Single-caster models have one (or a pair) of steerable wheels that can change direction, and one (or a pair) of driven wheels that provide power. These can be the same (or a pair) of wheels, or they can be two (or two pairs) of different wheels. If they are the same wheel, then it belongs to the front-drive single-caster vehicle; if they are different (pairs of) wheels, then it is an Ackermann chassis vehicle.