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Originally Posted by Ether
We were discussing a "good" and a "bad" mecanum drive. Who said the wheels on the "good" one were the same part number as those on the "bad" one? And even if they were the same part number, who said they were all assembled and adjusted with the same care and attention to detail?
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Well that's what I was assuming; that both teams had the same wheels and simply assembled their drivetrains differently (e.g. different tolerances, different gearing, etc...). Aside from some play in the mounting of the wheels they shouldn't behave any differently. I think we're debating different situations.
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Differing amounts of friction in the rollers (due to wheel design, manufacturing tolerances, and care in assembly and adjustment) affects the wheel-to-floor traction of the wheels. A detailed explanation of the physics can be found here.
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I was assuming that the wheels didn't have differing amounts of internal friction in the rollers or on the axles. If they did then I would definitely agree with you that that would make a difference.
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We were talking about the rollers, not the wheels. The rollers need to spin in order for the bot to translate at 45 degrees.
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I'm not talking about the motion of the bot, just the motion of the wheels. Mecanum wheels can obviously only spin in two directions. Both of which are applying force at a 45 degree angle to the rollers. In practice it's impossible to make your robot move while keeping the rollers on all 4 mecanum wheels static, which is why I pointed that out. You will always have a slight loss in efficiency.
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Read Note 1 at the bottom of the post to which you were responding.
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I'm making all of my statements assuming this is done in practice, not in theory.
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It sounds like you are saying that the tread pattern on commonly-used FRC wheels results in isotropic friction. Was that your intent?
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Yes. Blue nitrile tread(what I have the most experience with and what is generally accepted to be the best tread material) interacting with carpet will have a relatively isotropic interaction resulting in fairly uniform friction in all directions assuming the same surface area and profile in the direction of motion each time.
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Understood, and appreciated.
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I think this debate is a largely a difference in interpretation of the conditions under which these tests are done. I'm assuming that two teams with either a bad or good mecanum drive assembled their wheels and axles identically. Also I'm comparing mecanum specifically to well built 6 wheel WCD on 1-2" blue nitrile traction wheels.
Hope that clears things up.