Locking differential 101

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Limited Slip Differentials (LSDs) are made in a few major types which are clutch-, torsen/gear-, viscous- and spool types. There are a lot of variants, but now we'll go through the basic differences in these types. An open differential is found on most cars from the factory, allthough some sport models have a factory equipped lsd.


Clutch-type LSD

This type is well suited for track and competition use where a linear and predictable lock in all situations is preferred over everyday comfortability.


Operation is based on internal clutch discs which by applied torque are pressed together to lock the differential. The higher the torque in acceleration or braking, the harder the unit locks. In neutral or when pressing the clutch, a clutch-type differential wont lock since there is no torque being applied to it.
Most brands offer different ramp angles, which are generally referred to as 1-way, 2-way and 1.5-way. A one-way differential only locks on acceleration, a two-way on both acceleration and braking, and a 1.5-way only locks a little on braking.


The "wayness" of a clutch type differential is defined by the ramp angles, illustrated in the image below. A 2-way unit has an identical ramp angle on both sides, a 1.5 way has a higher angle on the braking side, and a one-way only has an angle on the acceleration side. The lower the angle, the harder the lock engages when the rod pushes the two halves apart compressing the clutch discs.




Oils. A cltuch type differentials character can be fine-tuned with the choice of oil. Regular oil has no friction modifiers, whereas LSD oils are generally intended for use in clutch-type differentials and have an added friction modifier to make it more slippery. The slipperier the oil is, the smoother the engagement is as the clutch discs are allowed to slip a little before engaging. Friction modifiers are also available separately. At least Red Line sells it separately, and it can be used to fine-tune a differentials action by adding anything between 0 and 5 % of friction modifier to the oil. Viscosity should usually be the factory rating.


Usage. Due to it's structure clutch-type differentials are easily rebuildable, and most brands offer rebuild kits for their units. A rebuil kit consists of new discs and some neccessary hardware. This type of differential especially in competition use will require an overhaul at some point, so it is not completely service free. The time is however quite long, and units can last well over a decade in street use.


Banging and studdering are typical and completely harmless side-effects of a clutch-type unit resulting of the clutch packs slipping and re-engaging. This is the reason this is not the optimal unit for street use, as banging and studdering usually relates to tight turns on parking lots and traffic where the outer wheel travels way more than the inner. This results in either the clutch pack, or tires slipping - thus the noise and studdering.


Pros:
- Linear grip in all situations, thus best suited for competition cars.
- Can be serviced and parts are readily available.
- Characteristics can be tuned by ramp angles and choice of oil.


Cons:
- Usually noisy and rough, not the best choice for a street car.
- Especially in competition use, requires service for optimal grip.
- Low street driveability.


Suggested applications:
- Competition cars without reservation.
- Street racers if you dont mind a rough ride.


Brands we carry:
- Cusco
- Kaaz
- Tran-x
- Gripper


Torsen- and gear LSD

The gear type is an ideal choice for the streets thans to it's silent and smooth yet predictable operation.


The operation of a torsen differential is based on a geared mechanism which tries to keep the wheels spinning at the same rate. If one wheel slips, it's torque is directed to the wheel with less speed. A torsen never actually locks the wheels, it only tries to maintain identical speeds. This makes it an ideal choice for the street.




Zero-grip situation. A torsen works by multiplying the grip of the wheel spinning faster to the slower wheel. When multiplying, the faster wheel needs to have some grip, because zero multiplied by anything is still zero. A zero-grip situation can occur if one wheel is in the air. In such a situation, the unit behaves like an open differential.
Some advanced models have internal mechanisms which prevent or make a zero-grip situation less likely. Wawetrack has a wave-like profile which locks based on the difference in axle speeds, Quaife units have a preload, and there are models like the Detroit Locker which offer 100 % locking. The mechanics of these are past the scope of this article, but the point in all of them is to provide internal friction/fake-grip to allow the unit to transfer power to the wheel not experiencing zero-grip. In real life a zero grip situation is extremely rare, so this should not be considered as a main disadvantage.
Most traction control systems also cure zero-grip situations, as they provide "grip" by braking the wheel which is found to slip.

A more in-depth writing on the behaviour of a torsen can be found here (A Quaife model)


Oil choice. A torsen needs internal friction, so LSD oils should not be used in this type of differential. A slippery oil actually reduces ability to transfer power from the slipping wheel. Most oil brands offer a regular oil with no additives. Red Line for example offer a Non-Slip version of most viscosities. The viscosity should be chosen based on the car recommendation.


