Types of BMW LSDs Explained
I stock many types of Limited Slip Differentials in various configurations. These are explained, with some common myths explored.
1-Way vs 1.5-Way vs 2-Way
Original fitment into most BMWs is a 2-way LSD with 45 degree ramps. Only a few BMWs had anything different. This configuration yields 25% lockup on coast and drive, which offers a good balance with good predictability. Some wheelspin may occur at times, though it is rare. Good tyres are important, as is rear suspension. These are typically a 2-clutch setup with molybdenum coated steel friction plates. The coating lasts for 100,000Ks or more if looked after correctly. Spinning one wheel will damage the coating permanently and the disc/s will need to be replaced. Usually the thrust plates require re-surfacing as well. In order of preference, adding custom machined 1.5-way thrust plates, adding more clutches in even amounts per side, or increasing pre-load will reduce unwanted wheelspin.
2-Way: A standard 2-way rebuild unit is a great choice for most applications, and a good starting point if you are not sure what type of custom work you would require. A custom 2-way unit with added clutches is best for drift applications. Either standard ramps or more aggressive ramps could be ordered for up to 75% lockup. Breakway torque could be between 75 to 125 ft/lbs.
1.5-Way: A more versatile type is the 1.5-way unit. This LSD starts with 25% lockup on coast, and any amount of lockup on drive, up to 75%. My 1.5-way units are 40% on drive for 2-clutch units, and 75% on drive for 4-clutch units. The 1.5-way gives strong lockup on acceleration and and backs off on coast. Track vehicles benefit from stronger lockup, enabling more aggressive cornering and strong exit speed. The reduced lockup on coast preserves the vehicle's sharp turn-in characteristics. Cornering before the apex is not affected, but cornering exit speeds are improved as the vehicle will be more likely to hold traction. This is a very good choice for most vehicles.
1-Way: 1-way units are typically offered for FWD applications, but are used in certain RWD applications where the sharpest possible turn-in is desired, and no lockup on coast is needed. They have very little lockup on coast (only what is provided by the preload, which should be very little). Tight tracks and hillclimbs are possibilities for a 1-way LSD, though the choice to go 1-way would be made only by an experienced driver. The absense of lockup on coast can make the vehicle less stable at high speeds. Adding some pre-load to a 1-way can increase stability at the expense of some noise and harshness.
There are offerings of modified LSDs online, typically from the US, which are not heat treated. They also have no floating center. The ramps in these units will degrade over time and then the thrust plates will then be a throw-away item (and not available as a spare part). Often I find that these units are noisy, have excessive play, and do not have the specified lockup. My thrust plates are heat treated to 1300F and case hardened to ensure longevity. The Rockwell hardness is similar to the original ramps +/- 5 HRC. My thrust plates will not degrade over time. My LSDs are quiet and have no free-play. My LSDs will survive regular track days and races. I cannot say the same for other units.
Molybdenum VS Steel VS Carbon VS Sintered Bronze
Molybdenum: OEM BMW limited slip differentials are typically a moly coated type clutch or friction plate. These offer strong lockup with a low clutch count. Good results are obtained with 2 to 4 clutches. Moly friction plates are a good performer and are suitable for road and race. These plates will leave a lot of coating material in the oil, especially when new, but it is not considered harmful to other components so long as differential oil servicing occurs. Most OEM units feature oil grooves on the friction plate only, and some (210mm from E36 M3 3.2L) have no groves at all.
Steel: Steel friction plates offer less friction thus require the use of many more plates to obtain similar torque-holding ability of 2-4 moly coated plates. Steel plates can engage smoothly if they feature oil grooves, though some aftermarket units feature completely flat plates which can have some noise or harshness. Japanese-made limited slip differentials often use steel plates. These LSDs cost $2000-3000 and are only available new, as they were never fitted to a BMW as OEM equipment. Performance benefits for most people are not present. Competitive racers and track-day enthusiasts will usually notice improved lap times from the high-end units which we supply e.g. MFactory and OS Giken. Strength is similar, with the weak point of the differential still being the pinion gear teeth and output shafts, and not usually the LSD unit. Steel plates may be grooved, flat, or a combination of both. Grooved plates usually offer smoother engagement and less noise. OS Giken units are often grooved on one side only, and this offers very low noise. Cusco units offer steel plates in both grooved for road and flat for race. MFactory units are usually grooved on both sides, and on both plates.
Bronze: Sintered bronze friction plates are a more expensive aftermarket option with no real benefits over molybdenum. There is not usually a valid reason to fit these plates. It is typically the thrust plate that suffers the damage, and ruins the friction plate. It makes no sense to use an alternative friction plate which is more expensive than OEM and does not do a better job. For this reason Diff Lab mostly uses OEM moly coated discs.
Carbon: Carbon LSDs offer very smooth engagement. Oil servicing is more frequent, due to excessive carbon build-up in the oil. This cannot be avoided. Carbon LSDs were produced by one manufacturer (Carbonetic) which has ceased production. Used items may be available occasionally. They are a good performer, but replacement parts will no longer be available. There is new old stock available for a limited time, but manufacturing has ceased. Carbon plates can chip and become damaged prematurely. Excessive play in the output shafts was a problem with these carbon units too. I do not recommend them for any vehicle, due to the poor machine work inside and tendency for the carbon pucks to break.
