open differential
A type of differential gear used in automobile construction, allowing for output shafts to have different angular velocity, while equalizing torque applied to each output shaft.
open differential: the standard axle that lets wheels turn at different speeds
An open differential is a gearbox assembly mounted between the two drive wheels of an axle, containing a set of bevel gears that distribute engine torque equally to both wheels while permitting them to rotate at unequal speeds. In a car turning a corner, the outer wheel travels a longer path and must spin faster than the inner wheel; the open differential accommodates this kinematic requirement while keeping torque balanced.
The mechanism consists of a carrier, two side gears (one on each output shaft), and typically two or four pinion gears meshing between them. When driving straight, all gears lock together as a solid unit and both wheels receive equal torque. When the vehicle corners, resistance on the slower inside wheel allows the pinions to rotate around their own axes, transferring some of the rotational speed to the faster outside wheel without reducing torque delivery to either side.
Limitations in traction
The open differential's fundamental weakness emerges when traction is unequal. If one wheel loses grip, sits on ice, or encounters mud, that wheel becomes the path of least resistance. Since the differential enforces equal torque split, it cannot send more power to the gripping wheel; instead, it directs nearly all available torque to the slipping wheel, causing it to spin freely while the other wheel remains motionless. This is why a car can become immobilized with one wheel on pavement and one on gravel.
For this reason, open differentials are standard only on road vehicles where traction is generally predictable. Off-road trucks, agricultural machinery, and high-performance vehicles often use limited-slip or locking differentials, which restrict speed differentiation or mechanically lock the two sides together when slip is detected, restoring torque to the gripping wheel.
The term 'open' distinguishes this design from its alternatives: it places no mechanical restriction on the relative speeds of the output shafts under normal operation. The geometry and materials are straightforward, bevel gears in cast iron or ductile iron carriers, with rolling element bearings supporting the shafts. Service life is long provided the differential oil is changed at manufacturer intervals and bearings are not run dry.