How Does a Car Rear Differential Work?

Educational

Mar 21st, 2019

How Does a Car Rear Differential Work?

Have you ever looked at that thing which is located on your car’s rear axle? Yes, we are referring to that ball of gears which give you the ability to round the corner? Has it ever occurred to you to try to figure out how it works? Well, this article is here to explain everything to you about how a rear differential works.

The truth is that there are many experts out there who are great mechanics and have been involved in many car projects that believe that a differential is a more alien technology than a human invention. It works almost like magic, and many people cannot fathom how human minds could have come up with such an intriguing invention. Well, of course, we know that referring to the differential as an alien invention is kind of an exaggeration but if you really think about it, do you really understand how the differential works? 

We engage in learning every day, and this time around, it is time to get into another learning session as we will be going into the intricacies of the differential of a vehicle and how it functions. We will be taking the basics of the differential in order to make you understand the mechanics behind this great invention. 

Purpose of a Differential

The reason for designing the automatic differential is quite obvious. The purpose is to be able to transfer power to a car’s wheels while giving the wheels the ability to spin at varying speeds. This is a very important addition to any vehicle since the four tyres of any car rotate at varying speeds each time a turn is negotiated. 

Basically, we refer to the differential as that component which equalises the travel distance between the inside and outside wheels whenever you take your car around a bend. You will find the differential lodged in its own separate case in a rear wheel drive vehicle. 

What the differential does

If we all drove our cars in a straight line, then a differential would not be necessary. Since that's not possible at all, cars need a differential to navigate a bend, go around corners, and negotiate a turn. 

Your vehicle's wheels have different rotational speeds when you try to make a turn. The inside wheels of your car travel at a distance that is much shorter than the distance covered by the outside wheels. What this implies is that your car’s inside wheels will have to travel slower than the outside wheels and without a differential to balance the differences in speed, it will be incredibly difficult to complete a turn.

For cars with non-driven wheels, this problem does not exist since the wheels all spin independently and are not in any way connected to each other. However, when it comes to driven wheels, a differential has a very essential role to play. The wheels are linked to one another and are controlled by just one engine and transmission. If a differential is absent, it means that these wheels will have to be linked together to travel at exactly similar speeds and exactly similar distances and this will make it an impossible task for you to turn your car. 

Functions of a differential

So, basically, a differential works by performing three key functions.

  • It is responsible for sending the power that is generated from the engine to the car’s wheels.
  • It slows down the transmission's speed of rotation just before the power reaches the wheels.
  • It divides the power that is sent to the car’s wheels from the engine while still allowing them to spin individually.

A differential mechanism simply involves dividing the power which the wheels receive while enabling them to rotate at their individual speeds. Therefore, in the event that a particular set of wheels spins slowly and covers only a short distance, the other set of wheels will continue to rotate and not skid with the aid of the differential. This helps to prevent tyre shredding and loss of torque. 

Components of a Differential

In this section, we will be considering the different parts which make up a differential. In a standard differential, here are the components that you will find –

  • Differential Case

The differential case is the unit’s main body. Inside the case, you will find the spider gears. It also houses the differential of your car.

  • Differential Gears

The differential gears, aka spider gears, is responsible for transmitting power to your vehicle’s axle shafts. The principal reason that differential gears are included is to prevent wheel slips, which usually occur at the point where you try to make a turn. For example, if you attempt to turn your vehicle left, the inside wheels of your vehicle will experience slips. The difference in the rotational speeds of the inside wheels and the outside wheels is the reason that the slip occurs. That is to say that the inside wheel will rotate slower than the outside wheel.

So, you probably got it all figured out. It is the job of the differential to enable the wheels to have different rotational speeds via its gear arrangements. The differential gears split the power that is produced from one shaft evenly between both shafts while giving them the ability to rotate at their own individual speeds. So, when one shaft slows down, the other immediately picks up speed. 

  • Differential Pin

It is the role of a differential pin to ensure that the spider gears are very much aligned and also provide two spider gears with an axis point. This helps in splitting the power more effectively between the left and right wheels.

There are also other components inside the axle housing, aka the pumpkin housing. Named below are these parts.

  • Pinion Gear

This is the point where the driveline power from the axle house starts its internal transfer. Basically, when pairs of gears begin to mesh, the gear that is the smallest is referred to as a pinion gear. This is the cylindrical gear in the rack and pinion mechanism which meshes with the rack. This mechanism is responsible for the transformation of rotational motion into the needed linear motion.

  • Ring Gear

On the differential case, you will find a gear that is bolted to it. This gear is the ring gear. It serves as the second point where the transfer of internally-generated power takes place. The ring gear in a differential system is carved on a rim that has the shape of a ring. The ring gear is that large gear that is located in a vehicle’s differential which is powered by the pinion and is responsible for the live axle receiving power.

Ring and pinion gears share the responsibility of giving your vehicle's wheels the ability to roll forward or backward. This gear set consists of a ring that has grooves, and these grooves are linked to the rear wheel's axles. It is also made of a pinion that has grooves which are linked to your vehicle's front wheels.

