MFactory
04-23-2015, 09:10 PM
Mods: Would it be possible to make this a sticky please?
- Limited Slip Differential FAQ -
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 I thought it would be a good idea to put together this FAQ and have all the common questions (and answers) in one place :)
This FAQ will be constantly revised/updated. If you would like to see something added that is not here, please let me know.
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
Please Note: This is not a sales thread, so I cannot answer any sales-specific questions. Product MSRP's will be listed below, where relevant
- 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.
Differential Video (Torsen type):
http://www.youtube.com/watch?v=JEiSTzK-A2A
Differential Video (Salisbury type):
http://www.youtube.com/watch?v=WeLm7wHvdxQ
- Types of LSD (There are more, but the 2 main types are covered here) -
Torsen Type:
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: Non-applicable for MFactory and Quaife. Wavetrac has replaceable friction-discs that can wear out.
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.
Common Brands:
MFactory® Helical LSD™:
Availability - since 2008
Country of R&D - USA
Country of Manufacture – Taiwan (In-house)
Quality Control – ISO Regulated
Metallurgy – Housings are open-die forged from SAE4320 steel. Internals are cold-forged from SAE9310 steel
Origin of Barstock - Japan
Heat-Treatment – Double Tempered, Super Sub-Zero (Deep Cryogenic), Micro-Peened Surface
Bias Ratio (Max Torque Transfer): 75%
Warranty: Lifetime, Globally Transferable
Guarantee: 30day Money Back
MSRP (LSD only): $999.95
MSRP (Reconditioned Pumpkin): from $1499.95
Quaife® Automatic Torque-Biasing (ATB®) Differential:
Availability – circa 1980's
Country of R&D - UK
Country of Manufacture – UK (In-house)
Quality Control – ISO Regulated
Metallurgy – Housings are milled from EN36A steel. Internals are milled from EN36A steel
Origin of Barstock - India
Heat-Treatment – Standard
Bias Ratio (Max Torque Transfer): 66%
Warranty: Lifetime, Region-based
Guarantee: N/A
MSRP (LSD only): circa $1500 (All Authorised LSD's are imported from Birds Auto in the UK)
MSRP (Reconditioned Pumpkin): from circa $2300
Wavetrac® Differential:
Availability – since 2008
Country of R&D - USA
Country of Manufacture – USA (Outsourced)
Quality Control – Standard
Metallurgy – Housings are milled from SAE8620 steel. Internals are milled from SAE9310 steel
Origin of Barstock - Unknown origin (TBC) as manufacturing is outsourced
Heat-Treatment – Standard
Bias Ratio (Max Torque Transfer): 66%
Warranty: Lifetime, Region-based
Guarantee: N/A
MSRP (LSD only): $1295.95
MSRP (Reconditioned Pumpkin): from circa $2200
Salisbury Type:
Also known as Clutch type and Plate type.
Lock Capacity
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 (dependant 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, 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 slipless 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. This 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 (dependant 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.
Aggressiveness
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 crosspin 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 crosspin 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 slipless 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.
