What Causes a Rear Differential to Go Bad?

Most people do not stop to think about how different systems of their car actually work, but that shouldn’t be the case – car differentials are easy to understand and knowing how to repair them would save you lots of cash in the long run.

This time, we’ll be looking at differential systems in cars – how they work, why do cars need them and what causes rear differentials to go bad.

What Is a Differential and What Is It Used For?

By definition, a differential is “a drivetrain gear assembly that connects two axles and allows one shaft to rotate faster than the other”. In short, it basically controls the speed at which the individual wheels are turning at by using a clever gear system. The differential is crucial for an automobile to function properly.

Why Do Cars Need Differentials?

The differential has three main functions.

Now, in order to transfer power to the rear wheels, there has to be a mechanism that rotates the rotational force of the driveshaft 90 degrees so that the rear axle, and consequently, the wheels are turning the right direction. So, that is the first function of a differential.

The second task is a bit more complicated to explain, therefore, an example will be used.

Imagine you are driving front wheel drive a car in a circle. It is obvious that right hand side and left-hand side wheels do not travel the same distance. The inner wheel is always drawing a smaller circle and the outer wheel will be traveling a longer distance at the same amount of time.

So, for the car to turn properly, in this situation, the outer wheels have to rotate at a faster speed than the inner ones. This is particularly important for the drive wheels which transmit force since non-drive wheels are already independent from each other.

Without the differential, both of your driven car wheels would turn at the same speed. So, while on a straight road this configuration would be fine, in a turn like this, one wheel will always be slipping and understeering. With the differential, the two wheels are able to spin at different speeds when needed. For example, on the same straight road both driven wheels will be spinning at the same speed but in a turn, the rotational speed would be different.

While this example was pictured with front wheel drive car, the same principles apply to rear wheel drive configurations. Even though they are not steering the vehicle, the drive wheels must spin at separate speeds while cornering.

Moreover, differential also determines the gear ratio of a car. It is calculated by dividing the number of teeth on a ring gear by the number of teeth on a pinion gear. In this case, a higher number indicates a lower gear ratio. It is important because it changes how much power and torque the car is able to achieve. For example, a 4:1 gear ratio means that for 1 revolution of a ring gear (or the wheels), pinion gear has to rotate 4 times. Generally, lower gear ratios are used for off-road or in heavier vehicles and higher gear ratios are for every day, and highway usage.

How Do Differentials Work?

Basically, everything begins with the engine converting controlled combustion inside the combustion chamber into mechanical power and turning the flywheel. Then with the help of the clutch, the rotational force is transferred into the gearbox and later – into the driveshaft, if it’s a front engine vehicle it is fairly simple.

For rear wheel drive vehicles, it is more complex as the driven wheels are further away from the engine, thus, they require a driveshaft. If we would look at a picture of the insides of a differential, we would see that at the end of our driveshaft is a pinion gear that connects to a larger ring gear inside the differential. Moreover, a couple of spider gears are attached diagonally to the ring gear and move together with it. Side gears, which are attached to the axle shafts, are meshed with the spider gears.

Now, if the car is moving in a straight, forward manner, the two spider gears will only rotate along with the ring gear as the speed of the wheels must be the same.

As it was already described, the second function is vital for a car to turn. To reach this goal, spider gears rotate along their axes. Consequently, this dictates the speed that the side gears rotate – on one side the rotational speed is increased and on the other – decreased. Thus, we have independency between the two wheels.

Here is a great video on how differential steering works on YouTube:

History of Differential Systems

Differential gears were discovered long before the first automobiles. The first recorded differential gear systems were regularly used in 100BC. It was a device called the Antikythera Mechanism and it used to determine moon phases by defining the angle between the sun and the moon. Additionally, differentials were used in clocks, bicycles.

Only in 1827 Onésiphore Pecqueur, a watchmaker, patented a differential for a steam wagon. That was the start of the device that is used in every automobile today. So, the first automotive differentials were used in steam-powered vehicles. The first automobiles, such as Benz Patent-Motorwagen (patent motorcar) and 1908 Ford model T used differentials.

Types of Rear Differentials

Open (Standard Differential)

The most basic differential out there. It usually comes with a basic package when buying a car. It uses the basic configuration already described in the last section.

The main benefits would be that it is inexpensive, lightweight and requires little maintenance.

The major trade off would be that it is not ideal for tough roads when the wheels start to slip. The next section describes why.

LSD (Limited Slip Differential)

A limited slip differential is a type of differential that limits the independency of the driven wheels that are on the same axle. So, while open differential sends power to the wheel with the least amount resistance, LSD distributes the power and torque between the wheels and the car is able to move while the one with open diff would be stuck.

There are a few types of LSD’s out there. Characterized by when it locks 1-way, 2-way, 1.5-way. That basically shows how the differential acts while accelerating and braking. For example, 1-way diff only locks while accelerating. 2-way while accelerating and braking and 1.5-way does not lock completely whilst decelerating. Additionally, by internal components, there are mechanic or clutch disc, viscous fluid, and electronic types.

