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Limited slip differential
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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 an 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.
932, Ferdinand Porsche designed a Grand Prix racing car for the Auto Union company.
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Encyclopedia
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 an 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.
Early history
In 1932, Ferdinand Porsche designed a Grand Prix racing car for the Auto Union 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. In 1935, Porsche commissioned the engineering firm ZF to design a limited slip differential that would perform better.
Benefits
The 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 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.
Types
Two main types of LSD are commonly used on passenger cars; torque sensitive (geared or clutch-based) 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-sensitive
The 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 helical limited slip differentials. For road racing, many prefer a helical 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%.
Clutch type - driveshaft torque activated
Characteristics
The clutch-type LSD responds to driveshaft torque. The more driveshaft input torque present, the harder the clutches are pressed together, and thus the more closely the drive wheels are coupled to each other.
With no / little input torque (trailing throttle/gearbox in neutral/main clutch depressed) the drive wheels are still coupled somewhat as the clutches are always in contact to some degree, producing friction. The amount of preload (hence static coupling) on the clutches is determined by the general condition (wear) of the clutches and by how tightly they are shimmed.
Broadly speaking, there are three input torque states: load, no load, and over run. Under load, 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 forwards or reverse, it is a 2 way differential. Some drifters 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.
Mechanism
The 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.
Servicing
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 regularly (as opposed to the open differential, where the oil could be left unchanged for several hundred thousand kilometres).
Geared torque-sensitive differential
Geared, torque-sensitive mechanical limited slip differentials utilize worm gears to "sense" torque on one shaft. The most famous versions are:
- Torsen 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;
- Quaife differential, sold under the name Automatic Torque Biasing Differential (ATB), covered by European Patent No. 130806A2.
- Eaton Corporation differential, sold under the name Eaton Detroit Truetrac.
Geared LSDs are less prone to wear than the clutch type, but both output shafts have to be loaded to keep the proper torque distribution characteristics. Once an output shaft becomes free (e.g., one driven wheel lifts off the ground; or a summer tire comes over ice while another is on dry tarmac when the car goes uphill), no torque is transmitted to the second shaft and the torque-sensitive differential behaves like an open differential.
Geared LSDs are dependent on the torque and not on the speed difference between the output shafts. 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.
Speed-sensitive
Viscous
The viscous type is generally simpler because it relies on hydrodynamic friction from fluids with high viscosity. Silicone-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 style in 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. They do not stand up well to abuse. 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 60,000 miles or more will be functioning largely as an open differential; this is a known weakness of the original Mazda MX-5 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 pump
This 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.
The newest gerotor pump based system has computer regulated output for more versatility and no oscillation.
Electronic
Electronic limited slip differential systems use speed sensors, anti-lock brakes, and microcomputers to electronically monitor slipping. If any of the wheels on an axle is rotating unusually faster than the others, the computer will assume that it is slipping and will briefly apply brakes to it, slowing the spinning wheel down and causing the opposite wheels to gain speed and keep traction.
One advantage of this system over mechanical is that the vehicle steering and control is less affected. It also generates less stress on the drive train compared to a mechanical locking device, making it particularly suitable for the vehicles with independent suspension. It can also be tuned for specific applications on and off road and at different speeds.
A disadvantage is that it is less predictable when going over an obstacle, as the system needs time to react. Also, the wheel with traction will only have half of the available torque applied to it.
The Mitsubishi Active Yaw Control (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 performs the function of an LSD.
Other related final drives
Spool
A spool limits differential rotation to exactly zero. A spool consists of a pinion & ring gear only, the center is solid, the axle is one piece. A mini-spool is similar, replacing the usual differential center with a solid piece, retaining the factory axles. A spool is not a differential at all, but is used to achieve a similar effect to an LSD on some street and race cars. This arrangement is popular in the motorsport of drifting where copious wheelspin is seen to be an advantage. "Drifters" often generate their own "ghetto differential" by welding-solid a standard open differential to produce a home made spool.
Locking differential (Detroit Locker/Lokka)
A locker 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 the locker has the extremely unusual characteristic of applying drive torque through the inner wheel in corners. 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, giving difficult to control handling. As the vehicle goes down a straight line it is locked positive, and as the vehicle enters a curve it is still locked positive. As the vehicle goes farther into the curve, the gear unlocks causing it to jerk. It can be very noisy and is often used in off-road 4WD applications. The traditional American racing differential is a Detroit Locker.
Selectable 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. Generally used by street cars that also drag race, the car drives to the event open, and locks the differential on the strip. Selectable locking differential is often used together with electronic systems for off-road driving.
Factory names
In the 1950s and 1960s many manufacturers began to apply brand names to their LSD units. While Packard pioneered the LSD under the brand name "Twin Traction" in 1956, the most famous of these was Chevrolet's "Positraction". Since then, Positraction (often shortened to "positrac" or merely "posi") has become a genericized trademark for LSDs.
Other factory names for LSDs include:
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