The chassis is the frame or foundation on which the dental vehicle body is placed on. The only way to increase the weight capacity on a chassis is to add a TAG (extra axle.) Generally, commercial vehicles are equipped with air brakes, while lighter weight vehicles are typically equipped with hydraulic brakes. Vehicles are rated by gross vehicle weight (GVW). The GVW is the total weight capacity of the vehicle including the vehicle itself and the cargo. Vehicles less than 26,000 GVW don't require any special drivers license (see the staffing section for more information on who can drive the vehicles.) The drive train includes the engine, transmission, and differential--all the things that power the vehicle and make the wheels turn.