The main benefit against open differentials would be seen during difficult road conditions. Especially whilst driving on ice or mud. As open diffs always put out exactly the same amount of torque for both wheels, the wheel with less traction will start slipping, while the one with grip will not have enough torque to have any effect.

Furthermore, LSD’s are known for better handling performance. When a car enters a turn with speed, its balance changes – most of the weight is situated on the outer wheels, while the inner wheels have less weight and, thus, less grip. In this case, LSD’s are able to distribute power to the wheels, so that you can accelerate sooner and more evenly. Therefore, exiting the corner much quicker and more rapidly.

The main disadvantage would be that it adds cost and complexity. Firstly, if your car does not have LSD already installed, it might be a pricey purchase. Usually, the price correlates around a few hundred to a few thousand dollars. And that does not include the fitting expenses. Moreover, as it is a more complex contraption, the repair will be much more complicated and parts – more expensive. And that also means your pocket will be emptier than having a standard diff setup.

Welded Differential

These types of diffs are made by taking an open differential and welding the gears, so that they would not rotate around their axes, thus, canceling the independency between the two wheels.

It is similar to just having your driveshaft connected to the axle. It is usually used by drifters as it allows both wheels to lose traction and spin making it easier to oversteer and do burn-outs. It is also much cheaper than buying and installing an LSD kit.

The main disadvantage is that it is a pain to drive it daily. Driving at low speeds and parking is the worst. For example, while parking backwards on an open differential, the inside wheel is slowly rotating, compared to the outside wheel. Now, cars with welded diffs will be slipping and not making the turn as sharp which might be a problem for tight parking spots.

Moreover, tire wear is noticeable and more rapid. Then again, if you will be using it for drifting, you will burn tires quicker than they would naturally wear out, so that is not a big concern.

Constant stress to the axles is a problem. Especially when running wider tires and doing tighter turns. Sadly, this might not be avoidable, but it mostly depends on your driving style and your car.

How & Why Rear Differentials Go Bad

A stock differential failure is rare, especially if it is being maintained properly. However, once they do go bad – excessive mechanical wear between gears (due to lack of lubrication), excessive backlash, or inadequate backlash is to be blamed.

It is not easy to tell when your differential is starting to go bad as it is not always a sudden change in drivability or noise. It usually takes time for the problem to develop.

The first sign of a problematic diff you would notice would be the sound coming from the rear end. Whining, whirring, rumbling, clanking, or banging noises all might indicate an issue with the differential. If the problem is more severe, you can notice problems while driving, especially when turning. Of course, firstly, it is a good idea to rule out other potential problems like a worn wheel bearing or your suspension, as differential repairs tend to be more expensive.

Improper contact between the pinion and the ring gear might cause uneven wear or even broken teeth. One of the best ways to diagnose fitment problems is to mark the ring gear:

The perfect contact surface has to be aimed to be in the middle of the ring gear and the patterns on different sides have to match as closely as possible. Depending on where the patterns are situated, correct adjustments ought to be taken.

Differential backlash movement usually comes with a clunking sound when accelerating or decelerating.

The problems happen when there is an excessive or insufficient looseness between the gears. Of course, as the parts wear and get looser, we face extra play between them. Moreover, while excessive backlash usually happens between the pinion gear and the ring gear, it might happen between the spider and side gears or even half shaft ends.

The backlash should be within tight specs. It can potentially wear out your differential quicker or even break the teeth on the gears while accelerating hard.

As mentioned before, there is a risk of snapping an axle whilst driving with a welded diff. It’s usually a good idea to consult others about your specific model or buy an LSD and have a piece of mind.

And of course, there is the usual wear and tear of the differential. Modern differentials are quite sturdy, therefore, they are made to withstand extremely high mileages if they are properly taken care of.

So, most failures come from improper assembly, modifications, or poor maintenance.

Closing Thoughts & Tips

Maintenance is crucial for smooth operation of every piece of equipment. While most people only change their cars’ engine oil and washer fluids, in fact, all other fluids must be replaced as well.

Over time, oils lose their viscosity and ability to lubricate. This happens due to oxidation and heat. Also, oil collects metal and other particles from the system, and by changing it, we get rid of the abrasive particles that damage our cars – that’s one of the reasons why the initial oil changes are so important.

Remember to check the maintenance manual for info on when to replace your fluids and which type is best for your vehicle. Do not forget a limited slip additive if you have an LSD.

Lastly, some car repairs are to be left for the professionals. If you are a car enthusiast that sometimes tinkers with their vehicle, please consider leaving more serious repairs for professionals. They have the knowledge and tools required for the job.

Resources:

1. Why Can Ignoring Potential Car Diff Problems Become an Expensive Mistake? – Automatic Transmissions R Us 
2. Differential Noise & Vibration: Here’s How to Diagnose the Problem – Mechanic.com.au

Eddie Westheimer

Eddie is the co-founder of CarCareCamp.com, and the site’s primary contributor.

Under his belt, Eddie has a bachelor’s degree in Automotive Electronics Engineering and almost a decade of experience working as a  semi-truck technician (specializing in electrics).