bot B, both bots have X amount of weight. The coefficient of friction between the wheels and the ground is Us

Bot A has 6 wheels, 4 of which are powered, so, 2/3rdsX * Us is the max tractional force provided.

Bot B has 4 wheels, 2 of which are powered, so 1/2X*Us is the max tractional force.

This is, assuming, of course, that the distribution of weight and the wheel spacing is perfectly even.