When we think of the basic driveline elements of a car, the engine, gearbox, and driven wheels are our first and often only thoughts as critical elements responsible for driving. But there’s a lesser thought of device that plays a big, big role – the differential. The differential plays a big role not only in the whole driveline process, but in the handling of a vehicle. Power is nothing if you can’t use it, after all.
But what is a differential? How does it work? How does it benefit a car, and are there any drawbacks? Read on…
What is a differential and what is its purpose?
The differential is a unit comprising several different types of gears that sits between the gearbox and the drive shafts of the driven wheels. Its physical position is on the usually on the axle of the driven wheels – at least in its most basic configuration – essentially splitting the driven axle into two parts. It has three key functions:
- It directs torque towards the driven wheels – often converting drive through a 90°
- It acts as a final gear reduction, adjusting the rotational speed one final time before the rotation reaches the driven wheels.
- The third, and main function of a differential though – and the one from which it derives its name – is that it allows two driven wheels on the same axle to rotate at different speeds.
That last function is vastly important to the operation of a vehicle and its handling. When driving in a straight line – the wheels, left and right, travel the same distance and rotate at the same speed. However, when a vehicle is turning, the inside and outside wheel travels different distances.
Consider the example diagram below:
If a vehicle’s wheels are 1.5 metres apart (from the centre to the centre of each tyre), and the vehicle drives a complete circle of a steady radius – in this example 1.5 metres on the inner wheel – the outside wheel travels on a circle with a radius of 3.0 metres. The circumference of the two circles travelled – the total distance of the paths of the inner and outer wheels – can be calculated using the mathematical formula of: Circumference = 2 x π x radius
The distance travelled by the inner wheel is 9.4 metres, while the inner wheel travels 18.8 metres. The outer wheel needs to cover a greater distance – in this case double the distance – in the same time it takes the inner wheel to cover a shorter distance. To do this, the outer wheel needs to rotate quicker than the inner wheel. This is where a differential comes into play.
How does a differential work?
A standard differential – often referred to as an open differential – is the most simple and common type of differential. It is made up of four key components:
- A pinion gear on the input shaft
- A ring gear
- Spider gear/s
- Side gears connected to the shafts of the driven wheels
The pinion gear meshes with the ring gear to convert drive through 90° to direct power towards the driven wheels. The spider gear is mounted to the ring gear and rotates with it – in turn rotating the side gears and the driven wheels. In a straight line, the spider gear is in a fixed position and both wheels rotate at the same speed. However when turning, the spider gear can rotate on its own axis – a process that turns one side gear quicker while it slows the rotation of the other side gear. This allows the wheels to rotate independently and either side of the vehicle to travel at a different speed. In essence, this is the simplest form of torque vectoring – a topic we’ll discuss that in another episode of Tech Talk soon.
The video below simply shows the basics of a differential:
But an open diff – being the simplest type in its construction – does have a few drawbacks.
Downsides of an open differential…
The basic principle of an open differential favours the wheel with the least resistance, allowing it to spin quicker. In principle it sounds good – and in an ideal world, well within the limits of day to day use it would be fine – that’s why many commuter cars use open differentials. But, in scenarios where one wheel lacks sufficient grip on a surface, it can become problematic.
If one wheel starts to slip – as a result of rain, ice, or even loose sand – the open differential would allow the slipping wheel to spin freely, while depriving the other wheel of any drive at all. The same situation can occur when cornering at high speed where individual wheels may break traction due to the road surface or throttle inputs. In such situations, handling, and even forward momentum is impeded, and handling responses are not as sharp and direct as needed.
There are different types of diffs though, some purpose built to alleviate this issue. These are known as limited slip differentials and are just one of numerous types of differentials.
What other types of differentials are there?
