Although there is a lot of information scattered around online regarding Limited Slip Differentials, sometimes it can be very hard to find answers to specific questions, so we thought it would be a good idea to put together this FAQ and have all the common questions (and answers) in one place :)
Credits
What is an LSD and why you need one?
A limited-slip differential (LSD) is a type of automotive differential gear arrangement that allows for some difference in angular velocity of the output shafts, but imposes a mechanical bound on the disparity.
In an automobile, such limited-slip differentials are sometimes used in place of a standard open differential, where they convey certain dynamic advantages, at the expense of greater complexity.
The main advantage of a limited-slip differential is demonstrated by considering the case of a standard open differential in off-roading or snow situations where one wheel begins to slip or lose contact with the ground. In such a case with a standard open differential, the slipping or non-contacting wheel will receive the majority of the power, while the contacting wheel will remain stationary with the ground. The torque transmitted will be equal at both wheels, and therefore, will not exceed the threshold of torque needed to move the wheel with traction. In this situation, a limited-slip differential prevents excessive power from being allocated to one wheel, and thereby keeping both wheels in powered rotation. The advantages of LSD in high-power, rear wheel drive automobiles were demonstrated during the United States "Muscle-Car" era from the mid 1960s through the early 1970s. It soon became apparent that "Muscle-Cars" with LSD or "posi" were at a distinct advantage to their wheel-spinning counterparts.
How does an LSD work?
Automotive limited-slip differentials all contain a few basic elements. First, all have a gear train that, like an open differential, allows the output shafts to spin at different speeds while holding the sum of their speeds proportional to that of the input shaft.
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 Torsen style and Salisbury style LSDs. The amount of limiting torque provided by these mechanisms varies by design and is discussed later in this FAQ.
Types of LSD (There are more, but the 2 main types are covered here):
Credits
- Wikipedia: Rather than having to retype a lot of the general "how it works" information, some technically-correct information has been carried over from Wikipedia
- Youtube: Source of some great technical videos
What is an LSD and why you need one?
A limited-slip differential (LSD) is a type of automotive differential gear arrangement that allows for some difference in angular velocity of the output shafts, but imposes a mechanical bound on the disparity.
In an automobile, such limited-slip differentials are sometimes used in place of a standard open differential, where they convey certain dynamic advantages, at the expense of greater complexity.
The main advantage of a limited-slip differential is demonstrated by considering the case of a standard open differential in off-roading or snow situations where one wheel begins to slip or lose contact with the ground. In such a case with a standard open differential, the slipping or non-contacting wheel will receive the majority of the power, while the contacting wheel will remain stationary with the ground. The torque transmitted will be equal at both wheels, and therefore, will not exceed the threshold of torque needed to move the wheel with traction. In this situation, a limited-slip differential prevents excessive power from being allocated to one wheel, and thereby keeping both wheels in powered rotation. The advantages of LSD in high-power, rear wheel drive automobiles were demonstrated during the United States "Muscle-Car" era from the mid 1960s through the early 1970s. It soon became apparent that "Muscle-Cars" with LSD or "posi" were at a distinct advantage to their wheel-spinning counterparts.
How does an LSD work?
Automotive limited-slip differentials all contain a few basic elements. First, all have a gear train that, like an open differential, allows the output shafts to spin at different speeds while holding the sum of their speeds proportional to that of the input shaft.
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 Torsen style and Salisbury style LSDs. The amount of limiting torque provided by these mechanisms varies by design and is discussed later in this FAQ.
Types of LSD (There are more, but the 2 main types are covered here):
Also known as Helical type, Gear type, Worm type and Torque Biasing type.
Geared, torque-sensitive mechanical limited-slip differentials use worm gears and spur gears to distribute and differentiate input power between two drive wheels or front and back axles. This is a completely separate design from the most common beveled spider gear designs seen in most automotive applications. As torque is applied to the gears, they are pushed against the walls of the differential housing, creating friction. The friction resists the relative movement of the outputs and creates the limiting torque.
Unlike other friction-based LSD designs that combine a common spider gear "open" differential in combination with friction materials that inhibit differentiation (e.g Salisbury type), the torque sensing design is a unique type of differential, with torque bias inherent to its design, not as an add-on. Torque bias is only applied when needed, and does not inhibit differentiation. The result is a true differential that does not bind up like a locking type LSD, but still gives increased power delivery under many road conditions.
The amount of torque bias is generally adjusted by changing the angle of the teeth on the worm gears; with the higher torque bias offering a higher amount of torque transfer to the wheel with traction
e.g If there is a maximum 66% torque bias (your typical Torsen type LSD), this will allow 66% of the output torque to be transferred to the wheel with traction, whereas the spinning wheel will only receive 34% of the output torque. When driving in a straight line, the torque split will generally be 50%/50%.
- Application
- As Torsen type LSD's are gear-based and do not rely on “lock” to function, they are smooth in operation and noise-free. In addition, due to not having wearable parts (i.e clutch-discs), they are also maintenance free.
- Because of this, Torsen type LSD's are generally the best choice for a "streetable LSD" (under normal driving conditions, most would not even notice the LSD is there), yet still offer a good limited-slip experience under spirited driving and/or race conditions.
- Maintenance
- Parts: None required
- Oil: Regular 75w90 Synthetic Oil for Street/Spirited use. Heavier 75w140 for Race Applications. Recommended brands include Torco SGO and Redline NS. Limited-slip Friction Modifier must NOT be used, as Torsen style LSD's rely on friction in order to function.
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.
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.
Pre-Load: 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.
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.
- Application
- 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.
- Maintenance
- 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.
MFactory
CTE Carbon Composites
INFINITUDE
Swift Springs
Synchrotech