[Geek 2.0]

What are some possible points of failure when a mecanum drivetrain doesn't drive straight sideways (i.e. curves when strafing)?

Being a programmer, I'm prone to ruling out code as a problem, but feel free to question it.

Here are some possible issues:
1. Frame is not rigid enough, and wheels not keeping contact
2. Wheels not spinning the same speed (implement PID?)
3. LOTS of slipping (too much acceleration?)

Anything else you can think of? Or do any of these stand out as a common issue?