If you're looking for a base for your next robot, look no further than the Rover 5. This 4 motor monstrosity takes any terrain by storm.... just don't take it out in a storm... please!
The Rover 5 uses 4 independent motors, each with an optical quadrature encoder and gearbox. The entire gearbox assembly can be rotated at 5 degree increments for different clearance configurations. You can even replace the tank treads with traditional wheels.
This isn't a wimpy or flimsy robot base. Weighing in at over 2.5 pounds without batteries, this base is heavy duty and can drive over quite a bit. Check the manual below for dimensional drawings.
Quadrature Encoder Example