On average, the modern automobile is contains around 30,000 individual components. With this amount of parts involved, understanding how they all work can often be confusing. In this article you will learn what exactly a differential is, and how it works.
In the most basic terms, a differential is an assembly of gears that allow two shafts to rotate at different speeds, and in cars also helps to send power to 90 degrees of a rotating shaft.
How this actually works in practise is slightly more complex, but in fact uses very simple principles.
Why is a Differential Needed?
Imagine two circles, one inside the other. The outer circle is larger than the inner one, so if a person on each circle were to race each other around the circumference, the person on the outer circle has physically more distance to cover compared to the inner one.
If the person on the larger outer circle wants to keep up, they would have to run faster to cover the extra ground.
Now replace each runner on the circles with car tyres. This principle is the reason a differential exists.
When a car goes around a corner, like the runners, the outer tyre has more distance to travel and so must travel faster. However, if the wheels have to travel at the same speeds, the outer wheel will be dragged around the corner.
This wears tyres very fast, and affects the vehicles handling.
To solve this problem, it is clear the wheels must be able to rotate at different speeds.
On unpowered wheels this is easy, as they can simply be attached to the vehicle without a physical connection between them, letting them turn independently. The tricky part is allowing wheels powered by the engine to rotate at different speeds.
To do this, you need a differential.
In a basic rear wheel drive vehicle, the engine sends power length ways down the vehicle into the differential, where the power then flows 90 degrees to the wheels.
The differential contains three shafts; one input, and two output shafts, mounted in a T shape.
The engine rotates the input shaft, also known as the prop-shaft. The output drive-shafts then take this power to the wheels, moving the vehicle.
To rotate the drive-shafts, the prop-shaft has a pinion that meshes with the ring gear mounted perpendicularly to it. This meeting point is where power is transferred 90 degrees from the prop-shaft.
The ring gear has two spider gears mounted to it that rotate with the ring gear, but are also free to spin on their own axis. These spider gears mesh with the output drive shafts that connect to the wheels.
That all sounds rather complex, so how does this assembly of gears allow the two wheels to rotate at different speeds?
How it Works
The key is the path of least resistance, and spider gears.
Differentials always provide each tyre with equal torque (rotational energy), and maintain a balance of power (how fast that energy is used) between the two tyres; if one tyre slows down, the other must speed up.
When driving in a straight line, the car tyres are travelling at an equal speed and under equal resistance, so the spider gears, while rotating with the ring gear, are themselves stationary, providing each drive shaft with equal torque and power.
Once the car begins to turn, as we know, the outside tyre must travel faster, while the inside one must travel slower.
This increased resistance on the inside tyre means it slows down, so the the differential sends more power to the outside tyre with less resistance, speeding it up.
This is because the difference in resistance from each tyre forces the spider gears to rotate, maintain power transfer to both tyres that are spinning at different speeds.
Essentially, the inside wheel is harder to turn, so via the spider gear, the differential sends the power to the outside wheel, speeding it up.
To see this in action, watch the short animation below.
This is the operation of the basic differential, it is amazingly simple yet so complex.
Limited Slip Differentials
In every day circumstances, the basic differential is cheap, tough and safe. However they can be troublesome in extreme circumstances as they allow a large difference in wheel speeds and can limit torque.
For example, if a car has one wheel on tarmac while the other is on ice, the wheel on ice needs less torque to turn, but as differentials always supply the same torque to each wheel, this low amount of torque will also be applied to the wheel with traction. This can often mean the car cannot move.
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A limited-slip differential helps with this. Limited-slip differentials limit the difference in tyre speed by applying resistance where needed. This means you can send much more torque to the wheels, increasing performance.