[sciguy125]

Let's think about this for a second:

Let's make a sphere. Mount a motor, gearbox, and whatever else inside of this sphere. Drive a shaft throw the center of the sphere, perpendicular to the motor shaft. Mount the assembly to the robot using this new shaft. Remove any excess portion of the sphere that will not contact the ground during normal operation - the sphere has become a wheel. Now, you need some way of pivoting the assembly about the mounting shaft.

This configuration will make it so that the height of the chassis never changes. However, you can change this by moving the mounting shaft closer or farther away from the robot (relative to the assembly).