Quote:
Originally Posted by 114ManualLabor
...I've been shown the math on why surface area shouldn't influence traction, but here's why I disagree:
I built a kitbot with 4 andymark kit wheels, all powered from two BB gearboxes. It pushed a decent amount of stuff in our shop. Then I pulled the wheels off, and doubled up the andymarks. Now I had two wheels per shaft, effectively doubling the surface area on the ground. This bot pushed more, and could shove me across the ground when I was fully resisting. The other setup, with single wheels, could not shove me.
I have yet to take physics (that's next year for me), but I'm still basically familiar with the issue... Anyone care to hep me out here?
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Trust your experiment. Math does not control the world--it only attempts to describe it.
The friction equations in first year physics are basically only engineering approximations, and do not hold for sufficiently roughened surfaces, nor for surfaces of materials with unusual, non-linear properties--like rubber.
If you want to use the first year physics equations, then you can think of it like this--rubber has a non-linear "coefficient of friction." For most rubber material in most traction situations, the apparent (in other words, the experimental) coefficient of friction is higher when the rubber is more lightly loaded (less weight per area of the contact surface) than when it is more heavily loaded.
For a robot built to a maximum weight limit, more rubber surface on the ground means less pounds per square inch on the rubber surface, giving a higher apparent coefficient of friction and more grip for a given robot weight. So treads, or doubled wheels, have more grip than lower-surface-area alternatives using the same material.
Carlo Bertocchini, who built the combat robot Biohazard (and was on a winning FRC team if I remember right) answered this question several years ago (near the bottom of the linked page)
http://www.robotbooks.com/robot-materials.htm