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What is a differential and what does it do?

Latest car guides

September 1st 2022

The differential is a core component of modern-day cars. However, its function is largely misunderstood by most motorists. This is understandable, as differentials aren’t exactly a hot topic during car maintenance checks.

To help you understand a little more about your car, let’s take a look at the differential in a bit more detail.

What is a differential?

Sometimes called ‘the diff’, a differential is a mechanical system that regulates the speed of your car’s wheels, helping them to rotate at different speeds while cornering. Differentials are a necessity for our cars, as they help us take turns and quickly manoeuvre around winding roads.

The differential sits between the engine and the wheels, splitting torque output as needed. Depending on the type of transmission your car has, the differential will be placed in different areas in the chain of command, but the goal remains the same.

Types of differentials

There are two main types of differentials, each serving its own unique purpose:

Open differential

Most cars have what’s called an open differential. Open differentials allow wheels to turn separately, at the detriment of reduced traction. If one wheel loses grip during a skid or while driving over ice, power is still able to be transferred to it, worsening its effects.

One way this is rectified is through traction control. Traction control isn’t controlled via the differential, rather it is a function of modern braking systems wherein, if a wheel loses traction, its speed can be reduced to regain control.

Limited slip differential (LSD)

If you’re driving a project car built for race days, it’s likely you will have migrated towards a limited slip differential. In comparison to open differentials, limited slip differentials allow wheels to turn at the same rate while pulling away. In the event of a skid, power is sent to the opposing wheel in the hopes of negating the skid’s effect.

How does a differential work?

To explain how a differential works, let’s use an example from its simplest configuration.

Differentials are composed of three gears: the ring gear, pinion gear and spider gear.

The pinion gear sits at the end of the car’s drive shaft, and is connect to the outer diameter of the larger ring gear.

The spider gear rotates both on the inside of the ring gear, and on its own.

The rear wheels are then meshed up to the spider gear via side gears that sit alongside two independent axles.

When driving in a straight line, the spider gear doesn’t need to rotate. It is only when taking a turn that the spider gear begins to move in favour of whichever wheel needs more rotation.

Why do we need a differential?

When it comes to the existence of a differential, it helps to cast your mind back to childhood.

Imagine playing with a toy car. Moving it in straight lines is a doddle; it easily glides along the floor with minimal issues. However, when you try to turn the car around a bend or in a figure of 8, you’ll start to leave scuff marks on your floor or carpet. This is because toy cars, specifically those without axles, have wheels that spin at the same speed, all the time.

So, what does a differential do? In short, they help get your car around bends without scraping tyres along the floor, by regulating alternating speeds of two wheels.

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