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The differential is a crucial component in the transmission system of a vehicle. It makes the
driving wheels to rotate at different speeds during turns and manoeuvres, resulting in a smooth and steady drive. However, when it comes to maximise the traction and stability it is necessary to use more advanced devices, such as the limited-slip differential.
As we saw in the previous video, when steering, the two steering wheels of the car have a different inclination from one another because the semi-circle that they have to cover is not the same; in fact, it has a different radius. For the same reason, of the two axles, the inner wheels cover a shorter distance than the outer wheels.
The problem occurs if the wheels of the same axle are connected, as they are in the case of the driving wheels.
In this example we can see a car with a front-wheel drive in which the steering wheels are also the driving wheels (the most common configuration in a car).
It is easy to understand what great stress the front tyres have to withstand, having both to make the vehicle change direction and transmit motion to it.
If they were rigidly connected to each other, during turns (with the car’s load transfer to the external wheels) the internal drive wheel would slide, and it would spin more than necessary causing damage and loss of control.
The differential solves this problem, letting the driving wheels to spin at different speeds, thus distributing power properly.
The conventional differential, called open differential, is located immediately after the transmission. The ring gear that takes motion from the transmission is connected to two bevel gears called “spider gears” placed one in front of the other, which rotate together with the ring gear but can also rotate on their own axle.
The spider gears are coupled with two side gears which are both connected to a different wheel through the axle shafts.
If the vehicle goes straight, the wheels are affected by the same resistive force while driving; in this situation the spider gears rotate together with the ring gear, but not on their own axle (axial rotation), this way the spider gears drag the side gears and the entire unit spins at the same speed, as if they were a whole.
However, if the vehicle turns, let’s say to the left, the internal wheel is affected by a superior resistive force (since the wheel would spin more than necessary).
This force reduces the left side gear speed and now the spider gears rotate along with the ring gear and also start spinning on their own axle (axial rotation).
Since the spider gears are also connected to the right-side gear, this one will receive the same rotation but in the opposite direction, increasing the speed of the external wheel.
This way, the driving force of the engine is transferred to the external wheel more, while also transferring less power on the internal one, in a well-balanced game which perfectly aligns the two wheels to the right speed when turning.
Although the open differential is effective in many situations, it shows limitations when it comes to maximising traction, especially on slippery surfaces o under more extreme driving conditions.
If, for example, on a muddy or a wet track a wheel slips, or in an aggressive turn the internal wheel lifts or slips, its resistant force is automatically cancelled or at least decreased in comparison to the other wheel. This means that the functioning of the open differential transfers the driving force to that precise wheel with less grip, preventing the vehicle from moving forward.
Therefore, sports cars are equipped with LIMITED-SLIP differential which can be of different types, whereas off-roaders are often equipped with manual locking differential.
The limited-slip differential also known as self-locking differential is designed to improve traction and stability by selectively distributing power to the wheels according to driving conditions.
There are different types of limited-slip differentials, but in general, these systems aim to minimize the speed difference between wheels to improve traction. This can be achieved through mechanisms such as controlled clutch packs and viscous coupling unit, which limit wheel’s rotation that loses grip transferring more power to the wheels with better grip.
5 сен 2024
