This would be another thing we'd talk about at the programming conference...
I know that swerve steering (i think it's the same as crab... not entirely sure) is where all 4 wheels of your robot rotate on an axis. This is a normal robot:
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This would be a robot with crab steering moving at 90 degrees to it's front:
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Your wheels can rotate anywhere, so you can move laterally along the playing field without turning (your front stays pointing the front).
It's hard to control because you have to link the joystick with the amount of turn each of your wheels undergoes... I'm still clueless as to how teams can make this mechanically with all four wheels turning.
-Gui