A Limited Slip Differential (LSD) is a modified or derived type of differential gear arrangement that allows for some difference in rotational velocity of the output shafts, but does not allow the difference in speed to increase beyond a preset amount. In a car or automobile, such limited slip differentials are sometimes used in place of a standard differential, where they convey certain dynamic advantages, at the expense of greater complexity.

The main advantage of an LSD is found by considering the case of a standard differential where one wheel has no contact with the ground at all. In such a case, the contacting wheel will remain stationary, and the non-contacting wheel will rotate at twice its intended velocity - the torque transmitted will be zero and the vehicle will remain stationary. In normal everyday use on typical roads, such a situation can usually be assumed to have a vanishingly small probability, and so a normal differential suffices. For more demanding use however, such as driving off-road, or for high performance vehicles , such a state of affairs is undesirable, and the LSD can be employed to deal with it. By limiting the velocity difference between a pair of driven wheels, useful torque can be transmitted as long as there is some friction available on at least one of the wheels.

Types

Two main types of LSD have been generally used - mechanical (geared or clutch-based) and fluid based (viscous). The latter is gaining ground especially in modern all-wheel drive vehicles, and generally require less maintenance than the mechanical type.

Mechanical

In the mechanical clutch type, differential velocity is detected by some means, such as a centrifugal weighted rotor, and this applies a force to a clutch mechanism which links the two shaft together to a varying extent - the greater the differential velocity, the more force is applied. This forms a negative feedback loop which limits the slip to a preset degree. In some designs, the clutch is self-actuating. Often, small multi-plate clutches are used. One disadvantage of the mechanical type is that the limiting action tends to occur quite rapidly rather than gradually, and this in itself can create unsettling dynamic effects for the vehicle as a whole.

Another type of mechanical LSD is the geared torque-sensing type. This arrangement uses planetary gears to "sense" torque on one shaft. The most famous version is the Torsen differential invented by Vernon Gleasman in 1958, then sold to Gleason Corporation, who started marketing it in 1982. But there are many other types available as well. Geared LSDs are less prone to wear than the clutch type, but their torque distribution characteristics can be less than ideal.

Viscous

The viscous type is generally simpler, and relies on the properties of a dilatant fluid - that is, one whose viscosity varies as the inverse proportion of an applied force. Silicone-based oils are often used. Here, a chamber of fluid rotates with the normal motion of the output shafts, but a differential motion causes paddles or vanes to move through the fluid. The greater the speed of the vanes, the more resistance the fluid will put up to oppose this motion. In contrast to the mechanical type, the limiting action is much softer and more proportional to the slip, so for the average driver is generally much easier to cope with.

Viscous LSDs are less efficient than mechanical types, that is, they "lose" some power. However, they are less prone to breakdown as long as the fluid is changed regularly.