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Limited Slip, Lockers & Spools Explained

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There is more info in here then most will never need, I pulled together some of the better atricles that are out there to read about the 3 main types of rear differentials.

I would have typed up somthing on my own but i would never have the time and there is far to much to cover on one syle diff let alone all three!




The main advantage of a limited-slip differential is shown by considering the case of a standard (or "open") differential in off-roading situations where one wheel has no contact with the ground. In such a case, with a standard differential, the non-contacting wheel will receive 100% of the power while the contacting wheel will remain stationary. The torque transmitted will be equal at both wheels, therefore will not exceed the threshold of torque needed to move the wheel with grip. In this situation, a limited-slip differential prevents 100% of the power from being allocated to one wheel, and thereby keeping both wheels in powered rotation.

Basic principle of operation

Automotive limited-slip differentials all contain a few basic elements. First, all have a gear train that, like an open differential, allows the outputs to spin at different speeds while holding the average speed of the two outputs to be equal to the input speed.

Second, all have some sort of mechanism that applies a torque internal to the differential that resists the relative motion of the output shafts. In simple terms this means they have some mechanism which resists a speed difference between the outputs by creating a resisting torque between either the two outputs or the outputs and the differential housing. There are many mechanisms used to create this resisting torque. The type of limited-slip differential typically gets its name from the design of this resisting mechanism. Examples include viscous and clutch-based LSDs. The amount of limiting torque provided by these mechanisms varies by design and is discussed later in the article.


Several types of LSD are commonly used on passenger cars.

  • Fixed value
  • Torque sensitive
  • Speed sensitive
  • Electronically controlled

Fixed value

In this differential the maximum torque difference between the two outputs, Trq d , is a fixed value at all times regardless of torque input to the differential or speed difference between the two outputs.Typically this differential used spring loaded clutch assemblies.

Torque sensitivity

This category includes helical gear limited-slip differentials and clutch, cone (an alternative type of clutch) where the engagement force of the clutch is a function of the input torque applied to the differential (as the engine applies more torque the clutches grip harder and Trq d increases).

220px-Bauma_2007_ZF_Loader_Axle_MT-L3075_2.jpg magnify-clip.png

ZF LSD – clutch stack visible on left

220px-BAUMA_2004_ZF_Differentialgetriebe.jpg magnify-clip.png

ZF LSD – spider pinion shaft ramps visible

Torque sensing LSDs respond to driveshaft torque, so that the more driveshaft input torque present, the harder the clutches, 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 little or no 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.

Clutch, cone-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 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. In a cone type the clutches are replaced by a pair of cones which are pressed together achieving the same effect.

One method for creating the clamping force is the use of a cam-ramp assembly such as used in a Salisbury/ramp style LSD. 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. (See the discussion of 2, 1.5 and 1 way below)

An alternative is to use the natural separation force of the gear teeth to load the clutch. An example is the center differential of the 2011 Audi Quattro RS 5.[5]

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 C° in practice to avoid chipping) surfaces in a one-way LSD on overrun produces no cam effect or corresponding clutch stack compression.

2-, 1-, and 1.5-way LSD

Broadly speaking, there are three input torque states: load, no load, and overrun. 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 behavior on overrun (particularly sudden throttle release) determines whether the LSD is 1 way, 1.5 way, or 2 way.

A 2-way differential will have the same limiting torque Trq d in both the forward and reverse directions. This means the differential will provide some level of limiting under engine braking.

A 1-way differential will provide its limiting action in only one direction. When torque is applied in the opposite direction it behaves like an open differential. In the case of a FWD car it is argued to be safer than a 2-way differential.[6] The argument is if there is no additional coupling on overrun, i.e. a 1-way 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.[6]

A 1.5-way differential refers to one where the forward and reverse limiting torques, Trq d_fwd, d_rev , are different but neither is zero as in the case of the 1-way LSD. This type of differential is common in racing cars where a strong limiting torque can aid stability under engine braking.

Geared LSD

220px-Audi_quattro_AWD_system.jpeg magnify-clip.png

Audi Quattro Torsen Differential

Geared, torque-sensitive mechanical limited-slip differentials use helical gears or worm gears rather than the beveled spider gears of the clutch based differentials. As torque is applied to the gears they are pushed against the walls of the differential housing which creates friction. The friction resists the relative movement of the outputs and creates the limiting torque Trq d .

