Quote:
Originally Posted by kramarczyk
Should have posted this earlier...
The center axle in a 6WD system carries a disproportionate amount of weight on it. In a uniformly distributed frame the center axle carries about 62.5% of the weight vs. 18.75% for the outer axles. Since traction is proportional to weight, increasing the friction coefficient at the center axles has the biggest bang for the buck in terms of pushing force while minimizing the reduction in turning due to high skidding losses.
Assuming a uniformly distributed 120lb bot with 6 skyways (0.7 CoF) then the pushing force is 84 lbs. Swap the center wheels to a ridgetop material (1.3 CoF I think) and the pushing force jumps to 129 lbs (120*.625*1.3+120*.1875*.7*2), an improvement of over 50%. In reality most bots are not uniformly distributed, but instead biased to the center which makes this change even more significant.
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This makes sense, I think. I had neglected to consider that the weight would be distributed unevenly on 'typical' 6WD arrangements since they don't use all six wheels at a time and I presume that's where the 62.5% figure comes from.
Perhaps I'm conceptualizing something incorrectly or misunderstanding your math, but wouldn't it seem that the most torque the outer wheels could transmit before slipping in the scenario above is 120*.1875*.7 (for the pair), or ~15.75 lbs? Once the output torque of your gearbox exceeds that value, those wheels no longer transmit torque to the carpet. In such a case, the torque transmitted to the carpet then becomes only that of the center wheel and its higher coefficient of friction -- 120*.625*1.3 or ~97.5 lbs. This is still an improvement over a .7 coefficient of friction at all points of contact, but it's not as significant as 50%.