Common Brands (Note: there are far more available than this very short list):
MFactory® Metal Plate LSD™:
Availability - since 2008
Country of R&D - USA
Country of Manufacture – Taiwan (In-house)
Quality Control – ISO Regulated
Metallurgy – Housings are open-die forged from SAE4320 steel. Internals are cold-forged from SAE9310 steel
Origin of Barstock - Japan
Heat-Treatment – Double Tempered, Super Sub-Zero (Deep Cryogenic), Micro-Peened Surface
Lock Capacity: 20 clutch-disc setup, allowing an adjustable 20/40/60/80/100% Lock Capacity
Pre-Load: Spring type. Adjustable between 0-100% Pre-Load (~85lbfts)
Aggressiveness: Variable Cam with adjustable ramp angles
Warranty: Lifetime, Globally Transferable (excluding wear parts)
Guarantee: 30day Money Back
MSRP (LSD only): $999.95
MSRP (Reconditioned Pumpkin): from $1499.95
OS Giken® TCD LSD:
Availability - TBC
Country of R&D - Japan
Country of Manufacture – Japan (In-house)
Quality Control – Standard
Metallurgy – TBC
Origin of Barstock - Japan
Heat-Treatment – Standard
Lock Capacity: 20 clutch-disc setup, allowing an adjustable 20/40/60/80/100% Lock Capacity
Pre-Load: Spring type. Adjustable between 0-100% Pre-Load (~85lbfts)
Aggressiveness: Fixed Cam, Non-adjustable (Additional Cams may be available)
Warranty: 1-Year, Region-based
Guarantee: N/A
MSRP (LSD only): circa $2800
MSRP (Reconditioned Pumpkin): from $4100
Kaaz® LSD:
Availability - TBC
Country of R&D – Japan (USA for Super-Q)
Country of Manufacture – Taiwan (Outsourced)
Quality Control – Standard
Metallurgy – Housings are open-die forged from SAE8620 steel. Internals are cold-forged from SAE8620 steel
Origin of Barstock - Taiwan
Heat-Treatment – Standard. WPC Treated Plates (Super-Q)
Lock Capacity: 12 clutch-disc setup, allowing an adjustable 33/66/100% Lock Capacity
Pre-Load: Cone type. Non-adjustable
Aggressiveness: Fixed Cam, Non-adjustable (Additional Cams may be available)
Warranty: 1-Year, Region-based
Guarantee: N/A
MSRP (LSD only): $1695
MSRP (Reconditioned Pumpkin): from $3000
Drexler®:
Availability – circa 1980's
Country of R&D - Germany
Country of Manufacture – Germany (In-house)
Quality Control – ISO Regulated
Metallurgy – Housings are milled from EN36A steel. Internals are milled from EN36A steel
Origin of Barstock - TBC
Heat-Treatment – Standard. Moly-Coated Plates
Lock Capacity: Standard 16 clutch-disc setup, allowing an adjustable 25/50/75/100% Lock Capacity (Custom setup can be requested)
Pre-Load: Standard Zero Pre-load (Adjustable setup available)
Aggressiveness: Fixed Cam, Non-adjustable (Additional Cams available)
Warranty: TBC
Guarantee: N/A
MSRP (LSD only): Made-to-Spec. ~$3000+
MSRP (Reconditioned Pumpkin): Made-to-Spec. ~$3000+
- Terminology -
Differential – This refers to the differential unit (i.e The LSD or Open Differential)
Final Drive – This refers to the Ring Gear & Pinion. Divide the number of teeth on the Ring Gear by the number of teeth on the Pinion, and this gives the Final Drive ratio
Carrier/Pumpkin – This refers to the entire rear assembly which houses the Differential Unit and Final Drive
- Fitments -
- E46 325i/328i/330i, Manual/Automatic, 1998-2006. Small Bolted Diff (Various Ratios)
- E60 525i/528i/530i/535i, Manual/Automatic, 2003-2010. Small Bolted Diff (Various Ratios)
- E60 535i/540i/545i/550i, Manual/Automatic, 2003-2010. Large Welded 3.08 Diff*
- E8X 135i, Manual, All Production Years. Large Welded 3.08 Diff*
- E8X 135i, Automatic, All Production Years. Small Bolted 3.46 Diff
- E8X 135i, DCT, All Production Years. Large Welded 2.56 Diff*
- E8X Z4 35i, Manual, All Production Years. Large Welded 3.08 Diff*
- E8X Z4 35i, DCT, All Production Years. Large Welded 2.56 Diff*
- E9X 335i, Manual, Pre March 2007. Large Bolted 3.08 Diff
- E9X 335i, Manual, March 2007+. Large Welded 3.08 Diff*
- E9X 335i, Automatic, Pre August 2007. Large Bolted 3.46 Diff
