Limited slip differential
A limited slip differential (LSD) is a type of differential
A differential is a device, usually, but not necessarily, employing gears, capable of transmitting torque and rotation through three shafts, almost always used in one of two ways: in one way, it receives one input and provides two outputs—this is found in most automobiles—and in the other way, it...
gear arrangement that allows for some difference in angular velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
of the output shafts, but imposes a mechanical bound on the disparity. In an automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
, 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.
Early historyIn 1932, Ferdinand Porsche
Ferdinand Porsche was an Austrian automotive engineer and honorary Doctor of Engineering. He is best known for creating the first hybrid vehicle , the Volkswagen Beetle, and the Mercedes-Benz SS/SSK, as well as the first of many Porsche automobiles...
designed a Grand Prix racing car for the Auto Union
Auto Union was an amalgamation of four German automobile manufacturers, founded in 1932 and established in 1936 in Chemnitz, Saxony, during the Great Depression. The company has evolved into present day Audi, as a subsidiary of Volkswagen Group....
company. The high power of the design caused one of the rear wheels to experience excessive wheel spin at any speed up to 100 mph (160.9 km/h). In 1935, Porsche commissioned the engineering firm ZF to design a limited slip differential that would perform better. The ZF "sliding pins and cams" became available, and one example was the Type B-70 for early VWs.
BenefitsThe main advantage of a limited slip differential is shown by considering the case of a standard (or "open") 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 freely—the torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
transmitted will be equal at both wheels, but will not exceed the threshold of torque needed to move the vehicle, and thus the vehicle will remain stationary. In everyday use on typical roads, such a situation is very unlikely, and so a normal differential suffices. For more demanding use, such as driving in mud, 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 angular velocity difference between a pair of driven wheels, useful torque can be transmitted as long as there is some traction available on at least one of the wheels.
TypesTwo main types of LSD are commonly used on passenger cars; torque sensitive (geared or clutch-based or cone-based as shown in figure at top of page) and speed sensitive (viscous/pump and clutch pack). The latter is gaining popularity especially in modern all-wheel drive vehicles, and generally requires less maintenance than the mechanical type.
Torque-sensitiveThe use of the word mechanical implies that the limited slip differential is engaged by interaction between two (or more) mechanical parts. This category includes clutch and cone and helical gear limited slip differentials. For road racing, many prefer a limited slip differential, because it does not lock the two output shafts to spin at the same rate, but rather biases torque to the wheel with more grip by up to 80%.
General LSDs respond to driveshaft torque, so that the more driveshaft input torque present, the harder the clutches or cones or gears are pressed together, and thus the more closely the drive wheels are coupled to each other. Some include spring-loading to provide some small torque so that with no / little input torque (trailing throttle/gearbox in neutral/main clutch depressed) the drive wheels are minimally coupled. The amount of preload (hence static coupling) on the clutches or cones are affected by the general condition (wear) and by how tightly they are loaded.
Broadly speaking, there are three input torque states: load, no load, and over run. During load conditions, as previously stated, the coupling is proportional to the input torque. With no load, the coupling is reduced to the static coupling. The behaviour on over run (particularly sudden throttle release) determines whether the LSD is 1 way, 1.5 way, or 2 way.
If there is no additional coupling on over run, the LSD is 1 way. This is a safer LSD: as soon as the driver lifts the throttle, the LSD unlocks and behaves somewhat like a conventional open differential. This is also the best for FWD cars, as it allows the car to turn in on throttle release, instead of ploughing forward.
If the LSD increases coupling in the same way regardless of whether the input torque is forward or reverse, it is a 2 way differential. Some drifters
Drifting refers to a driving technique and to a motorsport where the driver intentionally over steers, causing loss of traction in the rear wheels through turns, while maintaining vehicle control and a high exit speed...
prefer this type as the LSD behaves the same regardless of their erratic throttle input, and lets them keep the wheels spinning all the way through a corner. An inexperienced driver can easily spin the car when using a 2 way LSD if they lift the throttle suddenly, expecting the car to settle like a conventional open differential.
If the LSD behaves somewhere in between these two extremes, it is a 1.5 way differential, which is a compromise between sportiness and safety.
