What is a Limited-Slip Differential, and what are the different types?

A differential of any type allows two output shafts to spin at different speeds. This is important when going around corners, as the inside and outside drive wheels are spinning at different speeds. Most differentials are of the "open" type, meaning they have no limitation on the difference in speeds between the two output shafts. Without a limited slip differential, when one drive wheel gets stuck in a ditch, it could spin 100mph while the other drive wheel stands still. By their very nature, open differentials send power to the wheel with the least grip. An LSD is a differential that prevents one rear wheel from spinning while the other just sits there, but still allows for a variation in speed between right and left wheels as the car goes through a turn. In other words, the LSD unit limits the speed difference between the two wheels, allowing torque to be applied to a wheel when the other is spinning without traction. Why would anyone want an LSD differential? They allow power to be applied through two tires instead of one, and often means you can apply the power sooner coming out of turns. It is important to remember that although we steer the front wheels of a car, the car actually rotates around the rear axle. Limiting the differential's slip limits this rotation somewhat. Drivers preferring to drive the rear end of the car (oversteer) prefer a locked or severely biased limited slip, whereas drivers that prefer to drive the front end (understeer) of the car prefer a looser differential setup. Both differential types can be fast but consider that the looser differential is gentler on tires and may be easier to drive. What are the downsides? An LSD installed into an already balanced chassis can cause a dramatic increase in understeer on dry pavement, and may even cause oversteer on wet pavement, requiring changes in swaybars, springs, and shock settings to return the car to neutral. There are several variations of LSD differentials, which are very well discussed on Gordon Glasgow's LSD tech page (http://www.gordon-glasgow.org/lsdtech.html) or at http://auto.howstuffworks.com/differential6.htm , but I will briefly list them below:

Welded Differential:

Actually, this is not an LSD, but the modification is a popular with road racers as a low-cost way to make a locked rear end. By welding together the side gears and the spider gears within the differential in several places, the rear wheels are both forced to spin at the same speed. By definition, this is no longer a differential, as the rear wheels cannot spin at different speeds. This is fine at higher speeds on the race track, but is really unsuitable for street driving, as the rear end of the car skips/hops across the pavement as you go through low-speed turns since one wheel cannot spin at a different speed from the other. It also can cause severe understeer, not to mention breaking at the worst possible moment when those welds let go.

Detroit Locker:

A popular option on the Detroit Muscle cars, this is mechanical differential that acts like an open differential until power is applied, at which point it locks up and gives power to both rear wheels simultaneously. Unfortunately, this differential either gives you complete lock-up or no lock-up at all. These are available for the rear wheel drive, but will not be discussed here.

Limited Slip Differentials:

These come in several varieties: Clutch-Pack (or Salisbury), like the Subaru LSD, Kaaz, Cusco, ATS, and Power Brute units, Viscous Fluid units (as used in the early Miata, 240sx, 300zx, and '91-92 Subaru Legacy turbo 4WD sedans, as well as all LSDs found in '00 and up Subaru Imprezas, WRXs, Legacies, and Forester GTs), and the Mechanical Torque-Sensing units like the Quaife and the Gleason-Torsen (the latter came in the '94 and newer Mazda Miatas and 3rd generation RX-7s, Lexus IS300s, HumVees, etc.). All of these combine the streetability of an open differential with the advantages of a locked differential, but there are differences between the three types of LSDs. A brief history of LSD technology and applications can be found here.

