Automotive Differentials

All About Differentials

A differential is an amazing piece of automotive equipment. Although there are many different types of differentials used by different automakers, they all serve the same purpose. They allow for a difference in speed between two output shafts. They can allow two wheels on the same axle to spin at different speeds or, in AWD or 4WD applications, they allow the front and rear wheels to spin at different speeds.

Some differentials, are limited slip differentials, which prevent the wheels from spinning at two much of a different speed. That helps in low traction situations. Limited slip differentials are integral to many AWD systems.

We'll explain why you might want your wheels to spin at different speeds, how the differential works, and what all the different types are.

The Wheel Speed Difference

As mentioned above, a differential allows the wheels on either side of a driven axle to spin at different speeds. If you only drove in a straight line, that wouldn't be necessary, and you'd be able to put a solid shaft between the two wheels. That would make turning very difficult, though, due to a phenomenon known as Ackerman steering.

In a turn, the outside wheels cover a greater distance than the inside wheels. Imagine that you and a group of friends held hands, forming a horizontal line. Now imagine you and your friend-chain start walking in a straight line. You'll all cover the same distance in the same time. Now, imagine you decide to take a turn, while keeping the line straight. Your friend at the inside of the line will cover much less distance than your friend at the outside of the line. These two represent the inside and outside tire respectively.

They cover different amounts of distance in the same amount of time. If you remember high school physics class, you know that velocity is distance divided by time. Since the outside tire goes a greater distance in the same time, it has to go faster.

Another way to picture this is to think of a marching band (since we're already bringing up fond memories of our school days). If you ever saw a section of the band perform a pinwheel turn (or were part of the maneuver, yourself), you may have noticed that the marcher on the outside of the turn had to take bigger steps than the marcher on the inside. Bigger steps in the band are comparable to a higher speed for your car's tires.

Now, let's talk about the wheel speed difference from the front to the rear of the car. On 2WD cars, this difference won't matter, since the non-driven wheels spin freely. In 4WD or AWD, though, the front and rear wheels spin together. Full time 4WD and AWD systems put a differential in between the two sets of axles (To learn more about AWD and 4WD, check out our article on the subject). That's because, in a turn, the rear wheels travel a greater distance than the front wheels.

Let's imagine your group of friends now forms a conga line, but instead of snaking, like a normal conga line, they want to stay straight from front to rear, just like your car does. When this rigid conga line comes to make a turn, the rear end is going to swing out far wider than the front which can navigate the turn fairly narrowly. Again, that means the people at the rear are going to have to take bigger steps, and once again, those bigger steps compare to higher wheel speed on your car.

Now that you know why differentials are necessary, let's talk about how they work. We'll start with the simplest type, the open differential.

Open Differential

Inside your differential cover, you will find the differential. It consists of a housing that has a ring gear, and the gears (and/or other mechanical parts) inside the housing. In a rear wheel drive car, it receives power from a bevel gear at the end of the driveshaft and sends it to the two wheels and allows them to spin at different speeds. In front wheel drive and AWD cars, the front differential may be housed in the transmission as part of what's called a transaxle. There, the transmission output turns the front differential housing directly.

An open differential has gears, called spider gears that are connected directly to the differential housing. These mesh with the gear on the end of each axle. In normal operation, driving straight, these spider gears move forward, with the housing, and push the axle forward. The spider gears do not turn on their axis during straight line driving.

In a turn, when one wheel has to spin faster, the gear on the end of its axle will start spinning faster. The spider gears allow this, by turning. When they turn, they move forward over the slower axle gear. That means the faster axle gear can move forward relative to the slower axle gear. This is good for letting the wheels move different speeds, which makes turning smooth.

This free movement is a liability in a slipping situation, though. Let's say one of your drive wheels is on ice, and it's spinning very quickly. Mechanical power always takes the easiest path, so it will go to the spinning wheel. The friction between the wheel and tire, what we usually call traction, will keep that wheel from spinning. The spider gears will allow the spinning axle gear to keep turning forward around the other axle gear. The slippery wheel will keep spinning and the wheel with traction won't turn, and you'll be stuck, or in a skid, depending on the situation you were in when you hit the ice.

To prevent the traction problems of open differentials engineers invented limited slip differentials. Limited slip differentials have some way to engage the wheel that has traction and send power to that wheel. There are a number of different ways to accomplish this task, which result in a number of types of limited slip differentials.

Clutch Pack Differentials

Many limited slip differentials use clutch packs to engage both axles to the differential housing. These differentials are more or less like an open differential, with axle gears and spider gears. In this case though, there are clutch packs between each axle and the housing. One way of engaging the clutches is with a spring or series of springs between the two axle gears to push the gears into the clutch packs. In straight line driving, the gears are engaged to the clutch packs and turn with the differential housing. In a turn, the wheels are capable of turning different speeds if the torque on one wheel is able to overcome the force of the clutch and the spring(s). This setup is well known from its use in GM Positraction system. It's so common, that many people will casually refer to any limited slip differential as a "Posi." Even if one wheel is spinning, the other is still engaged to the differential housing and receives power.

