Chief Delphi - Another Drivetrain Idea

For our purposes here, lets consider the maximum speed on a standard mecanum (full speed forward, not strafing) to be 100% (so accounting for the losses in that already).

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All you're doing here is combining two imperfect drives orthogonally, so instead of max speed in one direction and 60% in another, you're probably going to have 80% in both, as the fast drive will be "dragging" the slow drive.

This is somewhat true. Though you must remember, the 40% lost to strafing before (due to the weight of the robot on the mecanum wheels and the resulting friction (assuming a constant speed)) would be cut in half since the other (about) half would be supported by the remaining 4 wheels. So really you are combining a ~80% with ~100% to get something more like ~90%. As well, as with all mecanum, simply making the robot lighter will make it strafe faster (in this case faster in all directions).

As for the size of the wheels, it should work with 4" or 6" (or 8" or 10", but those are very large for FRC these days). 4" are lighter, cheaper, take up less space, and give a longer wheel base, but I'm more of a fan of 6" mecanum wheels. My limited personal tests show that more rollers mean better strafing abilities (your results may vary), and all the rollers on Vex Pro mecanum wheels are the same size. Also we have tons of them laying around here, so got to make use of them!