- E9X 335i, Automatic, August 2007+. Small Bolted 3.46 Diff
- E9X 335i, DCT, All Production Years. Large Welded 2.56 Diff*
- E9X 335d, Automatic, Pre March 2007. Large Bolted 2.81 Diff
- E9X 335d, Automatic, March 2007+. Large Welded 2.81 Diff*
- F2X M135i/M235i, Manual, 2012+. Large Welded 3.08 Diff*
- F2X M135i/M235i, Automatic, 2012+. Small Welded 3.15 Diff*
- F3X 335i/435i, Manual, 2012+. Large Welded 3.08 Diff*
- F3X 335i/435i, Automatic, 2012+. Small Welded 3.15 Diff*
*As of 2007 onwards, on the higher-end models, BMW decided to start welding the Ring Gears onto the Differential, instead of using bolts (less material = cheaper to manufacture). Because of this, in order to install an LSD, you only have two options:
1) Machine the Ring Gear off the OEM Open Differential, then do further machine work to the Ring Gear to turn it into a bolted setup. A spacer is generally required due to the thinner Ring Gear
2) Purchase a Final Drive that is already a bolted setup. This could be an OEM Final Drive, or an aftermarket Final Drive:
OEM: Used - Prices range from $400+ depending on ratio
BMW Motorsport: New - Prices range from $1100+ depending on ratio
MFactory: New - 2.56, 2.81 and 3.08 ratios available. MSRP: $799.95
Quaife: TBC (AFAIK, only available when purchased as a package with their LSD)
Drexler: TBC
- Installation Specs -
- Measure back lash and bearing preload and mark the OEM C-Clips prior to removal so they can be reinstalled on the same side during reassembly
- Bolt ring gear to LSD @ 80 ft. lbs. torque (E92 LSD) or 70 ft. lbs. (E46 LSD)
- Press bearing races on LSD
- Install LSD with bearings into housing using the OEM C-Clips
- Measure backlash (0.003” – 0.009” acceptable)
- Measure bearing preload (2-10 inch lbs. acceptable)
- Check with marking compound for acceptable gear pattern
http://www.teammfactory.com/images/other/diffinstall.jpg
If backlash needs adjustment, install shims under bearing races with thinner replacement C-Clip. Note: the (A+B+C+D) shims + C-Clip total thickness should equal the thickness of the old C-Clips (A+D) being replaced (+/- 0.002)
- To increase backlash add shim to position “B” with thinner C-Clip “D”
- To decrease backlash add shim to position “C” with thinner C-Clip “A”
*Please contact your LSD Manufacturer for replacement Bearings & Shim Kits
- Common Problems -
Diff Thump:
The two main causes of "thump", and both have to do with free play:
1) Worn or misaligned (during reinstallation) centre bearing of the propshaft. The only solution for this is to check the centre bearing, and replace if necessary.
2) Backlash. All LSD internals have free play in them (i.e backlash). This free play is a necessity because steel expands when heated. If there was no free play in the gears, they would end up welding themselves together due to the friction, and because of this, there is a compound effect i.e the backlash between all of the internal gears of the LSD (at least 12 of them for most Helical LSD's) plus the backlash of the Ring & Pinion.
What this means is that there is a small "lag" between turning the propshaft, and the axles locking (i.e turning), which results in the "thump" noise you are hearing. It is not noise from the diff that you are hearing, but noise due to the vibration through the chassis and the propshaft/axles locking into place. This is also why it is normally louder during hard acceleration, especially so if you've upgraded any chassis components (mounts, diff bushings, subframe bushings, braces, suspension etc) or upgraded your clutch.
There is no full solution for this, however, warming the diff up (thus reducing the overall backlash) will quieten it down slightly, as will using thicker gear oil. Baby'ing the clutch will also reduce it a lot, however, this will also increase the wear on your clutch.
