Imagine you have a main robot (wheels + cortex + claw) and another smaller robot (only wheels). You connect them by cords, and the little robot starts on the main one (to be within the 18 by 18 by 18 limit). Then the small one drives off during the game, still connected by wires. The wires could be raised to avoid entanglement. Is this leagal?
Provided no other rules are violated in the process, this type of design is legal.