How Differentials Work

The differential is a device that splits the engine torque two ways, allowing each output to spin at a different speed.

The differential is found on all fuel-burning cars and trucks and also in many all-wheel-drive (full-time four-wheel-drive) vehicles. These all-wheel-drive vehicles need a differential between each set of drive wheels, and they need one between the front and the back wheels as well, because the front wheels travel a different distance through a turn than the back wheels.

This center differential is often referred to as a transfer case. Some electric cars do not need differentials because they have multiple motors which drive the wheels independently of one another.

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. This is why these vehicles are hard to turn on concrete when the four-wheel-drive system is engaged. When the 4WD system is unlocked, the vehicle behaves like a rear-wheel-drive setup with one differential getting power.

We will start with the simplest type of differential, called an open differential. Essentially an open differential allows the rear wheels to spin independently of each other. The image on the left labels the components of an open differential.

When a car is driving straight down the road, both drive wheels are spinning at the same speed. The input pinion is turning the ring gear and cage, and none of the pinion gears in the cage are rotating — both side gears are effectively locked to the cage.

When a car is driving, the pinions in the cage start to spin as the car begins to turn, allowing the wheels to move at different speeds. The inner wheel rotates slower than the cage, while the outside wheel spins faster. Since 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.

This final drive ratio has a multiplicative effect with each gear ratio in the transmission. Drive ratios with a higher number will typically make a vehicle accelerate faster, but will reduce fuel economy and potential top speed. A lower ratio will have the opposite effect.

When a car makes a turn, the wheels must spin at different speeds.

The 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.

Driving On Ice

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.

Driving Off-Road

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.

Now, remember — as we mentioned, the open differential always applies the same torque to both wheels. 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.

The solution to these problems is the limited slip differential (LSD), sometimes called positraction. Limited slip differentials use various mechanisms to allow normal differential action when going around turns. When a wheel slips, they allow more torque to be transferred to the non-slipping wheel. LSDs are also often included in two-wheel drive performance cars to improve acceleration and mitigate wheelspin in low gears.

The clutch-type LSD is probably the most common version of the limited slip differential.

This type of LSD has all of the same components as an open differential, but it prevents traction loss and distributes power when needed. During normal driving conditions, the wheels spin at different rates. But if one needs extra power or traction, an LSD will transfer the extra torque to that wheel.

How? It has a spring pack and a set of clutches. Some of these have a cone clutch that is just like 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.

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 determines how much torque it takes to overpower it.

Getting back to the situation in which 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.

Many modern performance cars and 4x4s also employ what's known as an electronic differential or E-diff. Instead of mechanical linkages, the E-diff uses an array of speed sensors to detect wheelspin. When a tire slips, the ABS system or stability management applies braking force to the corresponding wheel and causes torque to be sent to the side with traction. This process is known as torque vectoring. How lax or aggressive the response is can often be adjusted within the vehicle's computer.

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. Mechanically, it works similarly to the torque converter that transfers power from an engine into an automatic transmission.

The viscous coupling has two sets of plates inside a sealed housing that is filled with a thick fluid. 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.

When a car is turning, the difference in speed between the wheels is not as large as when one wheel is 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 simple experiment with an egg will help explain the behavior of the viscous coupling. If you set an egg on the kitchen table, the shell and the yolk are both stationary. If you suddenly spin the egg, the shell will be moving at a faster speed than the yolk for a second, but the yolk will quickly catch up.

To prove that the yolk is spinning, once you have the egg spinning, quickly stop it and then let go — the egg will start to spin again (unless it is hard-boiled).

In this experiment, we used the friction between the shell and the yolk to apply force to the yolk, speeding it up. When we stopped the shell, that friction — between the still-moving yolk and the shell — applied force to the shell, causing it to speed up. In a viscous coupling, the force is applied between the fluid and the sets of plates in the same way as between the yolk and the shell.

The locking differential is useful for serious off-road vehicles. 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. A permanently locked differential can also be made by using a metal rod to weld both pinions to each other, but this is not advisable for street use.

This mechanism is usually activated manually by a switch and when activated, both wheels will spin at the same speed. Some modern lockers can activate automatically using input from wheelspin-detecting sensors. 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.

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 can apply up to five times more torque to the wheel that has good traction.

These devices are often used in high-performance and all-wheel-drive vehicles. Like the viscous coupling, they are often used to transfer power between the front and back 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.