[MrForbes]

Originally Posted by George1902

If this were correct a mecanum drivetrain couldn't produce diagonal motion. It's done by driving the corner wheels, but if what you say is correct, the robot would just drive straight anyway.

I was not talking about the case where the robot is moving sideways as well as forward (diagonal motion). In this case, the rollers do roll, as you say.

In ideal conditions a mecanum wheel will behave exactly like an omni wheel at 45*. Each system will be approximately 70% (sqrt(2)/2) efficient in forward-backward and side-to-side motion.

I disagree with this. I have a mecanum wheel in my hand, and it has loose rollers, they turn very easily. When I hold the wheel so that one roller is contacting the desk, and then move the wheel (without turning it) so that the roller rolls without slipping on the desk, the wheel body moves in a diagonal direction.

Now I'll turn the wheel body, pressing down towards the desk, but keeping the wheel moving only in a forward direction, in line with the direction the wheel is pointing, and perpendicular to the axle. This would be the same as just rolling a normal wheel across the desk. When I do this, the rollers on the wheel do NOT roll relative to the wheel, they act as a (segmented) wheel tread.

Any time that there is good traction and the rollers are rolling relative to the wheel body, then the wheel is moving sideways. The geometry of the wheel makes this a fact.

I guess I need to make a movie.....

Anyways, the way mecanum wheels work, they are NOT like omnis, and they do NOT have the same efficiency going forward and sideways/diagonal.

This is an advantage of mecanum wheels.