Limited slip differential 1

Limited slip differential

Cone Type LSD

A limited slip differential (LSD) is a type of differential geararrangement that allows for some difference in angular velocity of theoutput shafts, but imposes a mechanical bound on the disparity. In anautomobile, such limited slip differentials are sometimes used in placeof a standard differential, where they convey certain dynamicadvantages, at the expense of greater complexity.

Early history

In 1932, Ferdinand Porsche designed a Grand Prix racing car for theAuto Union company. The high power of the design caused one of therear wheels to experience excessive wheel spin at any speed up to100 mph (160 km/h). In 1935, Porsche commissioned the engineering firm ZF to design a limited slip differentialthat would perform better. The ZF "sliding pins and cams" became available,[1] and one example was the Type B-70for 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 remainstationary, and the non-contacting wheel will rotate freely—the torque transmitted will be equal at both wheels, butwill not exceed the threshold of torque needed to move the vehicle, and thus the vehicle will remain stationary. Ineveryday use on typical roads, such a situation is very unlikely, and so a normal differential suffices. For moredemanding use, such as driving in mud, off-road, or for high performance vehicles, such a state of affairs isundesirable, and the LSD can be employed to deal with it. By limiting the angular velocity difference between a pairof driven wheels, useful torque can be transmitted as long as there is some traction available on at least one of thewheels.

TypesTwo main types of LSD are commonly used on passenger cars; torque sensitive (geared or clutch-based orcone-based as shown in figure at top of page) and speed sensitive (viscous/pump and clutch pack). The latter isgaining popularity especially in modern all-wheel drive vehicles, and generally requires less maintenance than themechanical type.

Torque-sensitiveThe use of the word mechanical implies that the limited slip differential is engaged by interaction between two (ormore) mechanical parts. This category includes clutch and cone and helical gear limited slip differentials. For roadracing, 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%.

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Characteristics

ZF LSD - clutch stack visible on left

ZF LSD - spider pinion shaft ramps visible

General LSDs respond to driveshaft torque, so that the more driveshaftinput torque present, the harder the clutches or cones or gears arepressed together, and thus the more closely the drive wheels arecoupled to each other. Some include spring-loading to provide somesmall torque so that with no / little input torque (trailingthrottle/gearbox in neutral/main clutch depressed) the drive wheels areminimally coupled. The amount of preload (hence static coupling) onthe clutches or cones are affected by the general condition (wear) andby how tightly they are loaded.Broadly speaking, there are three input torque states: load, no load, andover run. During load conditions, as previously stated, the coupling isproportional to the input torque. With no load, the coupling is reducedto the static coupling. The behaviour on over run (particularly suddenthrottle release) determines whether the LSD is 1 way, 1.5 way, or 2way.If there is no additional coupling on over run, the LSD is 1 way. This isa safer LSD: as soon as the driver lifts the throttle, the LSD unlocksand behaves somewhat like a conventional open differential. This isalso the best for FWD cars, as it allows the car to turn in on throttlerelease, instead of plowing forward.[2]

If the LSD increases coupling in the same way regardless of whetherthe input torque is forward or reverse, it is a 2 way differential. Some drifters prefer this type as the LSD behaves thesame 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, expectingthe 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 compromisebetween sportiness and safety.

Clutch Type LSD

The clutch type has a stack of thin clutch discs, half of which are coupled to one of the drive shafts, the other half ofwhich are coupled to the spider gear carrier. The clutch stacks may be present on both drive shafts, or on only one. Ifon only one, the remaining drive shaft is linked to the clutched drive shaft through the spider gears. If the clutcheddrive shaft cannot move relative to the spider carrier, then the other drive shaft also cannot move, thus they arelocked.The spider gears mount on the pinion cross shaft which rests in angled cutouts forming cammed ramps. The cammedramps 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 clutchstack) are forced sideways by the pinion cross shaft trying to climb the ramp, which compresses the clutch stack. Themore the clutch stack is compressed, the more coupled the wheels are. The mating of the vertical ramp (80°-85° inpractice to avoid chipping) surfaces in a 1 way LSD on over run produces no cam effect and no corresponding clutchstack compression.The break-in period of clutch LSDs can be very specific. Manufacturers give detailed instructions on how to break the differential in.[3] If these are not followed, the LSD may be permanently harmed, in that it may engage and