Clutch Type LSDThe clutch type has a stack of thin clutch discs, half of which are coupled to one of the drive shafts, the other half of which are coupled to the spider gear carrier. The clutch stacks may be present on both drive shafts, or on only one. If on only one, the remaining drive shaft is linked to the clutched drive shaft through the spider gears. If the clutched drive shaft cannot move relative to the spider carrier, then the other drive shaft also cannot move, thus they are locked.
The spider gears mount on the pinion cross shaft which rests in angled cutouts forming cammed ramps. The cammed ramps are not necessarily symmetrical. If the ramps are symmetrical, the LSD is 2 way. If they are saw toothed (i.e. one side of the ramp is vertical), the LSD is 1 way. If both sides are sloped, but are asymmetric, the LSD is 1.5 way.
As the input torque of the driveshaft tries to turn the differential center, internal pressure rings (adjoining the clutch stack) are forced sideways by the pinion cross shaft trying to climb the ramp, which compresses the clutch stack. The more the clutch stack is compressed, the more coupled the wheels are. The mating of the vertical ramp (80°-85° in practice to avoid chipping) surfaces in a 1 way LSD on over run produces no cam effect and no corresponding clutch stack compression.
The break-in period of clutch LSDs can be very specific. Manufacturers give detailed instructions on how to break the differential in. If these are not followed, the LSD may be permanently harmed, in that it may engage and disengage erratically due to irregularities on and damage to the clutch surfaces. Essentially, the LSD must be worked hard to remove manufacturing imperfections, then drained of the metal-laden oil.
Servicing consists of changing the oil after hard sessions to remove metal particles, and eventually replacement of the clutches or the centre. In any case, the oil should be changed on a regular basis depending upon use. The fluid should be changed after about 70000 miles or 100000 kilometers (as opposed to the open differential, where the oil could be left unchanged for several hundred thousand kilometres).
Geared, torque-sensitive mechanical limited slip differentials utilize helical gears or worm gears to "sense" torque on one shaft. The most famous versions are:
- TorsenTorsenTorsen is a type of differential used in automobiles. It was invented by American Vernon Gleasman and manufactured by the Gleason Corporation. Torsen is a contraction of Torque-Sensing...
differential based upon the Dual-Drive Differential invented by Vernon Gleasman in 1958, then later sold to Gleason Corporation, who started marketing it in 1982;
- QuaifeQuaifeR.T. Quaife Engineering, Ltd. is a British manufacturer of automotive drivetrain products. It designs and manufactures replacement gearboxes and gear sets, some of which are for four-wheel drive systems....
differential, sold under the name Automatic Torque Biasing Differential (ATB), covered by European Patent No. 130806A2.
- Eaton CorporationEaton CorporationEaton Corporation is a global diversified power management company with 2010 sales of $13.7 billion. The company is a leading provider of electrical components and systems for power quality, distribution and control; hydraulics components, systems and services for industrial and mobile equipment;...
differential, sold under the name Eaton Detroit Truetrac.
Geared LSDs use worm gears rather than clutches of the clutch type and the cones of the cone type, and work by "multiplying" the torque from the slowest moving wheel to the fastest, rather than actively controlling slip.
In the case of slip, the wheel in contact can receive up to X times the torque applied to the wheel which is slipping, where X is the torque multiplication value for the differential. In this sense, torque sensitive differentials are not strictly limited slip - once an output shaft becomes free (e.g., one driven wheel lifts off the ground; or a summer tire
A tire or tyre is a ring-shaped covering that fits around a wheel rim to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground...
comes over ice while another is on dry tarmac
Tarmac is a type of road surface. Tarmac refers to a material patented by Edgar Purnell Hooley in 1901...
when the car goes uphill), no torque is transmitted to the second shaft and the torque-sensitive differential behaves like an open differential. A common trick in these situations, such as turning up a steep hill in a low sports car, is to apply slight pressure to the brakes (handbrake in the case of RWDs), allowing some torque to be applied to the raised wheel which will then be multiplied to the wheel in contact.
Some torque sensitive differentials feature a bias plate, which allows some torque to be transmitted to the wheel in contact even when the opposite wheel has no traction.
Geared LSDs are more dependent on the torque and not on the speed difference between the output shafts (however the speed difference plays a part). Such differentials may not be adequate on extremely slippery surfaces such as ice (or thin air, when a drive wheel loses ground contact altogether).
