pic: Pic 1: "caster wheel" crab drive combo

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One of our mentors came up with crab drive using this low cost and easy to make wheel/motor combo. The gearbox pivots on a standard caster-wheel body.

I would assume that you have two of these acting as powered casters for a bit more traction, and a pair of static wheels in back facing forward? Or will the back wheels rotate like conventional crab drive?