Lockup: 25%, 40% 45%, 75%
Ramp angles, internal preload and clutch count all affect the lockup to some degree. Many diff builders claim all sorts of wild lockup numbers. Some are correct, but most are not. Standard sprung units with no modifications to ramp angles are capable of 25%, 45%, and 75% lockup. Typical E30/E36 era units can accomodate 2 friction plates per side for a maximum of 45% lockup. Only the very early 8-bolt capsules can accommodate 3 friction plates per side for 75% lockup. Different designs which utilise 1.5-way modified ramps are capable of any amount of lockup up to 75%. Common 1.5-way configurations might include: 25/40, 45/50, 45/60, 45/75.
The use of shims to increase lockup on a 2-clutch unit is not recommended. Lockup is determined by the ramp angles and clutch count. Shimming an LSD increases pre-load on the plates but has little effect on the dynamic lockup. Pre-load is not lockup. These LSDs are highly stressed, prone to excess heat and will wear prematurely. They often chatter and have harsh engagement. These builds are often called an 80% or 90% lockup. Whatever the case, the lockup (which is actually 25% in a 2 plate setup) will deteriorate over time, unlike a 4-clutch or 1.5-way unit which will retain strong lockup for years, with increased service life. The proper method of increasing lockup according to the original manufacturer specifications is to utilise less steep ramp angles (such as 30 degrees) or add friction plates. Diff Lab units may be shimmed to set the desired pre-load, but never to increase lockup. Cheap shimmed high lockup LSDs are a bad choice, and will not last the test of time. A 1.5-way unit is far superior and offers vastly improved turn-in. Ask yourself why none of the high-end aftermarket manufacturers offer LSDs with massive pre-load. In fact the best aftermarket LSDs such as the MFactory and OS Giken feature very low pre-load, some of which are adjustable too.
2-Clutch, 4-clutch, Porsche Clutch (and the strange 3-Clutch, 5-clutch and 6-clutch) OEM BMW LSDs
2 Clutch: Standard BMW classic LSDs typically have 2 friction plates. Some models utilised 4-clutch and hybrid types like the E46 M3 and later. Certain machining modifications to the capsule and internals facilitates the use of more friction plates.
3 Clutch: The DIY 3-clutch build in a classic LSD is not a good one. Top caps are broken easily if the stack hight exceeds the capability of the carrier. Internal preload may increase to beyond acceptable limits also. Care need to be taken to ensure that the stack height remains the same after components are removed, and new components are added. This setup is half way between an average setup (2-clutch) and an excellent setup (4-clutch). For this reason there seems to be no point to this one.
4 Clutch: The 4-clutch build requires machining of internals. The lockup does not double when the friction area doubles. The effect is less than doubling. There are numerous ways the build can be achieved. The end result can be either sprung or unsprung. It is possible to complete this build with little to no static preload, but strong lockup on drive. More even wear on the friction plates is a benefit of this build, with the unsprung 4-clutch build showing the most even friction plate wear. Lockup is not as as smooth as a 2-clutch unit, but is not described as harsh. This is still a smooth, balanced and predictable build. They can be built with up to 100-120 ft/lbs of breakaway torque, though less is usually better. Generally the factory setup with around 75 ft/lbs suits most vehicles.
A 4-clutch 1.5-way build is possible, and offers any amount of lockup on drive from 45% up to 75%. This is quite aggressive and not recommended for most people. It would be an ideal LSD for a drift car or rally car. Most track cars should utilise less pre-load and lockup around 45%.
5 Clutch: 5 clutch builds are a strange one offered by some diff builders. They are not specified by the manufacturer. Utilising 1.5-way ramps in a 2 or 4 plate unit is a better option. If the driver believes more lockup is needed beyond a 4-clutch unit with 60% lockup on drive, there may be something wrong with the setup of the vehicle. A balanced LSD configuration which provides just enough lockup to control wheelspin, without affecting turn-in is usually best. More is not usually better.
6 Clutch: 6 clutch builds are specified by the manufacturer but with 45 degree ramps only. Lockup is 75% on coast and drive. There is no specification for a 6 clutch unit with 30 degree ramps. There once was an aftermarket manufacturer of 1mm plates which enabled a 6 clutch or 8 clutch build. These units would have been a big failure on the track. They do not seem to be available any more. Although this build is actually specified by the manufacturer, it still falls into the strange category because there are few valid reasons to assemble one. If you want high lockup, you can achieve this much more easily with 4 clutches and modified ramps. You also have the benefit of being able to machine custom ramps to give anywhere from 45% to 75% lockup independently on drive and coast. It would have smoother engagement and decreased breakaway torque. This is a much better option than chasing more clutch plates.
Porsche 2.4mm Clutch: This is a very bad idea. These discs are oversize and can cause damage to the top cap. The BMW LSDs are designed to have 2.0mm discs, and are built accordingly. If you increase the stack height by 0.8mm bad things will happen.
These are made by MFactory, Eaton, Quaife, Zexel, Wavetrac and others. The internal designs are all similar, save for a few minor differences. They bias torque mechanically, smoothly, and with good predictable lockup. If one wheel loses traction though, all lockup may be lost and excessive wheelspin may occur. Maintanence is minimal. Oil changes are typically all that is required. Diff Lab stocks MFactory helical units and they are a good thing. Diff Lab stocks Zexel Torsen units occasionally. They are in short supply. Despite their limitations they are a great LSD option which is quiet, smooth and requires little maintenance. They are OEM equipment in lots of vehicles for a good reason: they work well and last forever. They also handle lots of torque and offer good longevity. Customer satisfaction with helical units is very good for road applications. Most find that a MFactory or Wavetrac helical gives much more control on the road, with standard vehicles. Modified vehicles would benefit from a clutch type unit. Race vehicles should only consider a clutch type. There are no helical LSDs suitable for a BMW in a race environment, save for the Mini.