A locking differential, differential lock, diff lock or locker is a variation on the standard automotive differential. A locking differential may provide increased traction compared to a standard, or "open" differential by restricting each of the two wheels on an axle to the same rotational speed without regard to available traction or differences in resistance seen at each wheel.
A locking differential is designed to overcome the chief limitation of a standard open differential by essentially "locking" both wheels on an axle together as if on a common shaft. This forces both wheels to turn in unison, regardless of the traction (or lack thereof) available to either wheel individually.
When the differential is unlocked (open differential), it allows each wheel to rotate at different speeds (such as when negotiating a turn), thus avoiding tire scuffing. An open (or unlocked) differential always provides the same torque (rotational force) to each of the two wheels, on that axle. So although the wheels can rotate at different speeds, they apply the same rotational force, even if one is entirely stationary, and the other spinning. (Equal torque, unequal rotational speed).
By contrast, a locked differential forces both left and right wheels on the same axle to rotate at the same speed under nearly all circumstances, without regard to tractional differences seen at either wheel. Therefore, each wheel can apply as much rotational force as the traction under it will allow, and the torques on each side-shaft will be unequal. (Unequal torque, equal rotational speeds). Exceptions apply to automatic lockers, discussed below.
A locked differential can provide a significant traction advantage over an open differential, but only when the traction under each wheel differs significantly.
All the above comments apply to central differentials as well as to those in each axle: full-time four-wheel-drive (often called "All Wheel Drive") vehicles have three differentials, one in each axle, and a central one between the front and rear axles (transfer case).
Automatic lockers lock and unlock automatically with no direct input from the driver. Some automatic locking differential designs ensure that engine power is always transmitted to both wheels, regardless of traction conditions, and will "unlock" only when one wheel is required to spin faster than the other during cornering. These would be more correctly termed "automatic unlocking" differentials, because their at-rest position is locked. They will never allow either wheel to spin slower than the differential carrier or axle as a whole, but will permit a wheel to be over-driven faster than the carrier speed. The most common example of this type would be the famous "Detroit Locker," made by Eaton Corporation, also known as the "Detroit No-Spin," which replaces the entire differential carrier assembly. Others, sometimes referred to as "lunchbox lockers," employ the stock differential carrier and replace only the internal spider gears and shafts with interlocking plates. Both types of automatic lockers will allow for a degree of differential wheel speed while turning corners in conditions of equal traction, but will otherwise lock both axle shafts together when traction conditions demand it.
Some other automatic lockers operate as an open differential until wheel slip is encountered and then they lock up. This style generally uses an internal governor to monitor vehicle speed and wheel slip. An example of this is the Eaton Automatic Locking Differential (ALD), or Eaton Automatic Differential Lock (ADL), developed by the Eaton Corporation and introduced in 1973 for GM's Rounded-Line C/K Series pickups and utilities. The Eaton ADL is sometimes incorrectly called the "Gov-Lok", despite neither GM nor Eaton ever calling it by that name. "Gov-Lok" is rather an unofficial name of unknown origin that gained popularity over the years. Both Eaton and GM do not know where the name came from, and Eaton has made several unsuccessful attempts in the past to debunk the Gov-Lok name. An updated version of the old Eaton ADL design is still available from Eaton, now known as the Eaton mLocker mechanical differential lock.
Some other automatic lockers operate as an open differential until high torque is applied and then they lock up. This style generally uses internal gears systems with very high friction. An example of this is the ZF "sliding pins and cams" available for use in early Volkswagens.
Selectable lockers allow the driver to lock and unlock the differential at will from the driver's seat. This can be accomplished many ways.
A spool is a device that connects the two axles directly to the ring gear. There is no differentiation side to side, so a vehicle equipped with a spool will bark tires in turns and may become unmanageable in wet or snowy weather. Spools are usually reserved for competition vehicles not driven on the street.
Mini-spool uses the stock carrier and replaces only the internal components of the differential, similar in installation to the lunchbox locker. A full spool replaces the entire carrier assembly with a single machined piece. A full spool is perhaps the strongest means of locking an axle, but has no ability to differentiate wheel speeds whatsoever, putting high stress on all affected driveline components.
The internal spider gears of an open differential can also be welded together to create a locked axle; this method is not recommended as the welding process seriously compromises the metallurgical composition of the welded components, and can lead to failure of the unit under stress. If it is desirable to have a spooled axle, the better option is to install either a mini-spool or a full spool.
Because they do not operate as smoothly as standard differentials, automatic locking differentials are often responsible for increased tire wear. Some older automatic locking differentials are known for making a clicking or banging noise when locking and unlocking as the vehicle negotiates turns. This is annoying to many drivers. Also, automatic locking differentials will affect the ability of a vehicle to steer, particularly if a locker is located in the front axle. Aside from tire scuffing while turning any degree on high friction (low slip) surfaces, locked axles provoke understeer and, if used on the front axle, will increase steering forces required to turn the vehicle. Furthermore, automatically locking differentials can cause a loss of control on ice where an open differential would allow one wheel to spin and the other to hold, while not transferring power. An example of this would be a vehicle parked sideways on a slippery grade. When both wheels spin, the vehicle will break traction and slide down the grade.
Limited-slip differentials are considered a compromise between a standard differential and a locking differential because they operate more smoothly, and they do direct some extra torque to the wheel with the most traction compared to a standard differential, but they are not capable of 100% lockup.
Traction control systems are also used in many modern vehicles either in addition or as a replacement of locking differentials. One example is that offered by Volkswagen under the name of electronic differential lock (EDL). This EDL is not in fact a differential lock, but operates at each wheel. Sensors monitor wheel speeds, and if one is rotating more than 100 RPM more than the other (i.e. slipping) the EDL system momentarily brakes it. This transfers more power to the other wheel, but still employs the open differential, which is the same as on cars without the EDL option. Electronic traction control systems may be integrated with anti-lock braking systems, which have a similar action on braking and use some similar components. Such systems are used for example on the most recent Nissan Pathfinder, Land Rover Defender, Land Rover Freelander, the McLaren P1 and the McLaren 650s.