Examples include:

Speed sensitivity

Speed-sensitive differentials limit the torque difference between the outputs, Trq d , based on the difference in speed between the two output shafts. Thus for small output speed differences the differential’s behavior may be very close to an open differential. As the speed difference increase the limiting torque increases. This results in different dynamic behavior as compared to a torque sensitive differential.


220px-S13HICAS.jpg magnify-clip.png

Nissan 240SX Viscous LSD

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. In particular, any sustained load which overheats the silicone results in sudden permanent loss of the differential effect.[7] They do have the virtue of failing gracefully, reverting to semi-open differential behavior. Typically a visco-differential that has covered 60,000 miles (97,000 km) or more will be functioning largely as an open differential;[citation needed] this is a known weakness of the original Mazda MX-5 (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 behavior deteriorates, the VLSD center is replaced.

Gerotor pump

This style limited-slip differential works by using a gerotor pump to hydraulically compress a clutch to transfer torque to the wheel that is rotating the slowest. The gerotor pump uses the differential carrier or cage to drive the outer rotor of the pump and one axle shaft to drive the inner rotor. When there is a difference between the left and right wheels' speed, the pump pressurizes the hydraulic fluid causing the clutch to compress. thereby causing the torque to be transferred to the wheel that is rotating the slowest. These pump-based systems have a lower and upper limits on applied pressure which allows the differential to work like a conventional or open differential until there is a significant speed difference between the right and left wheel, and internal damping to avoid hysteresis. The newest gerotor pump based system has computer regulated output for more versatility and no oscillation.


An electronic limited-slip differential will typically have a planetary or bevel gear set similar to that of an open differential and a clutch pack similar to that in a torque sensitive or gerotor pump based differential. In the electronic unit the clamping force on the clutch is controlled externally by a computer or other controller. This allows the control of the differential’s limiting torque, Trq d , to be controlled as part of a total chassis management system. An example of this type of differential is Subaru’s DCCD used in the 2011 Subaru WRX STi.[8] Another example is the Porsche PSD system used on the Porsche 928.

Edited by 330CubeGt
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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 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 where the inside wheel rotates at a slower velocity than the outside wheel - think of the rotational speed at the center of a merry-go-round compared to that at the outside of the merry-go-round)), 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.


Automatic lockers

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. 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," 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.

  • Pros: Automatic action, no driver interaction necessary, no stopping for (dis-) engagement necessary
  • Cons: Increased tire wear and noticeable impact on driving behavior. During cornering, the automatic locker is characterized by heavy understeer which transitions instantly to power oversteer when traction is exceeded.

Some other automatic lockers operate as an open differential until wheelspin is encountered and then they lockup. This style generally uses an internal governor to sense a difference in wheel speeds. An example of this is the Eaton Automatic Locking Differential (ALD), also called Eaton Automatic Differential Lock (ADL), developed by the Eaton Corporation for GM's rounded-line C/K Series pickups and utilities.[1] The Eaton ADL is sometimes incorrectly called the "Gov-Lok", despite the fact that neither GM nor Eaton never called it by that name. Gov-Lok is rather an unofficial nickname of unknown origin that gained popularity over the years.[2] An updated version of the old Eaton ADL design is still available from Eaton, now known as the Eaton mLocker mechanical differential lock.[3]

Some other automatic lockers operate as an open differential until high torque is applied and then they lockup. This style generally uses internal gears systems with very high friction. An example of this would be ZF "sliding pins and cams" available for use in early VWs.

Selectable locker

Selectable lockers allow the driver to lock and unlock the differential at will from the driver's seat. This can be accomplished many ways.

  • Compressed air (pneumatics).
  • Cable operated mechanism as is employed on the "Ox Locker."
  • Electronic solenoids and (electromagnetics) like Eaton's "ELocker." However, OEMs are beginning to offer electronic lockers as well. Nissan Corporations electric locker found as optional equipment on the Frontier (Navara) & Xterra. 2011 Ford Super Duty F-250 and SRW F-350 4x4 models have a electronic locker as 390.00 USD option. [4]
    • Pros: Allows the differential to perform as an "open" differential for improved driveability, maneuverability, provides full locking capability when it is desirable or needed
    • Cons: Mechanically complex with more parts to fail. Some lockers require vehicle to stop for engagement. Needs human interaction and forward-thinking regarding upcoming terrain. Unskilled drivers often put massive stress on driveline components when leaving the differential in locked operation on terrain not requiring a locker DisadvantagesBecause 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. The disadvantages of selectable locking differentials are not mentioned due to the ability to function as an open differential as needed.
      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.