  • Miscellaneous Hardware

Inside the housing, you will also find things like bearings, shims, as well as bearing caps. It is the work of the bearing caps to ensure that the differential case stays in place. Then the bearings ensure that every component has the freedom of movement. As for the shims, they are very much needed in order to set the ring-and-pinion backlash. 

Now, let us discuss the different kinds of differentials and how they operate.

How Rear Wheel Differentials Work

Just like every other component that you’ll find on a modern car, the differential has continued to undergo innovations and refinements. This has resulted in the development of a wide range of differentials with each of them having their specific benefits and downsides. 

Basically, the differential has a very simple concept which is to give wheels that are fixed on one axle the ability to rotate at their own pace independently. According to history, the differential is not a new invention and was used in the first Millennium BC. Even though the differential was invented long before automobiles came into existence, other forms of transport, such as wagons and chariots, also experienced the similar difficulty of wheels slipping while negotiating turns. This increased depreciation and caused damage to roads.

The invention of engine-powered wheels one worsened this problem. It was a dilemma that drove automakers to figure out the best way to power the two wheels while allowing them to rotate independently. This problem could not be fixed in the very first automobiles as issues with off-road traction proved to be an issue.

Types of Differential

Open Differential

With time, the open differential was developed to tackle this problem. Let us talk about the open differential and how it works.

The most standard and basic form of a differential consists of an axle that has two halves which both have a gear on their ends. These gears are linked by another gear, and there is usually a fourth gear that is used to supplement this assembly in order to provide extra strength. Also, the ring gear is used to augment this unit by adding it to the casing that houses the core gears. 

Through a pinion, the ring gear connects to a drive shaft, and this enables your wheels to receive power. It is this arrangement that comprises the open differential, which is the most popular type of vehicle differential. The other complicated differential systems are inspired from the open type.

Open differentials are designed in a way that they divide the power generated by the engine into two equal parts and sends each part to the two wheels linked to it. Where any wheel loses contact with the surface or is on an off-road surface, the torque that is transferred to the other wheel becomes zero. 

That is to say that despite the fact that a high traction level is present on the other wheel, no torque will be sent to it. Once the combined traction limit for the two wheels is attained, you will notice that the wheel which happens to have the lowest traction level begins to spin. If that limit is reduced even more, the wheel that already spins will have a much lower resistance.

Basically, this type of differential has a limited benefit. It mostly focuses on giving the axle the ability to corner better as it helps the outside wheel move faster relative to the inside wheel. Also, the open differential is relatively cheaper to manufacture.

The disadvantage of the open differential is that since there is an even division of torque between the two wheels, the level of torque which can be sent through the wheels is limited due to the wheel that has very low traction. This limitation led to the invention of other types of differentials.

Locking Differential

Cars that usually go off-road mostly have a locking differential. This type is like an open differential that is equipped with a locking ability to produce an axle that is fixed rather than an independent axle. Depending on the technology involved, this process can either be manual or automated.

A locked differential has its own benefits. It can produce a relatively higher level of traction compared to an open differential. Since the division of torque is not even, it has the ability to send increased torque to any of the wheels that have more traction. If the other wheel seems to have no traction, it still does not limit this process. Also, since you will not be travelling in high speed due to uneven terrain, you will unlikely encounter the problems of tyre depreciation around the fixed axle corners. 

This type of differential has a disadvantage, referred to as binding, which occurs when so much torque builds up and needs to be released. To release the excess power, the wheels need to lose contact with the ground for a position resetting. Also, you could release the locks if you do not need them.

Binding is created due to the different speeds at which the wheels move. It causes the axles to twist and causes so much pressure on your vehicle's gears. However, since there are load and enhanced traction, it prevents the tyres from slipping in order to release pressure. 

Spool Differential

Spool differentials are similar to a locking differential. The only difference is that it is welded permanently into an axle that is fixed. This is usually carried out in special circumstances that require the desirable features of a locking differential, such as in vehicles which are meant for drifting. Experts do not really recommend this since the strength of the components can be compromised due to the heat that comes from welding. This may enhance the possibility of parts failure. 

Limited Slip Differential

A slip differential is built to fuse the desirable qualities of open and locking differentials in a complex system. There are two kinds of limited slip differentials –

  • Mechanical Clutch

The mechanical clutch limited slip differential still surrounds a similar core gearing that you can find in the open differential that comes with some pressure rings. The core gearing exerts pressure on both clutch plate sets which you find alongside the gears. Thus, the independently rotating wheels become resistant, and the effect is transformed from an open differential to that of a locking differential. Also, increased traction is provided unlike in an open differential. This kind of limited-slip differential is subdivided according to how they behave with the exertion of pressure on your vehicle's clutch plates. 

The first one is the one-way limited slip differential where pressure exertion occurs only during acceleration. Therefore, when you turn a corner, the differential acts just like an open differential and lets your wheels rotate independently. However, when you accelerate, the first differential rotation leads to clutch plate friction, thereby, making them have more traction by keeping them locked in place.