Geared LSDs may be used:
- to reduce torque steer in front-wheel driveFront-wheel driveFront-wheel drive is a form of engine/transmission layout used in motor vehicles, where the engine drives the front wheels only. Most modern front-wheel drive vehicles feature a transverse engine, rather than the conventional longitudinal engine arrangement generally found in rear-wheel drive and...
- as a center differential in four-wheel driveFour-wheel driveFour-wheel drive, 4WD, or 4×4 is a four-wheeled vehicle with a drivetrain that allows all four wheels to receive torque from the engine simultaneously...
(e.g., on the Audi Quattro);
- in rear-wheel drive vehicles, to maximize traction and make oversteer easier to manage (as in driftingDrifting (motorsport)Drifting refers to a driving technique and to a motorsport where the driver intentionally over steers, causing loss of traction in the rear wheels through turns, while maintaining vehicle control and a high exit speed...
). Although, for extreme drifting, a geared LSD is less effective compared to a clutch type LSD. Torque sensitive differentials are rarely used in Rally motorsports, however.
The viscous type is generally simpler because it relies on hydrodynamic friction from fluids with high viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
Silicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....
-based oils are often used. Here, a cylindrical chamber of fluid filled with a stack of perforated discs rotates with the normal motion of the output shafts. The inside surface of the chamber is coupled to one of the driveshafts, and the outside coupled to the differential carrier. Half of the discs are connected to the inner, the other half to the outer, alternating inner/outer in the stack. Differential motion forces the interleaved discs to move through the fluid against each other. In some viscous couplings when speed is maintained the fluid will accumulate heat due to friction. This heat will cause the fluid to expand, and expand the coupler causing the discs to be pulled together resulting in a non-viscous plate to plate friction and a dramatic drop in speed difference. This is known as the hump phenomenon and it allows the side of the coupler to gently lock. In contrast to the mechanical type, the limiting action is much softer and more proportional to the slip, and so is easier to cope with for the average driver. New Process Gear used a viscous coupling of the Ferguson
Ferguson Research Ltd.
Harry Ferguson Research Limited was a British company founded by Harry Ferguson who was mostly known as "the father of the modern farm tractor"...
style in several of their transfer case
A transfer case is a part of a four-wheel-drive system found in four-wheel-drive and all-wheel-drive vehicles. The transfer case is connected to the transmission and also to the front and rear axles by means of drive shafts...
s including those used in the AMC Eagle
The AMC Eagle is a compact-sized four-wheel drive passenger vehicle that was produced by American Motors Corporation . The AMC Eagle line of vehicles inaugurated a new product category of "sport-utility" or crossover SUV....
Viscous LSDs are less efficient than mechanical types, that is, they "lose" some power. In particular, any sustained load which overheats the silicone results in sudden permanent loss of the differential effect. They do have the virtue of failing gracefully, reverting to semi-open differential behaviour. Typically a visco-differential that has covered 60000 miles (96,560.4 km) or more will be functioning largely as an open differential; this is a known weakness of the original Mazda MX-5
The MX-5, also known as Miata in North America and Eunos Roadster in Japan, is a lightweight two-seater roadster, of front-engine, rear-wheel drive layout, built by Mazda in Hiroshima, Japan. The model was introduced in 1989 at the Chicago Auto Show...
(a.k.a. Miata) sports car. The silicone oil is factory sealed in a separate chamber from the gear oil surrounding the rest of the differential. This is not serviceable and when the differential's behaviour deteriorates, the VLSD centre is replaced.
Gerotor pumpThis works by hydraulically compressing a clutch pack. The gerotor pump uses the housing to drive the outer side of the pump and one axle shaft to drive the other. When there is differential wheel rotation, the pump pressurizes its working fluid into the clutch pack area. This provides a clamp load for frictional resistance to transfer torque to the higher traction wheel. The pump based systems have a lower and upper limits on applied pressure, and internal damping to avoid hysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
The newest gerotor pump based system has computer regulated output for more versatility and no oscillation.
ElectronicElectronic limited slip differential systems use speed sensors, anti-lock braking system
Anti-lock braking system
An anti-lock braking system is a safety system that allows the wheels on a motor vehicle to continue interacting tractively with the road surface as directed by driver steering inputs while braking, preventing the wheels from locking up and therefore avoiding skidding.An ABS generally offers...
