What Is A Rear Differential? (AN EXCELLENT GUIDE TO KNOWING DIFFS)
If you take a moment to look underneath your vehicle you will most likely notice a large round pumpkin-like housing located on the rear axle. This is a differential. “A differential is a type of gearbox (transmission) that transfers engine power to the wheels while allowing them to turn at different speeds. Let’s learn some more about the differential.
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What Is A Rear Differential?
As mentioned in the introduction, the differential is that large round component usually located on the rear axle of a vehicle. It is commonly referred to as a “diff” for short. its job is to allow the wheels to rotate at dissimilar speeds when cornering. This is important because the wheels on the outside of the turn have a larger circumference to cover compared to the wheels on the inside of the turn, so the differential allows for that difference in rotational speed. There are small and large gears inside and they are covered in a thick fluid called differential oil which is designed to keep the components well-lubricated and cool.
The function of a Differential
To understand how a differential works, think of a track race at an Olympic event. The runners on the inside lanes of the track oval have a shorter circumference to cover compared to the runners on the outer lanes. This is the reason why they stagger the runners on the outside lanes slightly forward to compensate for that difference in the distance they need to cover.
If the runners were to be lined up in a straight line, the runners on the outside lanes would have to run much faster just to keep up with the runners on the inner lanes, which would not be a fair race. Just like those staggering racers, the function of the differential is to allow the outside wheel to rotate faster than the inside wheel while taking a corner.
Types Of Differentials
There are various types of differentials for different applications. For example, four-wheel-drive vehicles or 4WDs have a front, center, and rear differential. Most modern 4WD have locking Differentials in the rear and front. It uses a front and rear driveshaft to send equal amounts of torque to the front and rear diffs. The center differential located in the transfer case connects the front and rear driveshafts to allow the engine power to be sent to the front and rear driving wheels. Many 4WDs have locking rear differentials that allow the diff to lock the side shafts located inside the axle to rotate as a single unit. This is an advantage when going off-road.
LSD (Limited Slip Diffs)
Certain off-road vehicles have an LSD type differential which stands for Limited Slip Differential. It is designed to lock the front or rear axle automatically when it detects wheel slips or when the wheels lose traction while going off-road. These LSDs are usually found on light trucks and medium-sized 4WD Utes designed to go off-road. A 4WD can have a rear locking differential and a front LSD configuration. Alternatively, it can have only a rear LSD and an open front diff. A 4WD can also have a locking front, center, and rear diff for maximum off-road capability. Manufacturers use combinations of these depending on the vehicle’s intended use.
A front-wheel-drive vehicle or FWD has a front differential located next to the transmission called a transaxle which sends the power directly to the front wheels via two side-shafts. a FWD does not have a center or rear differential.
A rear-wheel drive vehicle has one differential located on the rear axle which sends power to the rear wheels. It does not have a front or center differential.
AWD Vehicles
A traditional all-wheel-drive vehicle or AWD usually has a front and rear differential combined with open differentials. This means the diffs cannot be locked for added traction, however, it uses sensors and electronics to manage the traction and power being sent to the wheels. These are usually found in high-performance vehicles like Subarus, Audi, Mitsubishi, and many other AWD vehicles.
They manage the amount of power being sent to the wheels by means of sophisticated electronics and sensors that monitor wheel rotation on the road surface for maximum traction and stability at all times.
Parts Of A Differential
The big pumpkin-like ball you see underneath the vehicle is called the bell housing. Inside the bell housing are various gears that allow for variations in rotational speed on the left and right wheels of the axle.
Some of the gears inside the diff are called:
- Pinion gears
- Ring gears
- Side gears
Advantages And Disadvantages Of A Differential
The advantage of a differential is that it allows your vehicle to corner smoothly and effectively without causing any damage to your drivetrain components, transmission, or tires. The diff allows the wheels on the left and right sides of the axle to rotate at dissimilar speeds to corner effectively thus avoiding any drivetrain or tire damage.
There are no real disadvantages of a differential except in an off-road scenario. Since the diff housing cover is so large, it tends to restrict the vehicle in certain offroad scenarios since it drastically reduces the ground clearance. This is overcome by choosing the right driving line to avoid your Diffs getting damaged or hung up on an obstacle. Also, differentials and driveshafts are rather heavy components and since they are located below the suspension they form part of the unsprung weight of the vehicle. Heavy unsprung weight hurts your vehicle’s MPG.
| Advantages | Disadvantages |
|---|---|
| Smooth and effective cornering | Reduced ground clearance in off-road scenarios |
| Prevents damage to drivetrain components, transmission, and tires | Adds to the unsprung weight of the vehicle |
Note: “Diff” is a common abbreviation for “differential”.
A heavy unsprung weight can significantly impact your vehicle’s fuel efficiency and overall performance. Additionally, it can lead to increased wear and tear on the suspension system, as well as reduced ground clearance in off-road situations. However, the presence of differentials and driveshafts is crucial for smooth and effective cornering, as they help distribute power evenly between the wheels.
Diffs also play a vital role in preventing damage to other important drivetrain components, such as the transmission and tires. Despite the disadvantages associated with the added unsprung weight, the advantages of improved cornering and drivetrain protection make differentials and driveshafts essential components in a vehicle’s overall functionality.
The Basic Design Of A Differential
The basic design of the Differential is the bell housing located on the axle. Inside the bell housing is the differential gears. These include pinion gears, spline, ring gears, side gears, and differential oil. On the outside, usually located at the bottom of the diff is a drain plug. This allows you to drain out old diff oil. Above the diff housing is a fill-hole plug which allows you to top-up or replace the diff oil with new oil. If you hear a whining noise on your diff that is usually a sign to replace your differential oil immediately.
Four Main Functions Of A Differential
- Allow the left-side and right-side wheels to rotate at dissimilar speeds.
- Allows the front and rear axles to operate as a single unit (4WD)
- Protects the drivetrain from damage when cornering.
- Protects the tires from premature wear when cornering.
Resources
- Photo by cody lannom on Unsplash