The Clutch-Pack ( Salisbury) LSD is what we're interested in. This is the kind of R-160 unit you can buy new from Nissan Motorsports or Subaru for about $800, or used from a salvaged Subaru for about $300. These LSDs have an assortment of friction disks and shims inside, arranged so that the limited slip typically has a factory breakaway setting of 45 ft-pounds (allowing the rear wheels to turn at different speeds if they have to). See exploded diagrams of this differential from the Subaru Manual here and a close-up diagram here. The major downside is that as these clutch disk units wear, their breakaway torque setting gradually lessens, so they become more like an open differential until you rebuild them again to get the breakaway back up to 45 ft-lbs. Gordon Glasgow's web page (link URL is at the beginning of this FAQ) tells how to rebuild these units, but it is tricky, as the only way to adjust the breakaway is to use different thickness shims, reassemble the entire LSD, and then see what you've got. For reference, the 2.5 Trans-Am race series BRE 510s had breakaway settings adjusted to 150 ft-lbs. Too high a breakaway pressure will cause clutch-pack LSDs to function as locked differentials, as well as creating severe understeer at corner entry as well as power-on corner exit. Anything under 100 ft-lbs should be fine for a dual-purpose street car. Pro: Cheap (used Subaru R-160), easily bolts in. Con: Hard to find, limited ratio (3.7), unless you swap R&P, clutch discs wear out, hard to adjust breakaway setting. For serious on-track duty, these setups generate HEAT, so a differential cooler may be in your future.

The Quaife and Gleason-Torsen (stands for TORque-SENsing) units are often regarded as the best LSD. Their locking action is via a complicated worm gear setup (read the technical paper here or go to the Torsen site), so they have no clutch plates to wear out and do not have the delay in locking up that some report with the viscous fluid LSD units. In a no-slip condition, the differential splits the torque 50:50 between the two drive wheels. When wheel slip occurs, the unit sends more torque to the axle with more grip (via the torque-multiplying characteristic of the worm gear mechanism), which in an autocross or race track situation is the outside drive wheel. By design, these LSDs separate the speed differentiation and torque distribution functions of a differential, resulting in a proactive LSD that actually prevents excessive wheelspin under acceleration. Different applications come with different torque transfer ratios (torque bias ratio, or TBR), capable of transferring torque to the non-slipping wheel at a ratio of up to 9:1. The locking action of these units occurs only under acceleration, and is instantaneous and progressive in nature. Under braking, the differential behaves like an open unit. Quaife now has an LSD unit that fits in a Subaru R-160 case that sells for about $2,000 new. One thing to know about the Torsen or Quaife units: they require BOTH wheels to have some traction in order to work as an LSD, as the differential "senses" the difference in torque between the two. If a drive wheel comes off the ground, it will spin just like an open differential, as zero available torque (a spinning wheel) when multiplied, is still zero. Pro: Durable unit, smooth, quick action Con: Pricey, no traction if wheel comes of the ground, cannot "tune" locking action.

Viscous LSD units are popular OEM LSD solutions, as they are relatively simple and cheap to produce. They come in many performance Subarus, Nissans, Mazdas, Toyotas, etc. The LSD unit consists of stacks of thin plates with holes or slots, all suspended in a special silicone fluid. They have no clutches to wear out, and locking characteristics can theoretically be changed by varying fluid viscosity. Generally, however, these units are non-serviceable, and require no special maintenance. As the differential spins, the plates shear the fluid up to a point, after which the fluid provides some resistance to shear, allowing 15-25% torque transfer to the other wheel. The downside is that these units don't act like a limited slip until one wheel actually starts slipping (i.e., they don't prevent slippage), which means the VLSD action often kicks in after you've already exited the corner. Compared to the proactive nature of the Torsen LSDs, the VLSDs are reactive units. They do not prevent slippage, they merely sense differences in rotation, not torque. They also don't allow for very much torque transfer, compared to mechanical or clutch-pack LSDs. They do still work well for starting from a dead stop in slippery conditions. It is important to note that VLSDs locking characteristics occur both during acceleration AND braking, as it can't tell the difference between the two, but merely reacts to the rotational speed differences between the two drive wheels. Subaru USA lists the '91-'92 Legacy 4WD Turbo 4 dr sport sedans as having an R-160 viscous 3.9 LSD option. All OEM Subaru LSDs since 1991 are viscous R-160 units (Note: No U.S. Subarus between 1995 and 1999 came with LSDs). Pro: Readily available from newer Subarus (R-160), Nissans (R-200), smooth action, no special maintenance needed. Con: Delay in action.

