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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Hmm. I've not studied these sort of things (I want to though), but that seems to make sense, except I would imagine that traction is much more than the weight pushing down on top of the wheel, although that would have a noticeable effect. You also have to keep in mind the material used as tread (Say, a plastic/perspex "tread" would be much less efficient than normal traction tread) and the driving surface matters too (such as carpet, hardwood floor, concrete, cobblestone, sand, etc.).
How then, would you calculate the traction of a robot whose weight is mostly on their right side, and the rest is evenly distributed front to back? It's a 4-wheel bot, though. (I'm asking because 675 is using 4 CIM's/4 56mm Gearboxes @ 12:1 and we can push another bot AND the rack with normal traction tread on carpet, without losing traction)
Quote:
Originally Posted by Vikesrock
Also wouldn't changing the center wheels to omnis allow other robots to translate you sideways in addition to being able to spin you around the center axis? Then they wouldn't even have to hit you on the corner, any part of either side of your robot perpendicular to the drive direction would do.
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Assuming that the chassis is not a rocker chassis and also that the other 4 wheels on the bot are normal traction (which would be quite unintelligent, omnis in the center but tractions on the corners... wtf?), then
maybe. If the chassis was a rocker, then yes, and probably with ease.
The optimal omniwheel/traction wheel combination is omnis at the corners, tractions in the center. Rocker chassis or not, it offers the turning radius of omnis, with the traction of, well, tractions.
I say if you're going to try to make an interesting drive system, go with mecanum. I think that a bot that moves in all directions is pretty cool.
Quote:
Originally Posted by squirrel
just because they're slipping, does not mean that they are not transmitting force. The coefficient of sliding friction is usally less than that of static friction, but is still much higher than zero.
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Hmm, good point.
Anywho, if you break traction in a pushing fight and you're sliding around, it doesn't matter the torque you're transmitting to the carpet, it just matters that it's below the number you want.