When stationary and in neutral, your transmission gears are still turning, so when you put it into gear, this locks the gears together causing the propshaft to turn very slightly which causes the "thump" due to the free play between the propshaft turning and the axles turning. As the axles are basically locked into position and can't move due to not enough power being applied (i.e clutch is still disengaged), this is why the thump (i.e vibration) is more noticeable when putting it into gear from stationary, compared to changing gear when moving.
Most quality LSD's will be machined precisely to specification, based on a new set of "unworn" axle stubs i.e if new axle stubs are used, there should be very limited play between the meshing of the splines (there will be a little due to the fact that they are not press fit and just freely slide in and out). If old axle stubs are used, then there will be slightly more play due to wear/compression, and this will contribute to the overall backlash, thus the "thump".
Remember, the thump is not "noise" per se, it is vibration through the chassis.
The above is applicable to ALL LSD's, regardless of type.
No-Load Issue (Torsen type LSD's only):
A common misconception is that a Torsen type LSD is useless under a no-load situation (e.g One wheel in the air). This misconception has been further perpetuated by the marketing strategies of some LSD Manufacturers. Whilst there is truth to this theory, the theory only really applies to vehicles with no Traction Control (i.e This no-load situation does not apply to the majority of BMW users. For the racer who prefers to switch off their Traction Control, there is a simple solution to the problem; simply balance left-foot braking with right-foot throttle control). In any modern car with traction control, if one wheel does lift off the ground, the traction control will kick in by providing load to the spinning wheel (normally via braking), thus the LSD will continue transferring torque to the wheel with traction.
- Footnotes -
As this is an FAQ open-to-all, all feedback and suggestions are appreciated. The sole purpose of this FAQ is to provide the community with a one-stop resource to openly discuss the technical aspects of a Limited-slip Differential.
Whilst I have tried to be as technically-correct as possible with the above information, there may be some topics open to debate and/or variations of the explanations, especially on the more technical aspects being discussed here. I am the first to acknowledge and respect other peoples opinions and, if shown to be incorrect, I can and will update/revise this FAQ.
Please respect other peoples opinions, and do not troll in this thread.
- Limited Slip Differential FAQ -
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 I thought it would be a good idea to put together this FAQ and have all the common questions (and answers) in one place :)
This FAQ will be constantly revised/updated. If you would like to see something added that is not here, please let me know.
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
Please Note: This is not a sales thread, so I cannot answer any sales-specific questions. Product MSRP's will be listed below, where relevant
- 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.
Differential Video (Torsen type):
http://www.youtube.com/watch?v=JEiSTzK-A2A
Differential Video (Salisbury type):
http://www.youtube.com/watch?v=WeLm7wHvdxQ
- Types of LSD (There are more, but the 2 main types are covered here) -
Torsen Type:
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: Non-applicable for MFactory and Quaife. Wavetrac has replaceable friction-discs that can wear out.
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.
Common Brands:
MFactory® Helical LSD™:
Availability - since 2008
Country of R&D - USA
Country of Manufacture – Taiwan (In-house)
Quality Control – ISO Regulated
Metallurgy – Housings are open-die forged from SAE4320 steel. Internals are cold-forged from SAE9310 steel
Origin of Barstock - Japan
Heat-Treatment – Double Tempered, Super Sub-Zero (Deep Cryogenic), Micro-Peened Surface
Bias Ratio (Max Torque Transfer): 75%
Warranty: Lifetime, Globally Transferable
Guarantee: 30day Money Back
MSRP (LSD only): $999.95
MSRP (Reconditioned Pumpkin): from $1499.95
Quaife® Automatic Torque-Biasing (ATB®) Differential:
Availability – circa 1980's
Country of R&D - UK
Country of Manufacture – UK (In-house)
Quality Control – ISO Regulated
Metallurgy – Housings are milled from EN36A steel. Internals are milled from EN36A steel
Origin of Barstock - India
Heat-Treatment – Standard
Bias Ratio (Max Torque Transfer): 66%
Warranty: Lifetime, Region-based
Guarantee: N/A
MSRP (LSD only): circa $1500 (All Authorised LSD's are imported from Birds Auto in the UK)
MSRP (Reconditioned Pumpkin): from circa $2300
Wavetrac® Differential:
Availability – since 2008
Country of R&D - USA
Country of Manufacture – USA (Outsourced)
Quality Control – Standard
Metallurgy – Housings are milled from SAE8620 steel. Internals are milled from SAE9310 steel
Origin of Barstock - Unknown origin (TBC) as manufacturing is outsourced
Heat-Treatment – Standard
Bias Ratio (Max Torque Transfer): 66%
Warranty: Lifetime, Region-based
Guarantee: N/A
MSRP (LSD only): $1295.95
MSRP (Reconditioned Pumpkin): from circa $2200
Salisbury Type:
Also known as Clutch type and Plate type.