All Wheel Drive Systems

A simple concept:

-When driving in a straight line, all four wheels are turning at the same speed.
-When turning in forward and reverse the rear axle follows a shorter turning radius than the front axle and rotates at a slower rate.

From this simple concept we can derive an cardinal rule:

-The front axle will always rotate at the same or slightly faster rate than the rear axle.

Subaru's All Wheel Drive systems are engineered to insure this rule is always followed regardless of traction conditions.

Two principles

Two principle all wheel drive systems exist, permanently engaged and automatically engaged.

All Subaru all wheel drive systems are considered permanently engaged, while most of the others fall into the automatic variety.

Automatic systems detect slip and direct power to the other axle. But wheel slippage has already occurred!! It is always better to avoid wheel slippage rather than react to it. In the time that it takes for the system to react, loss of control is possible. The delay also means that the section of road that caused the front wheel to slip is now under the rear wheel when it receives power, so it too may slip.

Subaru's all wheel drive system is designed to always drive the front and rear axles at the same speed, regardless of wheel slippage conditions. If the front axle encounters ice and the system is designed to drive that axle at the same rate as the rear, wheel slip will NOT occur!

This is the Subaru advantage.

Subaru All Wheel Drive Systems

Subaru uses five different All Wheel Drive (AWD) systems.

System 1: Continuous All Wheel Drive

Used on manual transmission (5MT) vehicles

Uses a viscous coupled centre differential to balance the engine power 50/50 between the front and rear axles. Allows slight axle speed differences for easy turning. If wheel slippage occurs on either the front or rear axle, the viscous coupling reacts to match the speeds of both axles and eliminate the slippage.

This system has an excellent capability of dealing with wheel slippage, but because the default condition is to split the power 50/50 front/rear, slippage rarely occurs.

Some models are equipped with a viscous coupled limited slip rear differential (LSD) which prevents a single rear wheel from slipping. If the right tire begins to slip, the LSD reacts and increases power at the left wheel and reduces it on the right and vice versa.

System 2: Continuous All Wheel Drive with Driver Controlled Centre Differential (DCCD)

Used on manual transmission (6MT) WRX-STI models only.

Operates in an identical manner to the standard viscous coupled centre differential system, but has a dial control that allows the driver to adjust to default front/rear power distribution from 50/50 to 35/65. By directing more than 50% of the power to the rear axle, the car behaves more like a rear wheel drive vehicle which is advantageous for sporty driving.

Front and rear viscous coupled limited slip differentials (LSD) are used with this system. This configuration can cause understeer and rob engine power, however the 300hp WRX-STI has more than enough power to counteract these effects.

System 3: Active All Wheel Drive

Basic system used on automatic transmission (4EAT) equipped Subaru's.

Uses an electronically controlled multi plate clutch pack to adjust the amount of power directed to the rear axle. Normally splits engine power 90/10 front/rear and can adjust distribution up to a to 50/50 ratio.

The Transmission Control Unit (TCU) determines the vehicles centre of gravity and traction requirements and continuously varies the power sent to the rear axle. During acceleration, the vehicle centre of gravity moves to the rear, and power is increased to the rear axle. During braking, the vehicle centre of gravity moves to the front, and power is reduced to the rear axle. If front wheel slippage occurs, more power is sent to the rear wheels to compensate. In most cases this system can predict situation where slippage is likely to occur (ex. acceleration) and make the required adjustments BEFORE slippage can occur.

This system uses axle speed sensors within the transmission unit itself, not the Antilock Brake System (ABS) sensors, the ABS and AWD are two completely independent systems.

Some models are equipped with a viscous coupled limited slip rear differential (LSD) which prevents a single rear wheel from slipping. If the right tire begins to slip, the LSD reacts and increases power at the left wheel and reduces it on the right and vice versa.

