How will the ball roll?

This is proving very difficult for us! We have tried attaching a motor to an axel with wheels at either end. The idea was that if this was placed in a ball with some weight, it would cause the it to roll (like a hamster in a ball!) We thought that if the wheels were more round like beads, if the ball was to bang into anything the motor would keep it rolling as the wheels would just turn causing the ball to change direction.