(ABS) sensors, accelerometers, and microcomputers to electronically monitor wheel slip and vehicle motion. In some systems the computer limits slip by varying the degree of locking in a mechanical LSD; such as Porsche's PSD system, which uses electro-hydraulic control of a mechanical LSD.
The Mitsubishi Active Yaw Control
Active yaw control
Active Yaw Control is an automobile feature that uses an active differential to transfer torque to the wheels that have the best grip on the road. Unlike traditional mechanical limited slip differentials, an AYC is electronically controlled....
(AYC) electronically controlled rear differential uses a conventional open differential with an added planetary gear set to rotate two hollow shafts around the left hand drive shaft, one running at +15% speed, one at -15%. These can be progressively locked up to the left hand drive shaft via a hydraulic clutch pack under CPU control, increasing or decreasing the torque on that wheel in relation to the other. This allows a certain amount of rear wheel "steering" to provide stability control and perform the function of an LSD.
Many vehicles use a traction control system
Traction control system
A traction control system , also known as anti-slip regulation , is typically a secondary function of the anti-lock braking system on production motor vehicles, designed to prevent loss of traction of driven road wheels...
to simulate a limited slip differential. With this type of system, if either of the wheels on an axle is rotating unusually faster than the other, the computer will determine how much it is slipping and apply braking to it, slowing the spinning wheel down and thereby increasing torque to the wheel with more traction.
SpoolA spool rear end allows no differential rotation. A spool consists of a pinion and ring gear only, the center is solid, making the axles act as one piece. A mini-spool is similar, replacing the usual differential side gears and spider gears with a solid piece, retaining the factory differential carrier. This arrangement is popular in "drifting
Drifting refers to a driving technique and to a motorsport where the driver intentionally over steers, causing loss of traction in the rear wheels through turns, while maintaining vehicle control and a high exit speed...
", where drivers aim for flamboyance rather than speed. Those that use the car for "drifting purposes" often weld the gears in a standard open differential together to produce a homemade spool.
It is also preferred by drag racing enthusiasts for two main reasons. A straighter launch is achieved, because power is split evenly between the wheels. High horsepower vehicles with a spool generally aim straight even with wheel spin. They can also be made stronger, for a given size and weight. Because of the solid center design and lack of side gears, cross pin and spider gears, a spool can take much more abuse. This is especially important for drag racers that shock their drive trains with hard launches and sticky tires. However, turning is more difficult with the wheels locked together (the outside wheel has a greater distance to travel than the inside wheel and thus has to rotate faster to compensate).
A locking differential, 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...
locks both wheels under normal conditions. If a wheel is externally forced to rotate faster than the differential centre (i.e., the outer wheel in a corner) the mechanism unlocks that wheel and allows it to turn freely (but only so long as it rotates faster than the centre). Thus in contrast to other LSDs,
the locker has the unusual characteristic of only applying drive torque through the inner wheel in corners when decelerating or under neutral throttle.
Drive shaft input torque causes the pinion cross shaft to lock the centre more firmly, resisting the unlocking action. As the two actions of the mechanism are contradictory, the car will unpredictably alternate between one-wheel and two-wheel drive under power in corners, causing rapidly changing handling characteristics. As the vehicle goes down a straight line it is locked, and as the vehicle enters a curve it is still locked. As the vehicle goes farther into the curve, the gear unlocks causing it to jerk. The net effect is extreme understeer (as all of the engine torque is being applied through the inside wheel), followed by power-on oversteer when the rear wheels break traction. It can also be very noisy and is often used in off-road 4WD applications. The traditional American racing differential is a Detroit Locker.
Normally functioning as open differential, a selectable locker can be locked by the driver. Compressed air, mechanical cable, electric actuator or hydraulic fluid activates the locking mechanism.
Factory namesIn the 1950s and 1960s many manufacturers began to apply brand names to their LSD units. Packard pioneered the LSD under the brand name "Twin Traction" in 1956, becoming one of the first manufacturers.
Other factory names for LSDs include:
- Alfa RomeoAlfa RomeoAlfa Romeo Automobiles S.p.A. is an Italian manufacturer of cars. Founded as A.L.F.A. on June 24, 1910, in Milan, the company has been involved in car racing since 1911, and has a reputation for building expensive sports cars...
