Popdown casters are one of the best way to overcome the traction problems experienced when trying to turn doing skid steering. The easiest way to do a caster is to put it near the front of your robot with the center of mass preferably between your back two wheels to aid in turning although this is not neccessary. Casters take too long to turn and line up and continue allowing the robot to spin even after power is no longer applied. The best way to go is delrin, or teflon either in the stick or furniture slide variety. Be careful that your robot isnt going to be popping wheelies as ours is now
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-Aaron