    [*]Here is a Picture of what most lockers will look like.NoSlipLocker_5069.jpg

Edited by 330CubeGt
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[TABLE=width: 670]


[TD=class: postTitle]-Differential Full and Mini Spools-





Before we discuss the difference between Differential Full Spools and Mini Spools, it is very important to understand What are differential spools and why at all one needs to use spools in differentials? When a car takes a turn, the outer wheels of the car have to a travel a greater distance as compared to the inner wheels in the same time. Therefore, a mechanism called as differential is provided in the rear end of the car, so that the speed of the inner wheels reduces and the speed of the outer wheels increases and the car can take turn easily and does not skid. At the same time, differential helps in maintaining equal speed of the wheels when the car is going straight.

Various vehicles use different options to maintain the torque required to run the vehicle in different given situations, e.g. straight drive on street, driving in dirt, driving on ice, driving on rocks etc. Therefore the type of differential required will depend on the situation where we want to run the car.When the vehicle is moving off-road i.e. situations like rock crawling, steep hills, mud-blogging, sand dragging etc it would require such a mechanism, which delivers equal power and traction to both the left and right side rear wheels. Such a mechanism which is available in the market is known as locker, A Locker is mounted on the differential and thus controls the speeds of the rear wheels, as required, but the locker is heavy and expensive. Therefore, to minimize the cost of the equipment used and to make the vehicle lighter, a spool is used.A differential spool is known as the best alternative in place of a locker both in terms of value and weight. A spool is basically a piece of steel that the ring gear bolts to and the Rear Axle Shaft splines fit into the internal splines of the spool.

The spool does not have any internal gears. The simplicity of the spool is also the reason for its tremendous strength. No moving parts = brute strength. Thus a spool, completely replaces the differential center with a solid piece, but retains the Rear Axle Shafts. The basic function of a differential spool is to permanently lock both the left and right axles of the rear wheel together, so that they move together and there can be no loss of power due to one wheel spinning faster than the other wheel. The Differential Spools are also used as a support for supporting the ring gear much better than most other differentials. Spool is only recommended to be used in such vehicles where the vehicle has to remain most of the times, off the road. Full Spools are recommended to be used in the vehicles where a higher horse power is required.

Advantages of using differential spools:

  1. A differential spool is less costly and light weight as compared to Locker.
  2. A differential spool provides guaranteed traction at all times,
  3. As differential spool is a single unit and has no moving parts, which means that it has got lesser tendency to break.
  4. A differential spool does not have clutches, therefore, neither a question of wearing out of clutches arises and nor this will need adjustment.

Disadvantages of using Spool:

  1. Spools should not be used on the vehicles which are run on street, as this will cause the tyres to wear out prematurely and one of the tyre will skid while turning, as the speed of both the tyres is maintained constant.
  2. Using spools on street vehicles also reduces the rear axle shafts life considerably.

Difference between Differential Full Spools and Mini SpoolsFull Spools:The differential spools as explained above are commonly known as Full Spools. The full spools can further be characterized as Light weight full spools and Ultra Light Weight Full Spools. As mentioned above, one of the reasons, apart from saving money, why people go for full spools is to reduce weight as a result of which industry drilled some extra non functional holes in them to shed their weight and these full spools started being called light weight full spools.

The growing demand of racing enthusiasts forced the industry to think and further and come out with a further lighter version of full spools wherein the extra area close to the holes, fitting the ring gear, was scalloped. These spools are known as ultra light weight differential spools.

Mini Spools:

While the full spool replaces the differential case, the function of a mini spool is to replace the spider gears and side gears / pinion gears in the differential case. These mini spools also come in two different designs. For Ford applications, the mini spools normally come as one piece but in the case of GM applications, they come in four pieces i.e. two parts with internal splines ( to replace spider gears) for axle shaft splines to fit and two slider blocks to replace the pinion gears. The purpose of full spool and the mini spool is the same i.e. to lock the axles together. In simpler words a mini spool is something which is inserted in the stock carrier to replace the spider gears and lock up the differential. A mini spool drives exactly like a full spool does, but they are not as strong as a full spool and do not add any extra support to the ring gear like a spool does. In terms of price also mini spools are far cheaper than full spools.

Here is a picture of what most FULL SPOOLS look like.


Here is a picture of what most MINI SPOOLS look like..




Edited by 330CubeGt
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