[ice.berg]

Originally Posted by sgreco

Mecanum are "bad" because they are inefficient. The theory of a mecanum wheel is that it behave in the same manner as having omni wheels at 45 degree angles (with respect to the front of the bot) on each corner of the robot.

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Sort of like that. When all the wheels spin "forward" they are generating force vectors in both the x and y direction (if you're looking down at the page, y up, x left and right). So you have a force vector from each wheel in the forward direction of Ftotal * sin45, all the x forces are lost because they cancel out. Sin45 (or cos45) is about .707. So you're only going to get about 70.7% efficiency with respect to what you could get if all the wheels were straight.

Also in order to drive "sideways" mecanum wheels have to be geared pretty low in order to have enough torque to actually move sideways. This means they will end up being reasonably slow in the forward direction in an FRC application.

Because mecanum wheels have rollers they don't have a lot of friction resistance to being pushed sideways, so they're easy to be defended. They also have trouble playing effective defense because they can be pushed out of the way sideways. In head on contact, they are limited by the 70.7% relative (and theoretical) efficiency referenced above.

Mecanum wheels also all have to be powered separately. This is not a huge deal, but it means 4 gear boxes. Then if you want to fix the low gear ratio problem and you put on 4 shifters, your robot gets really heavy really quickly.

Also, Mecanum wheels are heavy, and they come no smaller than 6 inches (unless you make you own). This means they have a high moment of inertia. A high moment of inertia in your wheels will restrict your acceleration.



(Note: I don't have any physical proof for the "70.7%", so maybe somebody has run a test and they can give you better information. Also, note that the 70.7% number is not the overall efficiency, it's a theoretical number relative to what you would theoretically get from normal wheels, all pointing straight, that have the coefficient of friction, the same weight, size and same moment of inertia).

OOOFTA!! That was a long analysis but it included all the important disadvantages of mechanums.
Well our team went with mech wheels this year for the third year in a row. But every year we change something up a little bit. Check out the live stream tonight to see what we did!