Differentials
The Basics
The automobile differential is a mechanical device typically consisting of gears that enable the axle shafts to spin at different speeds. This is necessary since less distance is traveled by the inside wheel when going around a corner.
As seen above, to make a right turn the outside wheel must travel a greater distance. If the differential did not have pinion gears that allowed the shafts to rotate at different speeds the inside wheel might hop. This binding of the axles would make turning quite difficult, possibly damage the drivetrain, and wear the tires very quickly.
The conventional differential is known as an "open" differential. It is the entire assembly, excluding the shafts, that sits between the drive wheels. Housed within the casing of the differential is the carrier which rotates as it is turned by the ring gear. The carrier is the heart of the differential and houses the small pinion gears that are held inplace by a shaft. These gears allow the axles to rotate at different speeds. When one axle rotates slowly the other will compensate by rotating faster. Under normal straight line operation the pinion gears within the carrier do not move. The carrier and ring gear are driven by a gear also known as the pinion, it is important to note that this is not the same gear as those found in the carrier. This gear connects to the driveshaft and ultimately the engine. Thus when power is applied it travels from the driveshaft, into the differential via the pinion, into the carrier via the ring gear, and ultimately to the axles through the small central pinion gears.
The addition of a differential makes turning convinient and possible, but there are drawbacks. An open differential, such as the one above, allows the outside tire to spin faster than the inside tire when needed. By default the torque of the engine will take the easiest path it can find to the ground. When one of the wheels provides more resistance to motion (say going around a curve), the engine's torque is sent to the opposite wheel. As soon as a wheel loses traction, almost zero torque is delivered to the wheel with grip, all of the power is instead transfered into spinning the wheel with no traction. Thus an open differential will only provide about 1/2 of the total possible traction at any given time. This severly limits acceleration and makes a high powered automobile quite difficult to drive. Spinning tires do little to accelerate a vehicle and even less to keeping the car on the road during a hard corner. To remedy this problem, the Limited Slip Differential or "LSD" was invented.
Limited Slip Differentials
Traditionally, limited slip differentials are found on more sporty or expensive vehicles. They come in three main varieties, each with their own positive and negative aspects. The main types of limited slip differentials are:
-Clutch Type
-Viscous
-Torsen
Clutch Type
The clutch type differential is one of the more popular performance desings. The carrier retains most of the standard open differential components, but adds in small clutches. Some of these small clutches are attached to the axles, the remainder are attached to the carrier. These clutches can be set up to provide a certain amount of resistance called "break away" torque. It is typically between 30 and 80 ft-lbs. A rating of 50 ft-lbs means that it takes a weight of 50lbs at the end of a 1ft bar to cause the axle to rotate. This resistance can be felt to the driver as the car not wanting to turn (understeer). This can be positive as it provides increased stability, or a negative as it reduces total cornering ability.
Since axle torque can be in excess of 4000ftlbs for a high powered vehicle, obviously the break away torque is not sufficent to keep the axles locked under power. The clutch type limited slip tightens and increases resistance directly in proportion to how much torque is applied as the input. In a CLSD the shaft that holds the pinions is actually called a cross pin and is free to move within a set of angled "ramps". The softer the angle of these ramps the harder the resistance to slipage known as "lock up".
As more torque is applied to the LSD, the cross pin is forced against the ramps which inturn forces the clutches against eachother. This causes the axle to slow down in relation to the carrier and provides more equal force between the drive wheels. By changing the ramps and the cross pins, not only can the force of lockup be altered, but so can the type of lock up.
In light blue one can see the cross pin of the LSDs. A 2-way LSD, typically considered the most performance orientated, forces the clutches out on both acceleration and deceleration with equal torque multiplication (note that almost no differential locks as hard on deceleration as on acceleration since the amount of input torque when decelerating is greatly reduced). This provides good traction when accelerating and also helps to stabilize the car when braking. The 1.5-way LSD provides a stronger lock up on acceleration but reduced lock up on braking. This makes it easier to turn the car when slowing down at the cost of braking stability. A 1-way LSD will lock on acceleration but on deceleration it will behave almost as an open differential, only limiting as much as the break away torque provides.
Torsen Type
Torsen stands for Torque Sensitive. The torsen LSD, also known as a helical LSD, is a popular performance design just like the CLSD. A well known brand of the torsen differential is Quaife who sells their "Automatic Torque Biasing" unit, or ATB for short. Internally the torsen LSD is unlike a standard differential. They do use gears, but they are usually of the helical/worm variety. These gears transfer torque from one axle to the other in the opposite direction of an open differential.
