Powertrain and what is a differential and where is it located on the car?

Car Differential, Driveshaft, and Gearbox: Overview and Working Principles

Figure 1. Rear-Wheel Drive Powertrain

1. Differential
2. Driveshaft (also known as propeller shaft or Cardan shaft)
3. Gearbox
4. Clutch
5. Engine

 

Differential

The differential is a crucial part of a vehicle’s drivetrain, allowing the wheels to rotate at different speeds while maintaining the torque transmitted from the engine. This is especially important when turning, as the inner wheels travel a shorter distance, while the outer wheels must cover a longer distance.

Main Functions of the Differential

Compensating for wheel speeds: Ensures that the car’s wheels can rotate at different speeds when cornering, improving handling.

Power distribution: Directs engine power from the gearbox to the driven wheels.

Stability and safety: Enhances vehicle traction and stability, particularly on slippery surfaces.

Components of the Differential

Figure. Structure of a car differential

1. Bevel gears
2. Drive pinion (bevel drive pinion)
3. Driveshaft connection
4. Stub shaft
5. Bevel gears
6. Ring gear
 

• Drive pinion

The drive pinion is the small gear in the differential that drives the ring gear. In most differentials, this gear is integrated with a shaft that extends out of the differential housing and connects to the driveshaft.
Because the gear is usually part of the shaft itself, mechanics and technicians often refer to it using alternative terms in everyday language, such as:

• Pinion shaft

• Pinion (the most commonly used term)

• Worm shaft (although technically a worm gear is a different type of gear mechanism, this term is sometimes used informally)

How does this system work? The small gear often referred to by mechanics as the “pinion” (drive pinion) receives rotational motion from the driveshaft. It then drives the large disc-shaped gear called the ring gear.

Together, the drive pinion and the ring gear form the vehicle’s final drive. In everyday mechanical terminology, this gear pair is often simply referred to as the “pinion and ring gear”.

• Stub Shaft

The stub shaft (which in Estonian is sometimes also referred to as the differential output shaft, short intermediate shaft, or informally by mechanics as a short “stub”) is a short but extremely strong metal shaft.

Its role is to act as a connection between the internal components of the differential and the long drive shafts (half-shafts) that transfer power to the wheels.

How does it work and where is it located? Imagine the differential housing. The stub shaft is the short shaft end that extends about 20–30 cm out of the differential.

• Inner end: The inner end extends deep into the differential and locks into the internal gears (side gears) using splines. Through this connection it receives torque from the differential.
• Outer end: The outer end extends outside the differential housing. The main driveshaft (half-shaft) that transmits power to the wheel is connected to it. This connection is usually implemented in one of two ways: A bolted round flange (common on many BMW and other rear-wheel-drive vehicles).Splines, where the inner CV joint of the driveshaft slides onto the shaft and locks in place with a retaining clip

Why is a stub shaft used in vehicle design?

Easier maintenance: Thanks to this short intermediate shaft, the long drive shafts attached to the wheels (which wear faster because of CV joints) can be removed without dismantling the differential itself. In many cases, removing them does not even cause differential oil to leak out.

Protection against oil leaks: The oil seal (shaft seal) in the differential housing seals tightly around the smooth surface of the stub shaft. Because this short shaft is rigidly mounted in the differential and rotates steadily around its axis, it provides a much more stable sealing surface than the long driveshaft, which constantly moves up and down with the suspension. This design significantly improves oil sealing reliability.

Common Types of Differentials

Open differential: The simplest and most common type. However, it has reduced traction on slippery surfaces.

Limited-slip differential (LSD): Improves traction by limiting the difference in wheel rotation speeds.

Locking differential: Used in off-road vehicles and extreme conditions where maximum traction is needed.

Torque-vectoring differential: A computer-controlled system that optimizes power distribution between the wheels.

 

Driveshaft

The driveshaft (more accurately, the Cardan shaft) is a key component in rear-wheel-drive and all-wheel-drive vehicles. It connects the gearbox or transfer case to the rear differential, transferring engine-generated torque to the drive wheels.

Main Components of the Driveshaft

Tube-shaped shaft: Made from steel or aluminum to balance weight and strength.

Universal joints (U-joints): Allow the driveshaft to move at different angles, compensating for suspension movement.

Slip yoke: Enables the shaft to move longitudinally, adjusting to changes in axle length as the suspension operates.

Center bearing: Stabilizes long driveshafts (e.g., in vans).

Driveshaft Maintenance and Issues

Common problems: Vibration due to an unbalanced shaft, worn U-joints, or damaged bearings.

Maintenance: Regular inspections and lubrication of joints help prevent wear and failures.

 

Gearbox

The gearbox is a critical vehicle component that adjusts engine speed and torque before transferring it to the drive wheels. It plays a central role in vehicle operation.

Main Functions of the Gearbox

Speed adjustment: Adapts engine speed to the vehicle’s speed and road conditions.

Torque multiplication: Helps the vehicle accelerate efficiently and tow loads.

Reverse gear: Enables the vehicle to move backward.

Power transmission: Transfers engine torque to the driveshaft or drive wheels.
 

Types of Gearboxes

Manual gearbox: The driver shifts gears using a clutch and gear lever.

Automatic gearbox: Gears shift automatically using a torque converter or continuously variable transmission (CVT).

Semi-automatic gearbox: A hybrid of manual and automatic, allowing manual gear shifting when necessary.

Dual-clutch transmission (DSG): Offers quick and smooth gear shifts using two clutches.

You can read about the different types of automatic transmissions here.

Gearbox Maintenance

Transmission fluid replacement: Should be done regularly according to the manufacturer’s recommendations.

Leak checks: Ensuring the gearbox is free of fluid leaks to maintain performance.

Smooth gear shifting: Difficulty in shifting gears may indicate potential gearbox issues.

You can find differential oils here!