[fritzdejongh]

I believe Ether is right. I think you can see the "extra" force as coming from the sideways forces holding the wheels together (these forces must be there, or the wheels would fly apart). This force also gets reduced to the component along the roller, which then has a forward component. The bottom line is, if nothing is slipping sideways, the full torque is being converted to a forward force. It's kind of reminiscent of tacking in a sailboat.

If you're going at 45 degrees, two wheels slip completely so you only get power from half the wheels, so a swerve drive is probably better for a general angle.