This plastic roller ball is a great third contact point for a small, differential-drive robot. They are sold in packs of three .
This small roller is a perfect third point for a small, differential drive robot. Using a ball castor provides better performance than just having a part of the robot scrape the ground. And, unlike regular castor wheels, a ball castor will not override your drive motors and steer your robot off course. This unit is made entirely of plastic and is therefore very light. To mount the ball castor, just drill a 0.390" hole in your chassis and push in the roller.
We also offer a line of Pololu ball casters and the Tamiya ball castor. This castor is sold in threes.