pic: 1918 NC Gears

[Oblarg]

Originally Posted by Wayne TenBrink

(reply to this and other posts)

We have never used VersaWheels individually - only as duals. 2014 was the first time we used them in any configuration, so we can't really compare the performance and durability of "single" vs. "stacked" wheels. This robot has played about 100 matches. We did a complete wheel change after about 50 matches. The wheels on the mid axles wear differently than the ones on the corner axles. Mid wheels (which have .050" drop on this chassis) last longer and wear is uniform. The tips of the "W" treads become rounded off evenly along both edges of the tread tips. Corner wheels wear faster, and the tread tips take on a "sawtooth" appearance due to the rotation/scuffing that happens during turning. We rotated corner wheel sets left-for-right in between wheel changes to put the "sharp" edge on the "working" side of the tread.

We did some traction testing during 2014 build and measured about 200 lb of pushing force with new wheels ("sharp" treads) and a robot test weight of about 125 lb. We didn't measure current draw. I agree that tread on carpet does not completely follow Newtonian rules of friction, but I don't know what the actual physics is (are?). I don't think the wear is directly proportional to wheel load either - the material interaction that improves traction also increases wear. In my judgment, the VersaWheels bite the carpet very well when new, but probably revert to "Newtonian" traction as the treads get rounded off.

VersaWheels are $5 each, so a full set of 20 is $100. I wouldn't want to change them every day, but once or twice during the season is bearable. Right now, the treads are getting quite dull. They ran part of MSC, all of Championship, and the MARC. Only the new wheels are sharp.

There has been a lot of discussion on CD about the anti-T-Bone benefits of convex-sided robots (hex, octagon, round, etc.). I think bulged sides help, but I don't think it is mainly because of the reduced friction/contact area between the robots (which seems to be the consensus among many people). A robot with "true radius" sides that is up against a wall or another bot can spin without moving (translating) its geometric center sideways. A straight side bot has to move its center sideways (and perhaps another robot, too) in order for the corners to clear the barrier as it rotates (a real challenge for long axis bots). "Venus Octobot" is about half way between radius and straight. We put more wheels in the bulge space, which increased the moment arm that powers the turn. We did some simulated T-Bone testing last week (with students pushing the 2011 bot into it) and our driver noticed an improvement in his ability to spin out of it. This Saturday, we get to test it at WMRI.

Thanks for the info! Have you ever done any pushing force tests with the same robot without the stacked wheels for comparison?