Lock Capacity
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 (dependant 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, 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 slipless 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. This 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 (dependant 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.
Aggressiveness
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 crosspin 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 crosspin 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 slipless 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.
Common Brands (Note: there are far more available than this very short list):
MFactory® Metal Plate LSD™:
Availability - since 2008
Country of R&D - USA
Country of Manufacture – Taiwan (In-house)
Quality Control – ISO Regulated
Metallurgy – Housings are open-die forged from SAE4320 steel. Internals are cold-forged from SAE9310 steel
Origin of Barstock - Japan
Heat-Treatment – Double Tempered, Super Sub-Zero (Deep Cryogenic), Micro-Peened Surface
Lock Capacity: 20 clutch-disc setup, allowing an adjustable 20/40/60/80/100% Lock Capacity
Pre-Load: Spring type. Adjustable between 0-100% Pre-Load (~85lbfts)
Aggressiveness: Variable Cam with adjustable ramp angles
Warranty: Lifetime, Globally Transferable (excluding wear parts)
Guarantee: 30day Money Back
MSRP (LSD only): $999.95
MSRP (Reconditioned Pumpkin): from $1499.95
OS Giken® TCD LSD:
Availability - TBC
Country of R&D - Japan
Country of Manufacture – Japan (In-house)
Quality Control – Standard
Metallurgy – TBC
Origin of Barstock - Japan
Heat-Treatment – Standard
Lock Capacity: 20 clutch-disc setup, allowing an adjustable 20/40/60/80/100% Lock Capacity
Pre-Load: Spring type. Adjustable between 0-100% Pre-Load (~85lbfts)
Aggressiveness: Fixed Cam, Non-adjustable (Additional Cams may be available)
Warranty: 1-Year, Region-based
Guarantee: N/A
MSRP (LSD only): circa $2800
MSRP (Reconditioned Pumpkin): from $4100
Kaaz® LSD:
Availability - TBC
Country of R&D – Japan (USA for Super-Q)
Country of Manufacture – Taiwan (Outsourced)
Quality Control – Standard
Metallurgy – Housings are open-die forged from SAE8620 steel. Internals are cold-forged from SAE8620 steel
Origin of Barstock - Taiwan
Heat-Treatment – Standard. WPC Treated Plates (Super-Q)
Lock Capacity: 12 clutch-disc setup, allowing an adjustable 33/66/100% Lock Capacity
Pre-Load: Cone type. Non-adjustable
Aggressiveness: Fixed Cam, Non-adjustable (Additional Cams may be available)
Warranty: 1-Year, Region-based
Guarantee: N/A
MSRP (LSD only): $1695
MSRP (Reconditioned Pumpkin): from $3000
Drexler®:
Availability – circa 1980's
Country of R&D - Germany
Country of Manufacture – Germany (In-house)
Quality Control – ISO Regulated
Metallurgy – Housings are milled from EN36A steel. Internals are milled from EN36A steel
Origin of Barstock - TBC
Heat-Treatment – Standard. Moly-Coated Plates
Lock Capacity: Standard 16 clutch-disc setup, allowing an adjustable 25/50/75/100% Lock Capacity (Custom setup can be requested)
Pre-Load: Standard Zero Pre-load (Adjustable setup available)
Aggressiveness: Fixed Cam, Non-adjustable (Additional Cams available)
Warranty: TBC
Guarantee: N/A
MSRP (LSD only): Made-to-Spec. ~$3000+
MSRP (Reconditioned Pumpkin): Made-to-Spec. ~$3000+
- Terminology -
Differential – This refers to the differential unit (i.e The LSD or Open Differential)
Final Drive – This refers to the Ring Gear & Pinion. Divide the number of teeth on the Ring Gear by the number of teeth on the Pinion, and this gives the Final Drive ratio