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disengage erratically due to irregularities on and damage to the clutch surfaces. Essentially, the LSD must be workedhard 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 ofthe clutches or the centre. In any case, the oil should be changed on a regular basis depending upon use. The fluidshould be changed after about seventy thousand miles or one hundred thousand kilometers (as opposed to the opendifferential, where the oil could be left unchanged for several hundred thousand kilometres).

Geared LSD

Audi Quattro Torsen Differential

Geared, torque-sensitive mechanical limited slip differentials utilizehelical gears or worm gears to "sense" torque on one shaft. The mostfamous versions are:

• Torsen differential based upon the Dual-Drive Differential inventedby Vernon Gleasman in 1958, then later sold to GleasonCorporation, who started marketing it in 1982;

• Quaife differential, sold under the name Automatic Torque BiasingDifferential (ATB), covered by European Patent No. 130806A2.

• Eaton Corporation differential, sold under the name Eaton DetroitTruetrac.

Geared LSDs use worm gears rather than clutches of the clutch typeand the cones of the cone type, and work by "multiplying" the torquefrom the slowest moving wheel to the fastest, rather than activelycontrolling slip.In the case of slip, the wheel in contact can receive up to X times thetorque applied to the wheel which is slipping, where X is the torquemultiplication value for the differential. In this sense, torque sensitive differentials are not strictly limited slip - oncean output shaft becomes free (e.g., one driven wheel lifts off the ground; or a summer tire comes over ice whileanother is on dry tarmac when the car goes uphill), no torque is transmitted to the second shaft and thetorque-sensitive differential behaves like an open differential. A common trick in these situations, such as turning upa steep hill in a low sports car, is to apply slight pressure to the brakes (handbrake in the case of RWDs), allowingsome torque to be applied to the raised wheel which will then be multiplied to the wheel in contact. Some torquesensitive differentials feature a bias plate, which allows some torque to be transmitted to the wheel in contact evenwhen the opposite wheel has no traction.

Geared LSDs are more dependent on the torque and not on the speed difference between the output shafts (howeverthe 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 drive vehicles;• as a center differential in four-wheel drive (e.g., on the Audi Quattro);• in rear-wheel drive vehicles, to maximize traction and make oversteer easier to manage (as in drifting). Although,

for extreme drifting, a geared LSD is less effective compared to a clutch type LSD. Torque sensitive differentialsare rarely used in Rally motorsports, however.

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Speed-sensitive

Viscous

Nissan 240SX Viscous LSD

The viscous type is generally simpler because it relies onhydrodynamic friction from fluids with high viscosity. Silicone-basedoils are often used. Here, a cylindrical chamber of fluid filled with astack of perforated discs rotates with the normal motion of the outputshafts. The inside surface of the chamber is coupled to one of thedriveshafts, and the outside coupled to the differential carrier. Half ofthe discs are connected to the inner, the other half to the outer,alternating inner/outer in the stack. Differential motion forces theinterleaved discs to move through the fluid against each other. In someviscous couplings when speed is maintained the fluid will accumulateheat due to friction. This heat will cause the fluid to expand, andexpand the coupler causing the discs to be pulled together resulting in a non-viscous plate to plate friction and adramatic drop in speed difference. This is known as the hump phenomenon and it allows the side of the coupler togently 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 stylein several of their transfer cases including those used in the AMC Eagle.

Viscous LSDs are less efficient than mechanical types, that is, they "lose" some power. In particular, any sustainedload which overheats the silicone results in sudden permanent loss of the differential effect.[4] They do have thevirtue of failing gracefully, reverting to semi-open differential behaviour. Typically a visco-differential that hascovered 60000 miles (97000 km) or more will be functioning largely as an open differential; this is a knownweakness of the original Mazda MX-5 (a.k.a. Miata) sports car. The silicone oil is factory sealed in a separatechamber from the gear oil surrounding the rest of the differential. This is not serviceable and when the differential'sbehaviour deteriorates, the VLSD centre is replaced.

