Mecanum drive

[EricH]

Originally Posted by Ether

Half?? Wow, you must have been watching some bots with pretty beat-up wheels.

Quality mecanum wheels will have nearly equal forward and strafe speed, but they are very expensive. If they get hit and damaged so that the rollers don't spin freely under load, you'll start to see degradation in sideways speed.

Or if the rollers too tight, or are rubbing hard against the sideplates. Either of those will cause it. This is another of those cases where theory says one thing, and reality says to theory, "You need to come over to MY place tonight!" Just about every mecanum robot I've seen in FIRST is slower sideways than front-back. 357, 330's testbed, and many others. It's faster than 1/2 speed, but it's nowhere near full speed. Maybe about 3/4 speed.

That's just what I've seen at competitions; the root causes could be any of the ones we've mentioned, or something we haven't thought of.

[Ether]

Originally Posted by EricH

Or if the rollers too tight, or are rubbing hard against the sideplates.

I agree with the above, but will add this: a high-quality mecanum wheel's rollers will not be "too tight" or "rub hard against the sideplates". The rollers will have properly-designed bearings that can withstand the high axial thrust loads and still spin freely with no binding or rubbing. That's what makes them expensive.

This is another of those cases where theory says one thing, and reality says to theory, "You need to come over to MY place tonight!"

There's no reason that theory cannot include a consideration of the effects of friction. In many complex systems, a proper theoretical analysis of frictional effects is absolutely mandatory for developing a proper design.

[EricH]

Originally Posted by Ether

I agree with the above, but will add this: a high-quality mecanum wheel's rollers will not be "too tight" or "rub hard against the sideplates". The rollers will have properly-designed bearings that can withstand the high axial thrust loads and still spin freely with no binding or rubbing. That's what makes them expensive.

Which is probably 50% of the reason they aren't found on FRC robots.

The other 50% is that very few teams, if any, know they exist.