Car wheels spin at different speeds, especially when turning. As a vehicle goes through a turn, each wheel travels a different distance through the turn. The inside wheels (for example, the right wheels during a right hand turn) travel a shorter distance than the outside wheels. Since speed is equal to the distance traveled divided by the time it takes to go that distance, the wheels that travel a shorter distance must travel at a lower speed. Also, the front wheels travel a different distance than the rear wheels.
For the non-driven wheels on your car – the front wheels on a rear-wheel drive car, the rear wheels on a front-wheel drive car – this is not an issue. There is no connection between them, so they spin independently. But the driven wheels are linked together so that a single engine and transmission can turn both wheels. If the car did not have a differential, the wheels would have to be locked together, and therefore forced to spin at the same speed. This would make turning difficult and hard on your car: For the car to be able to turn, one tire would have to slip. With modern tires and concrete roads, a great deal of force is required to make a tire slip. That force would have to be transmitted through the axle from one wheel to another, putting a heavy strain on the axle components.
The differential is a device that splits the engine torque two ways, allowing each wheel to spin at a different speed so that the car can make turns. The differential is located between the two wheels, and is attached to each wheel by a half-shaft rotated through a bevel gear. Four-wheel drive cars have a separate differential for each pair of wheels, and many have a differential between the front and rear wheels (for a total of 3 differentials) to facilitate turning on pavement. Part-time four-wheel-drive systems don’t have a differential between the front and rear wheels; instead, they are locked together so that the front and rear wheels have to turn at the same average speed, which makes these vehicles are hard to turn on concrete when the four-wheel-drive system is engaged.
A grooved, or splined, axle side gear is positioned on the splined end of each axle. The side gears are driven by “spider” gears, which are little gears mounted on a shaft attached to the differential case. As it is supported by the differential case, the side gear can turn inside the case.
The differential case can be turned, revolving around the axle gears. The differential pinion (a pinion is a small gear that either drives a larger gear or is driven by one) shaft turns the ring gear, which is fastened to the differential case. The propeller shaft (drive shaft) connects the transmission output shaft to the differential pinion shaft. The turning differential case is mounted on two large bearing holders. These bearings are called carrier bearings.
The propeller shaft rotates the ring gear pinion, and the pinion turns the ring gear. The ring gear then turns the differential case and pinion shaft, but the axle side gears will not turn. Note that the input pinion is a smaller gear than the ring gear; this is the last gear reduction in the car. You may have heard terms like rear axle ratio or final drive ratio. These refer to the gear ratio in the differential. If the final drive ratio is 4.10, then the ring gear has 4.10 times as many teeth as the input pinion gear. By passing the differential pinion shaft through two differential pinion gears that mesh with the side gears, the case will turn and the axle side gears will turn with it. During turns, the side gears turn at rates dictated by the radius of the turns, and the spider gears then turn to allow the outer wheel to turn faster than the inner one.
For lubrication fluid, a very heavy oil, must be used in rear axle housings. Special hypoid oils are used in the differential case. Even another type of fluid, or oil must be used in a positraction type differential.
The oil is circulated by the ring gear, and flung all over all the parts. Special troughs, or gullies are used to bring the oil back to certain spots, like the ring and pinion area and the piston bearings. The fluid is kept in with gaskets and oil seals. The bottom of the housing has a drain plug, and another filler plug is located part way up the housing. The housing must never be filled above this plug.
The housing fluid lubricates some of the outer bearings, but others have lubrication fittings for the injection of wheel bearing grease. A hand gun, not a pressure grease gun must be used to grease these bearings (sparingly). A pressure grease gun could inject grease into the brakes– greasy brakes are inefficient at best!
Finally, some bearings are filled with grease at the factory and are sealed. These never require attention unless they are defective.
Types of Differentials
The most common type of differential found on cars and trucks are known as Open Differentials. An open differential always applies the same amount of torque to each wheel. There are two factors that determine how much torque can be applied to the wheels: equipment and traction. In dry conditions, when there is plenty of traction, the amount of torque applied to the wheels is limited by the engine and gearing; in a low traction situation, such as when driving on ice, the amount of torque is limited to the greatest amount that will not cause a wheel to slip under those conditions. So, even though a car may be able to produce more torque, there needs to be enough traction to transmit that torque to the ground. If you give the car more gas after the wheels start to slip, the wheels will just spin faster. If you’ve ever driven on ice, you may know of a trick that makes acceleration easier: If you start out in second gear, or even third gear, instead of first, because of the gearing in the transmission you will have less torque available to the wheels. This will make it easier to accelerate without spinning the wheels.
Now what happens if one of the drive wheels has good traction, and the other one is on ice? This is where the problem with open differentials comes in. Remember that the open differential always applies the same torque to both wheels, and the maximum amount of torque is limited to the greatest amount that will not make the wheels slip. It doesn’t take much torque to make a tire slip on ice. And when the wheel with good traction is only getting the very small amount of torque that can be applied to the wheel with less traction, your car isn’t going to move very much.
Another time open differentials might get you into trouble is when you are driving off-road. If you have a four-wheel drive truck, or an SUV, with an open differential on both the front and the back, you could get stuck. If one of the front tires and one of the back tires comes off the ground, they will just spin helplessly in the air, and you won’t be able to move at all.