Gerotor pump

This works by hydraulically compressing a clutch pack. The gerotor pump uses the housing to drive the outer side ofthe pump and one axle shaft to drive the other. When there is differential wheel rotation, the pump pressurizes itsworking fluid into the clutch pack area. This provides a clamp load for frictional resistance to transfer torque to thehigher traction wheel. The pump based systems have a lower and upper limits on applied pressure, and internaldamping to avoid hysteresis. The newest gerotor pump based system has computer regulated output for moreversatility and no oscillation.

Electronic

Electronic limited slip differential systems use speed sensors, anti-lock braking system (ABS) sensors,accelerometers, and microcomputers to electronically monitor wheel slip and vehicle motion. In some systems thecomputer limits slip by varying the degree of locking in a mechanical LSD; such as Porsche's PSD system, whichuses electro-hydraulic control of a mechanical LSD.The Mitsubishi Active Yaw Control (AYC) electronically controlled rear differential uses a conventional opendifferential with an added planetary gear set to rotate two hollow shafts around the left hand drive shaft, one runningat +15% speed, one at -15%. These can be progressively locked up to the left hand drive shaft via a hydraulic clutchpack under CPU control, increasing or decreasing the torque on that wheel in relation to the other. This allows acertain amount of rear wheel "steering" to provide stability control and perform the function of an LSD.

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Many vehicles use a traction control system to simulate a limited slip differential. With this type of system, if eitherof the wheels on an axle is rotating unusually faster than the other, the computer will determine how much it isslipping and apply braking to it, slowing the spinning wheel down and thereby increasing torque to the wheel withmore traction.

Other related final drives

Spool

A spool rear end allows no differential rotation. A spool consists of a pinion & 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 gearswith a solid piece, retaining the factory differential carrier. Spool rear ends are used to achieve a similar effect to anLSD on some street and race cars. This arrangement is popular in 'drifting' where drivers aim for flamboyance ratherthan speed. Those that use the car for "drifting purposes" often weld the gears in a standard open differential togetherto produce a homemade spool.It is also preferred by drag racing enthusiasts for 2 main reasons. A straighter launch is achieved because power issplit 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 sidegears, cross pin and spider gears, a spool can take much more abuse. This is especially important for drag racers thatshock their drive trains with hard launches and sticky tires. However, turning is more difficult with the wheelslocked together (the outside wheel has a greater distance to travel than the inside wheel and thus has to rotate fasterto compensate).

Locking differential

Detroit Locker/Locker

A locker locks both wheels under normal conditions. If a wheel is externally forced to rotate faster than thedifferential 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,[5] the locker has the unusualcharacteristic of only applying drive torque through the inner wheel in corners when decelerating or under neutralthrottle.[6] Drive shaft input torque causes the pinion cross shaft to lock the centre more firmly, resisting theunlocking action. As the two actions of the mechanism are contradictory, the car will unpredictably alternatebetween 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 vehiclegoes farther into the curve, the gear unlocks causing it to jerk.[7] The net effect is extreme understeer (as all of theengine torque is being applied through the inside wheel), followed by power-on oversteer when the rear wheelsbreak traction. It can also be very noisy and is often used in off-road 4WD applications. The traditional Americanracing differential is a Detroit Locker.

Limited slip differential 6

Selectable locker

ARB Air Locker

Normally functioning as open differential, a selectable locker can belocked by the driver. Compressed air, mechanical cable, electricactuator or hydraulic fluid activates the locking mechanism. Generallyused by street cars that also drag race, the car drives to the event open,and locks the differential on the strip. Selectable locking differential isoften used together with electronic systems for off-road driving.

Factory names

In the 1950s and 1960s many manufacturers began to apply brandnames to their LSD units. While Packard pioneered the LSD under thebrand name "Twin Traction" in 1956, the most famous of these in the USA was Chevrolet's "Positraction". Sincethen, Positraction (often shortened to "positrac" or merely "posi") has become a genericized trademark for LSDs inthe USA.

