An automobile's differential gear (often abbreviated "diff", and most commonly referred to as the "differential") is a device which, potentially among other things, allows the wheels to be driven at different speeds. In a turn, the wheels (all of them, but we are currently interested in those which are driven, meaning that power is applied to the road or other surface through them) travel different distances in the same period of time, so this is important. Generally speaking, nearly every car needs a differential of some sort.
The differential is sometimes called a "rear end" (as in,
nine inch rear end.) In a front wheel drive (FWD) car, the differential is built into the transmission (or "transaxle".) On a rear wheel drive (RWD) car with a front-mounted engine, it is separate; Either a distinct unit in cars with independent rear suspension (IRS) or built into the middle of the live axle on cars without. On mid-engine and rear engine cars, it is built into the transaxle. In the case of four wheel drive (4WD) vehicles there are typically front and rear differentials built into fixed axles while a transfer case rests in between, and in all wheel drive (AWD) vehicles there are not only front and rear diffs but also a center differential, which is typically integrated into the transmission.
It is not strictly true that every car needs a differential. In some forms of racing (primarily drag racing) it is advantageous to have a "locked differential", also known as a spool because both wheels turn the same, and only the same. Technically, a spool is not a differential (though you can make one by welding some types of differentials together) but a transfer case, as is usually used in the center of four wheel drive (4WD/4x4) vehicles -- but more on that later.
The most commonly employed type of differential (because it is simple and thus inexpensive and durable) is the open differential, which simply consists of a series of gears. Provided that they have minimal friction and deflection, open differentials are an effective method of avoiding wheel hop of the driven wheels in turns. They do have some serious disadvantages, however; If one wheel is off the ground, then it will spin freely, and the other wheel will not apply any meaningful amount of power to the ground. Also, they tend to slip in wet conditions, which can be hazardous; When they begin to slip, they tend to continue to slip. Because the slipping wheel gets all the power they are not useful for off-road use, either.
There are two solutions to this problem; the limited slip differential (or LSD) and the locking differential. Both types come in automatic and non-automatic versions.
Limited Slip Differentials (LSD)
Limited slip differentials further come in four types; viscous, clutch-type or clutch pack, or mechanical. "Posi-traction" is a generic GM-used name for limited slip differential, and can apply to a variety of methods but in the case of GM usually means a clutch-type limited slip.
Viscous LSD works on the same principle as an automatic transmission, which is that the fluid inside of it heats up as it shears, changing its viscosity. When a wheel slips, power is transferred to the other wheel, so power is applied smoothly. There is very little loss under power in such a system, but there is some loss of power during slip. Viscous limited slip units effectively never wear out unless a seal fails -- the fluid could theoretically be burned up, but the engine would probably die first. Some Japanese sports cars (especially from Nissan) have been known to feature a viscous LSD as an option.
Clutch-type LSD uses clutches (high-friction pads under tension) to allow a specified amount of slip. They come in 1, 1.5, and 2 way variants, which number tells you whether they slip only under power, or also during engine braking. They are the favorites of drift racers because of their consistent output and tunability but the clutches wear out and must be replaced periodically.
Mechanical LSD such as the Torsen differential (torque-sensing) uses only gears. In the torsen diff's case, it is an open differential with a plus; there are gears which, when they bind, will transfer power to the wheel with traction. By changing the ratio of these added gears one can set the ratio of transferred power, so if they are geared to 5:1, the wheel with traction will have five times the power of the wheel without. However, if a wheel loses too much traction (The larger this ratio is, the less power it can afford to lose) then the torsen diff will not transfer any power, because N times 0 is always 0. Torsen diffs are used in the HMMWV/Hummer and in Quattro-model Audis and 4Motion Volkswagens. Mechanical LSDs are most effective when used with traction control.
There are basically two categories of locking differentials, automatic and manual or externally actuated. Most locking diffs use an electric solenoid or a pneumatic actuator, and are manually controlled. This is most popular for off road use, particularly on dirt. You can lock the halves of the differential together so that you know that you will always apply the same amount of power at both sides.
Tractech, Inc. designed the "Detroit Locker" which is locked most of the time, but in a turn the gears unmesh, which can cause a loud chattering sound that sounds like your diff is about to explode during cornering. They also now have a Detroit Gearless Locker which uses clutch tension to engage or disengage the sides of the differential, though the power is not actually transmitted through the clutch as it is in a limited slip system.
Sort of an anti-differential, a spool can be a made piece which is installed, or created by welding the spider gears in an ordinary open differential. It forces both wheels to move in the same way. Obviously this means it is not much use in cornering, but it is used in drag racing, both on the street and the drag strip|strip.
All (and Four) Wheel Drive
As for systems with more than one set of drive wheels, the situation becomes more complicated. There are two types of systems in which all four wheels of an automobile are driven.
Four Wheel Drive
Four wheel drive (4WD) systems are usually part-time, meaning that they must be engaged manually, either by shifting a lever or by pushing a button. Sometimes the hubs of the wheels also must be engaged, either manually (by spinning) or electrically. Four wheel drive systems have a transmission intended for rear wheel drive use. Most of them have a transfer case in the center, which differs from a differential in that it drives the output shafts (which go to the front and rear differentials) at the same rate, or at different, specified rates. Sometimes these can be replaced with a differential, which means the front and rear wheels can be driven at different rates.
All Wheel Drive
AWD, or all wheel drive, generally utilizes a transmission which is designed to drive two sets of wheels, with a differential built in. The fast majority of these are modified versions of front wheel drive transmissions. As is typical of modern unibody cars, the engine is mounted transversely and the transmission is closely mated to it, using as little space as possible. A drive shaft goes to the rear differential, and there is a differential for the front wheels built in.
Typically, an all wheel drive system features a limited slip differential in the rear (usually clutch-type or viscous) and an open differential in both the center (controlling front/back balance) and the front. It is generally possible to replace all differentials with superior models, however. The aforementioned (see above) Subaru "VCD" which is electronically controlled works in conjunction with the ECU (Engine Control Unit) to maintain maximum traction. In vehicles with limited slips fore and aft and a variable or limited slip diff in the center, this means that the individual wheel with traction receives the majority of the power. When combined with ABS this gives unparalleled traction control as power can be delivered to any given wheel while braking force is modulated at all others.
Special thanks to Transitional Man.