[=Martin=Taylor=]

What about wheels like this?

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When you want to turn, spin the front and back wheels (wheels on short sides). When you want to drive forward spin side wheels.

Another thing we've considered is "toe."

If the wheels are angled slightly at each corner, spinning all of them in the same direction will allow you to spin in place.

It will also take advantage of the superior transverse friction when driving forward. And will provide some braking to prevent the robot from coasting when making a hard stop.