Other factory names for LSDs include:• Alfa Romeo: Q2• American Motors: Twin-Grip• Buick: Positive Traction• Chevrolet/GMC trucks (after 1973): Gov-Lock• Chrysler: Sure Grip• Dana Corporation:Trak-Lok or Powr-Lok• Ferrari: E-Diff• Fiat: Viscodrive• Ford: Equa-Lock and Traction-Lok• International: Trak-Lok or Powr-Lok• Jeep: Trac-Lok (clutch-type mechanical), Tru-Lok (gear-type mechanical), and Vari-Lok (gerotor pump)• Oldsmobile: Anti-Spin• Pontiac: Safe-T-Track• Porsche: PSD (electro-hydraulic mechanical)• Saab: Saab XWD eLSD• Studebaker-Packard Corporation: Twin Traction• TVR: Hydratrak

References[1] The Motor Vehicle K.Newton W.Steeds T.K.Garrett Ninth Edition pp549-550[2] Donnon, Martin et al. (2004). High Performance Imports 48. Express Motoring Publications. pp. 77–80. "...being able to run the driven

wheels almost fully open under deceleration. In a powerful front wheel drive scenario where torque steer is a constant enemy, this approach [1way LSD] has some definite advantages."

[3] KAAZ Technical Help (http:/ / www. kaazusa. com/ techhelp2. html) - retrieved 2009-02-09[4] Donnon, Martin et al. (2003). Zoom 67. Express Motoring Publications. pp. 45–48. "...the gel used can quite suddenly alter with massive

temperature, and lose its ability to generate torque transfer."[5] Smith, Carroll (1996). Drive To Win. Carroll Smith Consulting Inc.. Section 7 Page 26. ISBN 0-9651600-0-9. "...All of the limited slips or

torque sensors function by delivering the majority (or all) of the input torque to the wheel with the most traction..."[6] Smith, Carroll (1996). Drive To Win. Carroll Smith Consulting Inc.. Section 7 Page 34. ISBN 0-9651600-0-9. "...When the vehicle is

decelerating or under neutral throttle in a turn, it is driven by the inside wheel only..."[7] Smith, Carroll (1996). Drive To Win. Carroll Smith Consulting Inc.. Section 7 Page 34. ISBN 0-9651600-0-9. "...a somewhat crude device,

noisy and upsetting as it cycles between "on" and "off" at partial or changing throttle... The transition from one wheel to two wheel drive canbe particularly upsetting if the driver decides to play with the throttle on corner exit."

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External links• Limited slip differentials explained (http:/ / www. drivingfast. net/ technology/ Differentials. htm)• What is a Quaife ATB differential - R.T. Quaife Engineering Limited (http:/ / www. quaife. co. uk/ differentials)

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Image Sources, Licenses and ContributorsImage:Limited slip diff cone.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Limited_slip_diff_cone.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors:RB30DEImage:Bauma 2007 ZF Loader Axle MT-L3075 2.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Bauma_2007_ZF_Loader_Axle_MT-L3075_2.jpg  License: Creative CommonsAttribution-Sharealike 3.0  Contributors: AconcaguaImage:BAUMA 2004 ZF Differentialgetriebe.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:BAUMA_2004_ZF_Differentialgetriebe.jpg  License: Creative CommonsAttribution-Sharealike 3.0  Contributors: Aconcagua (talk)Image:Audi quattro AWD system.jpeg  Source: http://en.wikipedia.org/w/index.php?title=File:Audi_quattro_AWD_system.jpeg  License: GNU Free Documentation License  Contributors:NocturnalA6 2.7Image:S13HICAS.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:S13HICAS.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Saen1978Image:ARB RD93 Locker Installed.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:ARB_RD93_Locker_Installed.jpg  License: Creative Commons Attribution-ShareAlike 3.0Unported  Contributors: Michael Robinson (unixxx)

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