Carrier/Pumpkin – This refers to the entire rear assembly which houses the Differential Unit and Final Drive
- Fitments -
- E46 325i/328i/330i, Manual/Automatic, 1998-2006. Small Bolted Diff (Various Ratios)
- E60 525i/528i/530i/535i, Manual/Automatic, 2003-2010. Small Bolted Diff (Various Ratios)
- E60 535i/540i/545i/550i, Manual/Automatic, 2003-2010. Large Welded 3.08 Diff*
- E8X 135i, Manual, All Production Years. Large Welded 3.08 Diff*
- E8X 135i, Automatic, All Production Years. Small Bolted 3.46 Diff
- E8X 135i, DCT, All Production Years. Large Welded 2.56 Diff*
- E8X Z4 35i, Manual, All Production Years. Large Welded 3.08 Diff*
- E8X Z4 35i, DCT, All Production Years. Large Welded 2.56 Diff*
- E9X 335i, Manual, Pre March 2007. Large Bolted 3.08 Diff
- E9X 335i, Manual, March 2007+. Large Welded 3.08 Diff*
- E9X 335i, Automatic, Pre August 2007. Large Bolted 3.46 Diff
- E9X 335i, Automatic, August 2007+. Small Bolted 3.46 Diff
- E9X 335i, DCT, All Production Years. Large Welded 2.56 Diff*
- E9X 335d, Automatic, Pre March 2007. Large Bolted 2.81 Diff
- E9X 335d, Automatic, March 2007+. Large Welded 2.81 Diff*
- F2X M135i/M235i, Manual, 2012+. Large Welded 3.08 Diff*
- F2X M135i/M235i, Automatic, 2012+. Small Welded 3.15 Diff*
- F3X 335i/435i, Manual, 2012+. Large Welded 3.08 Diff*
- F3X 335i/435i, Automatic, 2012+. Small Welded 3.15 Diff*
*As of 2007 onwards, on the higher-end models, BMW decided to start welding the Ring Gears onto the Differential, instead of using bolts (less material = cheaper to manufacture). Because of this, in order to install an LSD, you only have two options:
1) Machine the Ring Gear off the OEM Open Differential, then do further machine work to the Ring Gear to turn it into a bolted setup. A spacer is generally required due to the thinner Ring Gear
2) Purchase a Final Drive that is already a bolted setup. This could be an OEM Final Drive, or an aftermarket Final Drive:
OEM: Used - Prices range from $400+ depending on ratio
BMW Motorsport: New - Prices range from $1100+ depending on ratio
MFactory: New - 2.56, 2.81 and 3.08 ratios available. MSRP: $799.95
Quaife: TBC (AFAIK, only available when purchased as a package with their LSD)
Drexler: TBC
- Installation Specs -
- Measure back lash and bearing preload and mark the OEM C-Clips prior to removal so they can be reinstalled on the same side during reassembly
- Bolt ring gear to LSD @ 80 ft. lbs. torque (E92 LSD) or 70 ft. lbs. (E46 LSD)
- Press bearing races on LSD
- Install LSD with bearings into housing using the OEM C-Clips
- Measure backlash (0.003” – 0.009” acceptable)
- Measure bearing preload (2-10 inch lbs. acceptable)
- Check with marking compound for acceptable gear pattern
http://www.teammfactory.com/images/other/diffinstall.jpg
If backlash needs adjustment, install shims under bearing races with thinner replacement C-Clip. Note: the (A+B+C+D) shims + C-Clip total thickness should equal the thickness of the old C-Clips (A+D) being replaced (+/- 0.002)
- To increase backlash add shim to position “B” with thinner C-Clip “D”
- To decrease backlash add shim to position “C” with thinner C-Clip “A”
*Please contact your LSD Manufacturer for replacement Bearings & Shim Kits
- Common Problems -
Diff Thump:
The two main causes of "thump", and both have to do with free play:
1) Worn or misaligned (during reinstallation) centre bearing of the propshaft. The only solution for this is to check the centre bearing, and replace if necessary.