Another type of clutch pack limited slip differential uses pressure rings. The spider gears, instead of being connected to the housing, are on a shaft that sits between two pressure rings. Each pressure ring can press into a clutch pack on either side of the differential. The pressure rings have a notch cut in them where the spider gear shaft rests. The notch allows limited movement of the shaft. One wheel can spin faster than the other, but only until the spider gear shaft reaches the limits of its notch. When the shaft reaches that point, the force pushes the pressure rings slightly apart, which pushes them into the clutch packs. Then both axle gears will be engaged to the differential housing and both wheels will spin together.

The notch can be designed differently to achieve different effects. In a one-way limited slip differential, the notch is triangular and only allows the spider gear shaft to move forward. That means the differential only locks up under acceleration. A two-way limited slip differential has a diamond shaped notch that allows the shaft to move either forward or backwards. That means it can engage during deceleration as well. There is also something called a 1.5-way differential. This means the notch is longer in the forward direction. It takes more force to lock up the differential while accelerating than decelerating.

Some automakers use electronically controlled differentials. These are usually clutch pack differentials where the clutch is engaged by a signal from the vehicle's computer if wheel slipping is indicated by a difference in readings from the wheel speed sensors.

Planetary Gear Differentials

Another way to create a limited slip differential is by using planetary gear sets. In a basic system, the axles are connected to the sun gear. The planetary gears of each set stick out somewhat and can engage each other. The wheels can spin separately, up to the point where the planetary gears engage each other and lock the two axles together.

A more complicated version of this idea is the helical gear differential. This system is well known by its trademarked name Torsen. Audi often makes use of Torsen differentials.

In the Torsen differential, each axle ends in a helically cut worm gear. A set of cylindrical worm wheels, connected to the differential housing, rides on each worm gear. The worm wheels end in spur gears which engage each other. The worm gears can cause the worm wheels to spin on their axis, but the worm wheels can't cause the worm gears to spin. In straight line driving, the housing turns forward, and the worm wheels pull the worm gears around. The worm wheels don't spin on their axes during this process.

In a turn, one wheel spins slower than the differential housing. In that case, the worm gear on that axle turns the worm wheel backwards, since the worm wheel and the housing are moving forward over the worm gear. The other wheel is spinning faster than the housing, so the worm gear turns the worm wheel forward. The two spur gears spin opposite directions, which causes no binding.

If one wheel is slipping, then, just like the outside wheel in a turn, its worm gear will spin forward and spin the worm wheel forward. Except, in this case, the wheel with traction is not going backwards relative to the housing. That means the worm wheel is not spinning. So, the worm wheel of the spinning axle wants to engage the worm wheel of the other axle, which, in turn, would cause it to engage the worm gear of the other axle. Remember, though, that the worm wheel can't spin the worm gear. That locks everything up together, and causes the housing to rotate both worm gears, and both axles together. That gives torque to the wheel with traction.

Viscous Couplings

Some differentials may use fluid to connect the two axles. One type of viscous coupling, commonly used by Subaru, uses a series of fins and discs to move the fluid. One axle is connected directly to the differential housing. The housing has a series of fins inside of it. The other axle has a series of discs that sit in between the fins, but don't touch them. Hydraulic fluid fills the spaces between the discs and fins in the housing. Normally, the fluid causes the fins and discs to spin at the same speed, which cause the two wheels to spin at the same speed. In a turn, the wheels can spin at different speeds because the fluid produces only limited friction. If one wheel starts to spin, though, the spinning discs or fins speed up the fluid flow which increases the viscosity or thickness of the fluid. That exerts a great force on the fins or discs (whichever are connected to the wheel with traction) and causes them to spin, along with the connected axle, sending power to the wheel with traction.

Another way to achieve a similar affect is with georotor pumps. Essentially, the axle gear acts as the rotor in a pump to build hydraulic pressure. That pressure can then drive the gear for the other axle. Jeep has occasionally used this system.

Locking Differentials

Some vehicles, usually trucks meant for offroading, use selectable locking differentials. These differentials are a lot like clutch type limited slip differentials. In a selectable locking differential, though, the driver can select to lock the differential. Compressed air, a cable, or electronic solenoids engage the clutch packs.

Spools

A spool is not a differential at all. It makes a hard connection between the two wheels. This makes it easy to put power down, but is a major hindrance for turning. Spools are a performance upgrade intended for straight-line speed. They are ideal for drag racing and not much use in road driving.

Limited Slip Differentials in 4WD and AWD

As mentioned above, many 4WD and AWD systems use a limited slip differential to allow the front and rear wheels to spin at different speeds. Many types of differentials have been used in these systems. The inner workings remain the same as explained above, but the shafts lead to the front and rear differentials, rather than two wheels on the same axle.