As a result, a simplified Torsen differential will look as shown in the Fig. Now we will go through different driving scenarios and understand how the Torsen manages to operate the vehicle well. When the vehicle moves straight, the worm wheels will push and turn the worm gears. So both the drive wheels will rotate at the same speed. Please note here that, in this condition the worm wheels do not spin on its own axis.
In this condition, the whole mechanism moves as a single solid unit. When the vehicle is negotiating a right turn, the left wheel needs to rotate at a higher speed than the right wheel. This fact is clear from the Fig. This speed differential is perfectly supported in a Torsen.
Please note that the worm wheel is subjected to relative motion not the absolute motion. The worm wheel is fitted between the case and worm gear, so the relative motion between the case and worm gear is what makes the worm gear turn. The worm gear of the faster left axle will make the corresponding worm wheel spin on its own axis.
On the other side, relative to the case the slow right axle is turning in the opposite direction; thus the right worm wheel will spin in the opposite direction. The meshing spur gears at the ends of worm wheel will make sure that, the worm wheels are spinning at the same speed. If one wheel ends up off the ground, the other wheel won't know or care. Both wheels will continue to spin at the same speed as if nothing had changed.
As soon as one wheel starts to lose traction, the difference in torque causes the gears in the Torsen differential to bind together. The design of the gears in the differential determines the torque bias ratio. For instance, if a particular Torsen differential is designed with a bias ratio, it is capable of applying up to five times more torque to the wheel that has good traction.
These devices are often used in high-performance all-wheel-drive vehicles. Like the viscous coupling, they are often used to transfer power between the front and rear wheels. The advent of engines powering front or rear wheels to propel a vehicle instead of merely dragging them via horse added a new problem to overcome — how to allow independent rotation while still being able to power both wheels.
But this was far from ideal as it meant they were underpowered and encountered frequent problems with traction on anything other than firm, level ground. Eventually this led to the development of the Open Differential before other more complicated types were developed to overcome more complex driving conditions.
A differential in its most basic form comprises two halves of an axle with a gear on each end, connected together by a third gear making up three sides of a square. This is usually supplemented by a fourth gear for added strength, completing the square. This basic unit is then further augmented by a ring gear being added to the differential case that holds the basic core gears — and this ring gear allows the wheels to be powered by connecting to the drive shaft via a pinion.
In this example you can see the three sides of the internal gearing that make up the core mechanism, with the larger blue gear representing the ring gear that would connect to the drive shaft. The left image shows the differential with both wheels turning at the same speed, while the right image illustrates how the inner gears engage when one wheel turns slower than the other. This gearing arrangement makes up the open type differential, and is the most common type of automotive differential from which more complicated systems are derived.
The benefit of this type is mostly limited to the basic function of any differential as previously described, focusing primarily on enabling the axle to corner more effectively by allowing the wheel on the outside of the turn to move at a faster speed than the inside wheel as it covers more ground.
It does also benefit from its basic design being relatively cheap to produce. The disadvantage of this type is that because the torque is split evenly between both wheels, the amount of power able to be transmitted through the wheels is limited by the wheel with the lowest amount of grip. Once the traction limit of both wheels combined is reached, the wheel with the lowest amount of traction will begin to spin — reducing that limit even further as there is even less resistance from the already spinning wheel.
The locked or locking differential is a variant found on some vehicles, primarily those that go off road. It is essentially an open differential with the ability to be locked in place to create a fixed axle instead of an independent one.
This can happen manually or electronically depending on technology in the vehicle. The benefit of a locked differential is it is able to gain a considerably greater amount of traction than an open differential. Because you are unlikely to be travelling at speed and are usually travelling over uneven ground, the issue of tyre drag and wear around corners on a fixed axle is less of a problem.
One disadvantage of locked diffs is called binding, which occurs when excess rotational energy torque is built up within the drive train and needs releasing — typically achieved by the wheels leaving the ground to reset the position.
The Torsen differential and the plated limited slip differential serve the same purpose; remaining the maximum amount of torque possible to the wheels at all times, while still being able to go around corners during regular driving. Both use friction, but the way friction is created and used is very different. The Torsen, geared or helical differential does not contain spider gears and side gears like an open differential or a plated LSD.
Instead, there are worm gears on the driveshafts and worm wheels, which are connected to the differential casing. On the end of the worm wheel, there are spur gears which connect the worm wheels together. When the car is driving straight, the worm wheels just push against the worm gears and turn both wheels at the same speed. As both wheels are travelling at the same speed, there is no need for the worm wheels to turn around their own axis.
The Torsen works the same as an open differential when the torque applied to both wheels is equal. If one axle spins faster, it turns the worm gear. This worm gear is connected via the spur gears with the worm gear of the other axle and makes it turn in the opposite direction, therefore making it possible to have both wheels spin at a different speed in corners. The inside wheel turns slower by the exact amount the other wheel turns faster.
This difference in speed is dictated by the road, but what happens if the car is not in a corner? Illustration by Alex Y. If one of the wheels encounters a situation where it has wheelspin, the geared differential will bias torque to the wheel with grip. This happens because there is friction in the differential. There is friction between the worm gear and worm wheel, between both worm gears and between the worm wheels and the housing where they turn around their own axis.
This friction is created by design and is why the differential works as an LSD.
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