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Andrew -
What do you mean when you say you design around rolling contact force? Is there some formula to calculate this, and what exactly is it? (Rolling "friction" force, as mentioned above?)
Also, why don't you just take into account both things, or is that a bit more difficult?
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As Lloyd said (one post down), rolling contact force is created by static friction. It will be less than mu*N. Once it reaches mu*N, you slip (the wheels start to burn the carpet) and you have a much lower force exerted by the wheels, since kinetic friction is less than static friction.
The formula which we use (and which many other people also use) for calculating the desired force of contact is:
F = (motor torque at desired operating point [usually max power])*(GR1*.9)*(GR2*.9)*...*(GRn*.9)/(wheel radius)
GR1...GRn are the gear ratios of our various stages in between the motor and the load. The .9 is there to account for efficiency. It could be lower! But, short of doing a bunch of experiments on the gear stages themselves, this should get us into a ball park for pushing force.
Why don't we account for both things at one?
I was taught that you should separate your requirements and figure out how to satisfy each independently.
We decide what speed/pushing power we want (this is a trade off unless you add in a transmission or more than two drive motors). We also decide what level of complexity we can handle in the design/fabrication based on available man power and work done in the off-season.
This year (and hopefully in the future), we had access to unlimited skyway wheels. Although our design choices were quantized around the available diameters (6", 8", 10", I think), we figured out how to "double up" the tires. So, we could get more friction by adding another tire to the back or less friction by removing tires, by cutting grooves circumferentially, or by plastering short lived teflon tape over part of the tire. We have been toying with the idea of making replaceable tires which fit over our wheels, perhaps using the Small Parts rubber and a mold... We'll have to see how next year's rules changes impact this problem.
Anyway, per Lloyd's comments, we adjust friction between too much slip and motor stall. I think there is also another plateau on the "too much friction" side where you may see bits in your motor/gear box getting broken (although we have fortunately not found this plateau). That probably could only occur if you add weight to your drive system and get back driven and have fresh fuses so that they don't pop until you see 100 amps.
I would be interested in hearing comments on when is too much friction a bad thing. I remember that Woodie commented that you want your tire to slip at maximum push. However, we have designed to blow our circuit breakers as the weakest component in our system. If someone has an idea, I'd like to hear it before we learn the hard way.
Andrew
Team 356