The Limited Slip Differential (LSD) is the most common alternative style of differential – frequently used in high performance vehicles to improve handling ability and overall levels of grip. Within the LSD grouping, there are two main sub-sections:
- Mechanical LSD – Mechanical LSDs make use of spring loaded type system on the outputs shafts that responds to centripetal force (force acting in a straight line away from the centre of rotation). They allow slippage and different wheel speeds up to a point, but when one wheel’s speed is vastly different to another, the spring system is engaged, either locking the differential partially or fully. This system places a limitation on the least resistance principal – only allowing this up to a point. Mechanical LSDs have been put to good use in many cars such as the Renault Megane RS among others to improve handling.
- Electronic LSD – The electronic LSD, or e-diff, makes use of computer sensors that detect wheel speed. When one wheel is detected to be spinning too fast in relation to both ground speed (km/h or mph) and the speed of the other driven wheel, a computer triggers an electronic actuator that either partially or fully locks the differential, adjusting individual wheel speed accordingly. The Volkswagen Golf GTI has notably made use of the e-diff, starting with the Golf 7 GTI Performance Pack.
Locking differentials are another common type of differential, more commonly found on off-road vehicles such as the Jeep Wrangler and Mercedes-Benz G-Class. These are traditionally open differentials, but with the ability to lock the differential for a fixed torque split to either wheel. In muddy circumstances, these ensure that each wheel is constantly driven, which allows for maximum grip on slippery surfaces. Locking differentials can be had in either mechanically operated variants or electronically actuated ones.
Centre differentials are a type of diff traditionally used in either 4WD or all-wheel drive (AWD) applications. Where diffs are traditionally utilised to apportion torque delivery between wheels on a single driven axle, centre diffs allow for torque to be distributed between the front and rear axles. Centre differentials are commonly found in two main variants:
- In 4WD uses, the centre diff often allows the front or rear axle to be disengaged entirely, reducing fuel consumption and general wear and tear. The un-driven axle can be re-engaged – generally at a standstill – when poor terrain is encountered that necessitates additional driven wheels. This is most commonly found in bakkies/pick-ups and 4×4 off roaders.
- Not every all-wheel drive system makes use of a central differential. It’s most commonly a feature found on performance vehicles that make use of all-wheel drive. Audi RS models such as the RS3, RS4, RS6, and RS7 are models that make use of such a diff, and another famous one is the Subaru Impreza WRX STi. In these types of applications, either a mechanical or electronic differential can be used in various manners:
- Traditionally, all-wheel drive systems make use of a semi-fixed torque split, whereby both the front and rear axles are driven constantly, with the differential allowing additional torque to be apportioned from front to back or vice versa. The standard split, and the amount which can be adjusted to each axle differs from car to car. Though these systems are not traditionally lockable, some cars like the WRX STi feature the ability to lock the centre diff.
The controls for the centre differential in the Subaru WRX STi - Front-wheel drive biased systems, such as Haldex, and found on cars such as the VW Golf R, Audi RS3, and Mercedes-AMG CLA45, traditionally drive only the front axle. When slippage is detected on the front wheels, the central diff is engaged and distributes torque to the rear axle for better grip. By disengaging the rear axle automatically when not needed, these systems offer superior fuel economy. But, they can often be slower to respond than systems where both axles are at least partially driven.Audi has embraced this system for its latest version of quattro all-wheel drive on non-performance variants, making use of an electronically actuated diff. Older Haldex systems made use of mechanical diffs to achieve the same effect.
- Traditionally, all-wheel drive systems make use of a semi-fixed torque split, whereby both the front and rear axles are driven constantly, with the differential allowing additional torque to be apportioned from front to back or vice versa. The standard split, and the amount which can be adjusted to each axle differs from car to car. Though these systems are not traditionally lockable, some cars like the WRX STi feature the ability to lock the centre diff.
Conclusion
Though the basic concept of a differential might be quite simple, the numerous different types of differentials have all been conceived to tailor the basic design to the needs at hand. While all modern cars feature at least one differential, it’s not at all uncommon to find performance or off-road vehicles with two, or even three differentials. Though they may all do different things, their basic function remains the same – to allow torque to be distributed in varying amounts either front to back, or side to side.
Make sure to follow us on social media for our follow-up Tech Talk segment: What is Torque Vectoring? Coming soon…
Written by Roger Biermann