System 4: Variable Torque Distribution (VTD)

More advanced system used on some automatic transmission (4EAT) equipped Subaru's.

Unlike the standard Active All Wheel Drive system, this system allows more than 50% of the engine power to be directed to the rear axle. Normally engine power is split 45/55 front/rear, and during acceleration even more power is directed to the rear wheels. By sending more power to the rear wheels, a sporty feel is obtained. Aside from being able to send more than 50% of engine power to the rear axle, this system functions in an identical manner as the Active All Wheel Drive System.

This system is always equipped with a viscous coupled limited slip rear differential (LSD) which prevents a single rear wheel from slipping. If the right tire begins to slip, the LSD reacts and increases power at the left wheel and reduces it on the right and vice versa.

Some VTD equipped vehicles are equipped with a SPORTSHIFT feature which allows for manual shifting.

System 5: Variable Torque Distribution (VTD) with Vehicle Dynamics Control (VDC)

The standard Variable Torque Distribution (VTD) system with one of the most advanced traction control, anti lock brake system (ABS) and skid control systems available.

This system uses the antilock brake system's (ABS) wheel speed sensors to detect wheel slippage. If wheel slippage is detected, the VDC system can activate the brake on the slipping wheel to control its speed. This system can control slippage on 1 to 3 wheels simultaneously.

If wheel slippage is severe (more than one wheel), the VDC system will instruct the engine management system to reduce engine power output, which is accomplished by turning off one or more fuel injectors and retarding ignition timing.

In addition to traction control this system can take corrective action when understeer, oversteer or vehicle drift is detected. Understeer is a condition where the vehicle turns less than what is desired, while oversteer is a condition where the vehicle turns more than what is desired. Drift is the initial moment of a slide or loss of control.

The VDC system uses the ABS wheel speed sensors, steering wheel position sensor and yaw sensors to determine if the vehicle is responding to driver inputs, no more, and no less. If the VDC system detects that the car is not responding correctly to driver inputs, the system takes corrective action to restore control. Corrective action includes braking one or more wheels, adjusting engine power and front/rear torque split to restore directional control of the vehicle.

This system is NOT equipped with a viscous coupled limited slip differential, since the traction control system is more effective at controlling individual wheel slippage.

This system can correct many loss of control conditions at the instant that they occur, however it can not accommodate for driver stupidity.

Future advancements:

Subaru is continuously improving their all wheel drive systems. The following systems will be available in the next few years:

5 speed automatic transmission (5EAT)

-An extra gear for improved performance and economy.
-Available on 2005 models?

Semi Automatic Manual Transmission

-Uses a conventional manual transmission clutch mechanism, but instead of a clutch pedal to release the clutch an electronically controlled hydraulic system is used.
-Rather than a traditional H pattern shifter, a shift lever similar to an automatic transmission lever is used. The shift lever is used to select direction (park, forward and reverse), transmission mode (automatic or manual shifting) and to manually change gears (+ or -, usable in both automatic and manual shifting modes). Steering wheel mounted paddle controls will also control gear changes.
-A conventional manual transmission is used, but rather than controlling the shifting forks which engage and disengage the required gear by the manual H pattern shift lever, electronically controlled hydraulic motors are used to perform this function.
-This system is used in BMW and Ferrari vehicles and is becoming more popular. This system has been used in both the Formula One (F1) and World Rally Championship (WRC) circuits for many years.

This system has many advantages:

-Typically lighter in weight and smaller in size than automatic transmissions
-More reliable than automatic transmissions
-Typically more gears can be fitted to these transmission than with automatic transmissions
-Clutch abuse is eliminated
-Gear grinding is eliminated
-Shifts faster than any driver could, and almost as fast as automatic transmissions
-Integrated hill holder mechanism (detects incline, and if required holds brake engaged until clutch take up).