Tamiya’s steel-ball caster is a good way to give a “third wheel” to a small robot powered by a differential drive. The kit comes with enough parts to make two casters, either 25 mm or 35 mm in height.
The Tamiya 70144 ball caster from Tamiya’s educational construction series is a small steel ball that rests on metal rollers inside a plastic housing. The caster can be assembled in two different ways, so that it has a total height of either 25 mm or 35 mm. The kit contains two complete ball casters and various types of mounting screws as well as spacers for small height adjustments.