The trailer axle(s) are driven by a drive shaft exiting the rear of the transmission with splines and universal joints to accommodate the movement between them. All axles are portals with hub reduction and locking differentials. Initially the axle differentials were permanently locked but recently some models can run with open but lockable differentials for better high-speed capabilities. The usual twin back axles are combined in a separate frame that is can pivot in relation to the "trailer" frame, keeping all wheels on the ground, but until recently always unsprung (called a "bogie"). Likewise the less-common trucks with single rear axles usually are unsprung in the rear, while the front axle have suspension to give the operator a better ride. The reinforced cab is sometimes also sprung, as is done in modern cab-over-engine trucks, as is the drivers seat.