Traction control systems. Most TC-systems work by using the ABS sensors to read individual tire speeds, and if a wheel spins too fast it starts to brake the wheel. A torsen benefits from this behavoiur, and mostly eliminates the blinking traction control light in the dash. Torsens work flawlessly with these systems, and will not cause any fault codes.


Usage. A torsen has no wearable parts so it is service-free. It's also a completely silent and transparent unit as far as driving is concerned. These make the torsen differential an ideal choice for the a street car.


Pros:
- Silent and transparent, very good for daily driven street cars.
- Service-free. Install and forget.
- Compatible with traction control and ABS systems.


Cons:
- Not as linear as a clutch-type. Suffers from zero-grip situations to some extent which can cause a problem in race or offroad uses.
- Not tunable.
- Not serviceable (concerning race use mostly).


Suggested applications:
- Highly recommended for street cars and street racers.
- Not ideal for a pure track or competition machine.


Brands we carry:
- Quaife


A viscous differential

A viscous differential is the most common factory-install differential. It's not ideal by any means, and wears, but is a well suited as an OEM upgrade.



Operation. A viscous limited slip differential has two sealed viscous packs where discs rotate in a viscous fluid. One pack for each driveshaft. Every other disc is attached to the differential core, and every other to the driveshaft. When the driveshafts rotate at different speeds (ie. a wheel splips), the discs start rotating in the viscous packs. The fluid is thick, and starts to drag the slower shaft with it, providing traction to it as well. Some models also benefit of the fluid heating in the process, which can be used to engage the discs tighter together or actuate a separate clutch. The picture below hopefully clears this out a little.




Usage. A viscous LSD is not very well suited for track or providing high lock amounts. It is best suited for snow and ice conditions where extra traction is required. Viscous units wear over time, and usually units with 100.000km on the clock work more or less like an open differential. Depending on the type viscous units are quite unpredictable, locking at one instance and then behaving like an open unit.


Oil choice. The viscous units are in a sealed environment so the fluid can not be changed without special knowledge & tools. Diff oil should be used based on the recommendation of the car manufacturer.


Pros:
- A silent, usually OEM unit.
- Helps in slippery conditions.


Cons:
- Low locking force.
- Unpredictable behaviour.
- Limited lifespan, not serviceable.


Applications:
- 2nd hand units are a cheap upgrade.


Spools and welded differentials

A spool is a solid unit replacing the diff completely. A welded diff is usually an open differential which is welded to act as a solid unit instead. The wheels spin at the same speed in all conditions.


Operation. This type simple joins both driveshafts and wont allow any speed differences.




Usage. Spools are to some extent used in drag racing and quarter mile applications where turning and cornering are not a concern. Thanks to it's low (or free) price, this type is also used in low-cost race cars where budget needs to be saved. The benefit in drag racing is that power is in all situations applied evenly to both wheels, reducing the possibility of spinning and powersteering.


Road use. The road-legality of such units should be checked because in some countries and/or some year ranges, the differential is tested to allow spin in different directions.


Oils. Use the recommended oil for the original differential in the car.


Pros:
- Low cost, or even free in case of a welded unit.
- 100 % in all situations.
- A spool can be made very light-weight.


Cons:
- A 100 % lock unit is very hard to turn, and causes problems on the streets. In higher speeds makes the car very oversteerable.
- May not be road legal.


Applications:
- Quartermile and dragracing.
- Low-cost budget racers and funmachines.


Conclusion

This was a quick glimpse on the different locking differentials. These are only basic types, and most brands have their own variations with different characteristics and technical innovations. The same principles apply in most cases however.


To sum things up, we suggest a torsen for street use, clutch-type for competition, and spools or welded units for having fun or racing in a straight line. Viscous units are a cheap OEM upgrade, if available.


The guide is for general purposes only. Please use oils and other specifications based on the manufacturers recommendations.


We hope the article managed to shed light on the different types, their pros & cons and operation. If you want to know more, our customer support is happy to help you out. -Wille


All differentials we carry can be found here:
Quaife limited slip differentials
Cusco limited slip differentials
Gripper limited slip differentials
Kaaz limited slip differentials
All differentials & their parts

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