Also known as Clutch type and Plate type.
The clutch type has a stack of thin clutch-discs, half of which are coupled to the axles, the other half of which are coupled to the LSD housing. The number of discs used by the LSD directly affects the durability of the LSD, and how much you can adjust it (i.e The greater the number of discs, the more adjustability on offer)
If the option is available (dependent on the LSD), the clutch-discs can be re-stacked; in effect, activating/deactivating the outer clutch-discs (the discs coupled to the LSD housing), thus increasing/decreasing the lock capacity.
A very common misconception is that changing the number of clutch-discs will change the % of lock. For example, there is no such thing as "70% lock"
; all LSD's will lock 100%. What the % really means is the torque threshold of that particular LSD (i.e The Lock Capacity), so if the clutch-discs were re-stacked to 70% of the lock capacity of the LSD, if you were to go over that threshold, the clutch-discs will start slipping. This is why the best aftermarket LSD's on the market have a high number of clutch-discs, as not only does this allow greater adjustability, it also increases the durability of the LSD, thus allowing a slip-less lock in all driving conditions. Be wary of manufacturers marketing LSD's based on "lock %"
without offering further explanation of their choice of wording, as there is no such thing; the term "lock"
is not analogue; the discs are either locked, or they are slipping. "% lock"
is a contradiction.
Also known as Initial Torque, or Breakaway Torque, is essentially the amount of torque required by the LSD "before"
it will start slipping i.e The higher the initial torque, the longer the LSD will be "locked"
during low speed manoeuvring. This is generally why it is preferable to have a lower initial torque, especially if a Salisbury type LSD is to be used on a street car.
If the option is available (dependent on the LSD), Pre-Load may be adjusted to the drivers specific requirements:
- Cone Type: The most common method for setting Pre-Load is via the use of conical washers, typically one on either side of the LSD housing. Although economical to produce, this type of Pre-Load system is archaic and is like an on/off switch. Not only do most default setups have a high initial torque (80lbft+), unless spare washers are included, this type of LSD is generally non-adjustable to the end user.
- Spring type: The preferred method for setting Pre-Load is via the use of coil-springs, which allows for a much smoother and progressive transition from locked to unlocked. Most default setups have a low initial torque (50lbft+) which helps to alleviate some of the issues encountered during low speed manoeuvring and are easily adjusted simply by removing/adding coil-springs. The only downside of this type of Pre-Load system is the higher cost involved in manufacturing.
One method for creating the clamping force on the clutch-discs is the use of a cam/ramp assembly such as used in a Salisbury type LSD. The spider gears mount on the cross-pin 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 triangular (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). By changing the ramp angles on the cam, the driver can adjust the aggressiveness of the LSD.
An alternative is to use the natural separation force of the gear teeth to load the clutch-discs (e.g OEM and OEM-Based BMW LSD's)
As the input torque of the driveshaft tries to turn the differential center, internal pressure rings (adjoining the clutch-discs) are forced sideways by the cross-pin trying to climb the ramp, which compresses the clutch-discs. The more the clutch-discs are compressed, the more coupled the wheels are. The mating of the vertical ramp surfaces in a one-way LSD on overrun produces no cam effect or corresponding clutch-disc compression.
2-Way, 1.5-Way and 1-Way:
Broadly speaking, there are three input torque states: load, no load, and over run. During load conditions, 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.
- A 2-way differential will have the same limiting torque in both the forward and reverse directions. This means the differential will provide some level of limiting under engine braking.
- A 1.5-way differential refers to one where the forward and reverse limiting torque 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.
- 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. This is the preferred setup for a FWD vehicle.
- Due to the "locking" nature of a Salisbury type LSD, unless the aggressiveness of the LSD is very low (i.e OEM and OEM-Based BMW LSD's), they are generally unsuitable for a street car. Not only can they be very difficult with low speed manoeuvring (the locking/unlocking of the clutch-discs can cause jerkiness), the slipping clutch-discs are also the cause of the infamous "chatter" noise, which can make your street car sound like a train in the parking lot!
- However, for a race application, the ability to have a slip-less lock during extreme driving conditions (Auto-X, Tight Circuits, Rally etc) as well as precise adjustability to match both the driving style and the type of racing more than makes up for the "inconveniences" experienced by a street car.
- Parts: As a wearable part, clutch-discs will need to be replaced when worn. This can be determined by testing the Initial Torque of the LSD. Typically, if the value is under 50lbft's on a Pre-Loaded LSD, this would signal worn clutch-discs. If a Coil-Spring Pre-Load system is used, the coil-springs are also a wearable part, and should always be replaced at the same time as replacing the clutch-discs.
- Oil: Regular 75w90 Synthetic Oil for Street/Spirited use. Heavier 75w140 for Race Applications. Recommended brands include Torco SGO and Redline. Contrary to popular belief, there is no such thing as "Special LSD Oil"; this is just marketing, typically by the LSD manufacturer trying to sell their own-brand Oil. By using one of the recommended oils above, not only are you saving over the premium prices commanded by the "Special LSD Oil", your LSD will also perform better as you will not have a tonne of friction modifier pre-added to your oil (the higher the amount of friction modifier, the lower the torque threshold of the LSD). If you find the "chatter" too much, you can always add additional Friction Modifier (we recommend Torco Type-F) to "fine-tune" the LSD to your requirements.
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