This simple circular laser-cut plastic robot chassis makes a great base for small mobile robots.
It has a diameter of 5 inches and has additional holes and slots to mount sensors.
1 X Pololu Chassis
1 x Tamiya twin-motor gearbox
1 x Tamiya ball caster (contains two casters)
1 x Tamiya truck tires (contains four tires)
Add a Second Chassis and Gearbox and build a 2nd Robot!
Pololu Chassis Manual