Mini Plastic Ball Caster Wheel Pair (775)

Brand : Sumozade
Price : $1.20
​This small roller is a perfect third point for a small, differential drive robot. 
Using a ball caster provides better performance than just having a part of the robot scrape the ground. And, unlike regular caster wheels, a ball caster 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 caster, just drill a 12mm hole in your chassis and push in the roller.