The power takes the path of most resistance and is fed to the wheel with the most traction. The worm gears inside the carrier are loaded by the weight and input torque of the car and are consequently forced against the walls of the housing. As the gears are forced against the carrier housing they create friction thus limiting slip. The amount of torque that can be biased is refered to as the Torque Bias Ratio or TBR. A typical TBR would be from 4:1 to 8:1. To change TBRs the actual angle of the gear teeth must be modified, thus a helical LSD is non-adjustable. The presence of a TBR implies that there must be some torque at each axle. Thus if a wheel leaves the ground or slips, the other wheel will be rendered useless as well and the differential will behave as if it were "open". This type of differential is very useful in AWD applications as a center differential since almost never does the traction between front and rear become zero.
Viscous Type
The Viscous type limited slip differential typically is not a performance option. The carrier is filled with plates attached to the axles and the housing itself. These plates do not contact eachother and the entire carrier is filled with a viscous (thick) fluid. As the plates move in relation to eachother the fluid thickens and drags on the plates. This drag creates a resistance to slippage.
The viscous LSD is a speed sensitive unit. Thus it provides very little resistance to slippage while the plates are at or near equal speeds. The viscous LSD needs a speed difference between the plates in order to deliver torque between the axles. For this reason the VLSD acts open for most of the time until slipage occurs in which case it starts delivering torque to the other wheel. Obviously this is not ideal and severely hurts performance. This type of differential is well suited to luxury cars or cars that do not place an emphasis on total performance.
It is important to understand that an AWD system that uses an open or viscous differential essential is not "all wheel drive". Just as a two wheel drive car with an open differential can only deliver as much torque as one wheel can stand, the same applies to an AWD car. If all three of the car's differentials are open the car is only one wheel drive, there is no real gain in performance. A two wheel drive with a clutch LSD will out perform this type of car in poor weather. If just the center differential is viscous like old Subaru's there is a little bit better traction, but it is still hardly four wheel drive. A two wheel drive car with a CLSD would typically also out perform this setup in poor conditions. Fortunately viscous units are sometimes offered with a differential lock feature which delivers proper traction for snow or low speed conditions. Unfortunately the locking feature is often a rarity and is either on or off, not torque sensitive like a clutch or torsen LSD.
Positives and Negatives
| Clutch LSD | Torsen LSD | Viscous LSD | |
| Positives | -Torque sensitive |
-Torque Sensitive -Responsive -Not prone to wear -Relatively predictable |
-Cheap -Typically the easiest to maintain -Fails slowly -Simple |
| Negatives | -Clutches can wear out |
-Does not function unless both wheels have traction |
-Fluid loses its ability to lock up over time or when overheated |
Lockers and Welded Differentials
Lockers are typically used for off roading or drag racing. They allow axles to spin faster than the center but not slower. Thus they work fine on corners, but completely lock up when traction is lost or pushed to the limits. They are not useful for road racing as they are unpredictable (switches between full lock and open) and they can be excessively loud and clunky on the street. They are excellent for off roading where they can provide full lock with very little torque input unlike a CLSD.
A welded differential is an open differential that has had the pinions and planetary gears welded to the carrier so that they cannot move freely. This completely binds the two axles together. This type of differential is not good for anything but going in a straight line. It is simple and typically strong if welded correctly.
Traction Control
Traction control uses "E Differentials" to limit slip. Typically the differential is a standard or viscous unit. The car's computer gathers data from sensors to understand what is happening. If slip is detected, the car applies the brakes to that specific wheel which in turn sends torque to the other side. This type of system works very well in snow, better than a conventional LSD. It is also relatively cheap and simple. For road racing the technology typically is unable to duplicate the consistency and tractive ability of a torsen or clutch type LSD.
Summation
1. The best road racing LSDs are the Torsen and the Clutch type.
2. The Viscous differential is good for a daily driver, not much else.
3. AWD and two wheel drive cars can actually only be powered by essentially one wheel if they do not have a proper LSD.
4. Tip: In a RWD with an open/viscous differential in poor weather such as snow, apply the E-brake enough to slow the spinning wheel. This will essentially lock the two drive wheels and allow the car to be driven away.
Chris Moris
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