- American Motors: Twin-Grip
- BuickBuickBuick is a premium brand of General Motors . Buick models are sold in the United States, Canada, Mexico, China, Taiwan, and Israel, with China being its largest market. Buick holds the distinction as the oldest active American make...
: Positive Traction
- ChevroletChevroletChevrolet , also known as Chevy , is a brand of vehicle produced by General Motors Company . Founded by Louis Chevrolet and ousted GM founder William C. Durant on November 3, 1911, General Motors acquired Chevrolet in 1918...
/GMC trucks (after 1973): Gov-Lock and Positraction
- ChryslerChryslerChrysler Group LLC is a multinational automaker headquartered in Auburn Hills, Michigan, USA. Chrysler was first organized as the Chrysler Corporation in 1925....
: Sure Grip
- Dana Corporation:Trak-Lok or Powr-Lok
- FerrariFerrariFerrari S.p.A. is an Italian sports car manufacturer based in Maranello, Italy. Founded by Enzo Ferrari in 1929, as Scuderia Ferrari, the company sponsored drivers and manufactured race cars before moving into production of street-legal vehicles as Ferrari S.p.A. in 1947...
- FiatFiatFIAT, an acronym for Fabbrica Italiana Automobili Torino , is an Italian automobile manufacturer, engine manufacturer, financial, and industrial group based in Turin in the Italian region of Piedmont. Fiat was founded in 1899 by a group of investors including Giovanni Agnelli...
- FordFord Motor CompanyFord Motor Company is an American multinational automaker based in Dearborn, Michigan, a suburb of Detroit. The automaker was founded by Henry Ford and incorporated on June 16, 1903. In addition to the Ford and Lincoln brands, Ford also owns a small stake in Mazda in Japan and Aston Martin in the UK...
: Equa-Lock and Traction-Lok
- InternationalInternational HarvesterInternational Harvester Company was a United States agricultural machinery, construction equipment, vehicle, commercial truck, and household and commercial products manufacturer. In 1902, J.P...
: Trak-Lok or Powr-Lok
- JeepJeepJeep is an automobile marque of Chrysler . The first Willys Jeeps were produced in 1941 with the first civilian models in 1945, making it the oldest off-road vehicle and sport utility vehicle brand. It inspired a number of other light utility vehicles, such as the Land Rover which is the second...
: Trac-Lok (clutch-type mechanical), Tru-Lok (gear-type mechanical), and Vari-Lok (gerotor pump)
- OldsmobileOldsmobileOldsmobile was a brand of American automobile produced for most of its existence by General Motors. It was founded by Ransom E. Olds in 1897. In its 107-year history, it produced 35.2 million cars, including at least 14 million built at its Lansing, Michigan factory...
- PontiacPontiacPontiac was an automobile brand that was established in 1926 as a companion make for General Motors' Oakland. Quickly overtaking its parent in popularity, it supplanted the Oakland brand entirely by 1933 and, for most of its life, became a companion make for Chevrolet. Pontiac was sold in the...
- PorschePorschePorsche Automobil Holding SE, usually shortened to Porsche SE a Societas Europaea or European Public Company, is a German based holding company with investments in the automotive industry....
: PSD (electro-hydraulic mechanical)
- SaabSaabSaab AB is a Swedish aerospace and defence company, founded in 1937. From 1947 to 1990 it was the parent company of automobile manufacturer Saab Automobile, and between 1968 and 1995 the company was in a merger with commercial vehicle manufacturer Scania, known as Saab-Scania.-History:"Svenska...
: Saab XWD eLSD
- Studebaker-Packard CorporationStudebaker-Packard CorporationThe Studebaker-Packard Corporation was the entity created by the purchase of the Studebaker Corporation of South Bend, Indiana, by the Packard Motor Car Company of Detroit, Michigan, in 1954.Packard acquired Studebaker in the transaction...
: Twin Traction
- TVRTVRthumb|right|240px|TVR No.2, the oldest surviving TVR, located at [[Lakeland Motor Museum, Newby Bridge, Cumbria]]TVR was an independent British manufacturer of sports cars. Until 2006 it was based in the English seaside town of Blackpool, Lancashire, but has since split up into several smaller...