For the two-way limited slip differential, it levels up the process by creating clutch plate pressure during deceleration, too. This is done for stability enhancement while braking on various types of road surfaces.

The combination of both sub types results in the creation of the one-and-half way limited slip differential. 

Mechanical limited slip differentials have to be maintained very often and can easily depreciate rapidly. Thus, replacing its parts becomes expensive.

  • Viscous limited slip differential

In this limited slip differential, thick liquid performs the role of the clutches in creating resistance that is required for altering the open differential and locked differential behavior. Relatively, it has fewer moving parts compared to the mechanical limited slip differential. 

Viscous limited slip differentials have the ability to send torque a lot more effectively where there is more traction. Its fluid is speed resistant, and if a wheel happens to lose traction and spins, the speed difference that exists between both wheels in the differential leads to the creation of increased resistance on the wheel which moves slower. This channels more torque to that wheel.

As viscous limited-slip differentials continue to be used, their effectiveness reduces. With the heating up of the fluid, it loses its viscous property and begins to provide lesser resistance. It is also unable to lock like a mechanical limited slip differential. 

Its downside is that when cornering at a high speed, it does not transfer torque effectively every time. This is because it tends to see the outside wheel that moves more quickly as a sign of traction loss, transferring more torque to the inside wheel and causing oversteer or understeer.

Torsen Differential

This kind of differential uses clever gearing for the production of a similar effect to a limited slip differential system. Torsen differential does not need clutches and fluid resistance. An extra worm gearing layer is added to the main gear set that is found in open differentials. The added gears act on the individual axles for the provision of resistance which is needed to activate the transfer of torque. This is achieved by constantly meshing the worm gears together through spur gears that are connected.

As both sides of the differential continue to mesh, it enhances the immediate transfer of torque which helps it respond rapidly to changes in road and driving conditions. Unlike in open differentials where the torque is always shared equally among the wheels, Torsen differentials have the ability to send a greater amount of torque through a particular wheel based on gear ratio. This helps to eliminate the limitations of open differentials since the available levels of torque are not in any way inhibited by the traction of any of the wheels.

Additionally, it is possible to machine the gear in order to impart different resistance ratios as you accelerate and decelerate. All of these are implemented mechanically, and no electronics are needed and no part is sacrificed. in general, Torsen differential is a superior mechanical system that combines the desirable characteristics of the differentials that we have just discussed above. 

Active Differential

The active differential has so many similarities with the limited slip differential. It makes use of mechanisms for the supply of resistance which is needed for torque transfer between both sides. However, it doesn’t rely just on the mechanical force as it makes use of clutches which may be activated electronically.  

Therefore, it is possible to control and program them. With the aid of certain sensors in your car, a computer can be used to determine the wheels which need power at particular points in time. This helps to enhance power, and rally drivers usually love these kinds of cars since they drive under constantly changing conditions.

Torque Vectoring Differential

The torque vectoring differential steps up the electronic system even more. It uses vector manipulation as you make a series of turns. Basically, it improves your cornering effectiveness by helping particular wheels to get higher torque at certain times. 

It assists you as you steer the vehicle while creating more power at the same time to overcome the weaknesses of a limited slip differential system. As you enter a corner, a multi-way limited slip differential provides resistance to the wheels to get the axle locked to an extent and keep it stabilised while braking. Then it releases it with a drop in the speed of the tyres as you turn, thus giving your wheels the ability to spin at varying speeds.

This time, rather than getting the resistance of the two wheels released, the torque vectoring differential will keep the outside wheel clutch activated. This helps to increase that wheel's resistance, thereby causing more torque to be sent through it. Power imbalance created by this process helps to reduce understeer by allowing the car to make a sharper turn.

As more resistance is applied as you go through the corner, your car goes past the apex and will start to accelerate out. During this process, the multi-way limited slip differential will keep being overridden. As such, the outside wheel which moves faster will be interpreted to be experiencing a slip and torque will be diverted to the inside wheel since it is perceived to have a greater grip.

As the torque vectoring differential exerts more resistance on the clutch of the outside wheels, the system is deceived into torque diversion through it. This enhances the applied power level which reduces understeer that occurs when you try to accelerate out from a turn. Torque vectoring differentials have the ability to channel the entire torque to one particular wheel in extreme circumstances.

The disadvantage of a torque vectoring system is its complexity and high price. It is mostly used for racing cars due to its potential for cornering at very high speeds. 

Conclusion

 All rear wheel differential systems come with pros and cons. You will also notice that the more complicated the system is, the more superior it is, and this affects their cost in relation to simpler systems. When it comes to making a choice, you need to weigh things objectively. Ask yourself the reasons you need a vehicle for and the functions you expect a differential to perform. 

Did you learn from this article? Try How a Four Wheel Drive Works or How Does an Automatic Transmission Work. You may also visit our website if you wish to read more articles, find car part suppliers, and locate auto service providers.