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Front-wheel drive sports cars can suffer from poor traction due to the high engine torque transmitted to the wheels.
This is especially noticeable when exiting a bend, where the inner wheel is partially relieved of the weight acting upon it due to load transfer caused by centrifugal force: this results in one of the two wheels being able to transmit significantly less torque to the ground than the other. It's important to remember that an open differential always splits torque equally, meaning the maximum torque that can be transmitted to the ground is limited by the grip of the wheel with less load.
Many modern cars are already equipped as standard with an electronic system called TTC, which SIMULATES a limited slip differential: in practice, when one of the two drive wheels loses grip, the electronic control unit responsible for traction control sends a braking impulse to the brake caliper of the slipping wheel: the negative torque added to the wheel "fools" the standard differential, sending an equal amount of torque to the wheel with better grip. TTC is certainly a valid and safe system that ensures good vehicle control; however, we reiterate that it is not a limited slip differential and does not guarantee the same performance that only a good limited slip differential can provide.
If you regularly use your car on the track during track days, or you don't like suffering from the annoying slipping of the inner wheel every time you exit a corner a little too enthusiastically, then a true limited slip differential might be for you.
Our helical gear limited slip differential allows the engine's torque to be divided asymmetrically, meaning the wheel with more grip will always receive a surplus of torque compared to the wheel with less load. The locking percentage under acceleration is approximately 35%. We chose not to go higher given that the cars on which the differential can be mounted are road cars.
The helical gear differential operates very progressively and does not create undesirable effects in normal driving.
The locking percentage on overrun is limited to approximately 25%, so as not to affect braking and corner entry.
It's very difficult to explain the resulting effect in words, but the feeling of opening the throttle after the apex of a corner and not feeling the front end going off on a tangent, but pulling towards the inside, is something unknown to a front-wheel drive car with a standard differential.
Furthermore, our differential's operation is compatible with TTC.
How does a limited slip differential work? First, let's consider the operation of a normal standard front differential.
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The differential is a device that divides the engine torque coming from the gearbox exactly in half for each of the two half-shafts, regardless of the difference in rotation speed of the two wheels on the same axle. So, as shown in the figure, if the input torque to the differential is 100 (units are completely arbitrary), a torque of 50 will end up on each wheel.
However, there is another important parameter to consider: tire grip on the ground. Torque can be "transmitted" to the ground up to the grip limit of the wheel; this limit is therefore proportional to the fraction of load acting on the wheel and the coefficient of grip, which mainly depends on the consistency of the ground.
So if one of the two front wheels ends up in an area with reduced grip (e.g., on slippery asphalt), the torque that can be transmitted to the ground by that wheel decreases, as the wheel itself reaches a slipping condition. Due to the property of the differential stated above, which is to be a mechanism that divides the input torque into equal parts, we will have the same torque value on the other wheel, even if it maintains good grip.
As can be seen from the previous figure, if the slip limit torque value for the front left is 20, the right wheel can also only transmit a torque of 20 to the ground. Therefore, the total traction of the front axle is reduced to a value of 40, significantly lower than the 100 in the previous example.
If we install the Prometeo limited slip differential instead of the standard differential, traction improves in these cases. Let's see how and by how much.
Firstly, in conditions of good and homogeneous grip, the limited slip differential behaves like a normal differential, as shown in the following figure.
However, if one wheel loses grip, the limited slip differential transfers more torque to the other wheel. The total torque transmitted to the ground by the front axle is therefore greater than that transmitted by a common differential. The following figure is very illustrative of this mode of operation.
The 35% locking percentage is the best compromise found through numerous tests performed under various conditions. We must not forget that an excessive locking percentage would have made the vehicle too understeering in normal driving, with consequent driving problems. And, on the other hand, too low a locking percentage would have reduced the benefits of the limited slip differential, making it objectively useless.
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