Limited Slip Differentials
A Limited Slip Differential (also known as a LSD) attempts to address the problems of an Open Differential. A Limited Slip Differential is very similar to an Open Differential, but it adds a spring pack and a set of clutches. Some of these have a cone clutch that is similar to the synchronizers in a manual transmission. The spring pack pushes the side gears against the clutches, which are attached to the cage. Both side gears spin with the cage when both wheels are moving at the same speed, and the clutches aren’t really needed – the only time the clutches step in is when something happens to make one wheel spin faster than the other, as in a turn. The clutches fight this behavior, wanting both wheels to go the same speed. If one wheel wants to spin faster than the other, it must first overpower the clutch. The stiffness of the springs combined with the friction of the clutch determine how much torque it takes to overpower it.
Therefore, in the situation where one drive wheel is on the ice and the other one has good traction, With this limited slip differential, even though the wheel on the ice is not able to transmit much torque to the ground, the other wheel will still get the torque it needs to move. The torque supplied to the wheel not on the ice is equal to the amount of torque it takes to overpower the clutches. The result is that you can move forward, although still not with the full power of your car. This is why Limited Slip Differentials are popular in Drag Racing – they minimize wasteful wheel spin on a hard launch.
The locking differential is useful for serious off-road vehicles and for drag racing. This type of differential has the same parts as an open differential, but adds an electric, pneumatic or hydraulic mechanism to lock the two output pinions together. This mechanism is usually activated manually by switch, and when activated, both wheels will spin at the same speed. If one wheel ends up off the ground, the other wheel won’t know or care. Both wheels will continue to spin at the same speed as if nothing had changed. This maximizes the amount of forward motion, irregardless of wheel slippage – perfect for drag racing.
There are several types of locking differentials. An ARB Air Locker is a unique differential because it acts like an open differential until an on-board air compressor is activated by a switch. The air pressure is used to lock the differential. This allows a very high breakaway torque for racing but no compromises for daily driving.
A Detroit Locker, popular on muscle cars and some off-road trucks, is a ratcheting type of locking differential. It is very strong and will almost always provide equal torque application to each axle, but it is noticeable when cornering.
Finally there is the spool, which solidly connects the left and right axles with no slipping allowed. It is used for drag-racing applications only, since it maximizes forward acceleration, but makes the vehicle very difficult to turn and is very hard on the axles.
The Torsen differential is a purely mechanical device; it has no electronics, clutches or viscous fluids. The Torsen (from Torque Sensing) works as an open differential when the amount of torque going to each wheel is equal. As soon as one wheel starts to lose traction, the difference in torque causes the gears in the Torsen differential to bind together. The design of the gears in the differential determines the torque bias ratio. For instance, if a particular Torsen differential is designed with a 5:1 bias ratio, it is capable of applying up to five times more torque to the wheel that has good traction.
These devices are often used in high-performance all-wheel-drive vehicles. Like the viscous coupling, they are often used to transfer power between the front and rear wheels. In this application, the Torsen is superior to the viscous coupling because it transfers torque to the stable wheels before the actual slipping occurs.
However, if one set of wheels loses traction completely, the Torsen differential will be unable to supply any torque to the other set of wheels. The bias ratio determines how much torque can be transferred, and five times zero is zero. One novel solution is to apply the brakes and the gas at the same time. This will create a level of “traction” on the spining wheel, and allow the Torsen differential to shift power to the other wheel.
Vicious Coupling Differentials
The viscous coupling is often found in all-wheel-drive vehicles. It is commonly used to link the back wheels to the front wheels so that when one set of wheels starts to slip, torque will be transferred to the other set.
The viscous coupling has two sets of plates inside a sealed housing that is filled with a thick fluid, as shown in below. One set of plates is connected to each output shaft. Under normal conditions, both sets of plates and the viscous fluid spin at the same speed. When one set of wheels tries to spin faster, perhaps because it is slipping, the set of plates corresponding to those wheels spins faster than the other. The viscous fluid, stuck between the plates, tries to catch up with the faster disks, dragging the slower disks along. This transfers more torque to the slower moving wheels — the wheels that are not slipping. The faster the plates are spinning relative to each other, the more torque the viscous coupling transfers. The coupling does not interfere with turns because the amount of torque transferred during a turn is so small. However, this also highlights a disadvantage of the viscous coupling: No torque transfer will occur until a wheel actually starts slipping.
A positraction differential is a special traction differential. Its purpose is to improve the way your differential performs under adverse conditions. When one wheel starts to slip, these differentials transfer the torque to the wheel that is not slipping. The car can then continue to go forward. There are several different kinds of positraction differentials, but all of them are based on a friction device to provide resistance to normal differential operation.
A positraction differential provides better traction, which is handy when roads are slippery. It also lends itself to fast acceleration.
One type uses four differential pinions instead of two, with two pinion shafts. It also uses a series of four clutch discs. The differential pinions run into resistance when they try to turn the axle side gears. The resistance gets transferred to the pinion shafts driving the pinions. The shafts are forced to slide up little ramps. This action moves both shafts outward. The pinions cause the clutches to lock.
Other types use cone clutches, or disc clutches under pressure from coil springs. By restricting the differential action, torque is delivered to the slipping wheel.