2) Backlash. All LSD internals have free play in them (i.e backlash). This free play is a necessity because steel expands when heated. If there was no free play in the gears, they would end up welding themselves together due to the friction, and because of this, there is a compound effect i.e the backlash between all of the internal gears of the LSD (at least 12 of them for most Helical LSD's) plus the backlash of the Ring & Pinion.
What this means is that there is a small "lag" between turning the propshaft, and the axles locking (i.e turning), which results in the "thump" noise you are hearing. It is not noise from the diff that you are hearing, but noise due to the vibration through the chassis and the propshaft/axles locking into place. This is also why it is normally louder during hard acceleration, especially so if you've upgraded any chassis components (mounts, diff bushings, subframe bushings, braces, suspension etc) or upgraded your clutch.
There is no full solution for this, however, warming the diff up (thus reducing the overall backlash) will quieten it down slightly, as will using thicker gear oil. Baby'ing the clutch will also reduce it a lot, however, this will also increase the wear on your clutch.
When stationary and in neutral, your transmission gears are still turning, so when you put it into gear, this locks the gears together causing the propshaft to turn very slightly which causes the "thump" due to the free play between the propshaft turning and the axles turning. As the axles are basically locked into position and can't move due to not enough power being applied (i.e clutch is still disengaged), this is why the thump (i.e vibration) is more noticeable when putting it into gear from stationary, compared to changing gear when moving.
Most quality LSD's will be machined precisely to specification, based on a new set of "unworn" axle stubs i.e if new axle stubs are used, there should be very limited play between the meshing of the splines (there will be a little due to the fact that they are not press fit and just freely slide in and out). If old axle stubs are used, then there will be slightly more play due to wear/compression, and this will contribute to the overall backlash, thus the "thump".
Remember, the thump is not "noise" per se, it is vibration through the chassis.
The above is applicable to ALL LSD's, regardless of type.
No-Load Issue (Torsen type LSD's only):
A common misconception is that a Torsen type LSD is useless under a no-load situation (e.g One wheel in the air). This misconception has been further perpetuated by the marketing strategies of some LSD Manufacturers. Whilst there is truth to this theory, the theory only really applies to vehicles with no Traction Control (i.e This no-load situation does not apply to the majority of BMW users. For the racer who prefers to switch off their Traction Control, there is a simple solution to the problem; simply balance left-foot braking with right-foot throttle control). In any modern car with traction control, if one wheel does lift off the ground, the traction control will kick in by providing load to the spinning wheel (normally via braking), thus the LSD will continue transferring torque to the wheel with traction.
- Footnotes -
As this is an FAQ open-to-all, all feedback and suggestions are appreciated. The sole purpose of this FAQ is to provide the community with a one-stop resource to openly discuss the technical aspects of a Limited-slip Differential.
Whilst I have tried to be as technically-correct as possible with the above information, there may be some topics open to debate and/or variations of the explanations, especially on the more technical aspects being discussed here. I am the first to acknowledge and respect other peoples opinions and, if shown to be incorrect, I can and will update/revise this FAQ.
Please respect other peoples opinions, and do not troll in this thread.