Auto Academy: Mastering Torque With Limited Slip Differentials
In my earlier article on AWD vs 4WD, I mentioned the concept of a differential. This is the component of the drivetrain that allows each wheel to spin at a different rate as it goes around a turn. It’s an ingenious bit of mechanical design, but it does have limitations: The power created by an engine will always take the path of least resistance, which means that it will turn the wheel that is easiest to turn. If both wheels are evenly weighted and on the same surface then the power will be equally distributed. If, however, one of the wheels loses friction (snow, ice, water, a burnout, etc.), the more of the power will go to that wheel, which is exactly the opposite of what you want if you are trying to get the car to move. Luckily, there is a solution to our woes: LSD.
Of course, in this case, LSD stands for Limited Slip Differential. This is a type of differential that can use a variety of technologies, including mechanical, hydraulic, and electrical, to limit the torque difference between the two sides of the differential. When it is detected that one side, and therefore one wheel, is rotating more quickly than the other, the limited slip part of the name comes into play.
However, it has been designed, it goes to work to reduce or equalize the amount of torque that goes to both wheels, which effectively reduces the speed of the free-spinning wheel. A limited slip differential is a nice compromise between an open differential, which is what most cars have and which does not control relative speeds, and a fixed/locked differential, which forces both wheels to spin at the same speed no matter what.
Even as someone who studied to be a mechanic and has a degree in mechanical engineering, I still find limited slip differentials to be difficult to explain. Luckily, the Internet is able to provide visuals that simply erase this challenge. The following animation shows the operation of the most basic type of limited slip differential, the clutch pack:
Now remember, I said that was the MOST basic type of limited slip differential. This style of differential was made extremely popular when it was introduced in 1961 as the Positraction read end. If you have ever heard of a car having a “Posi rear end”, this is what they mean. In the muscle car era, when engines were big and tires were skinny, having a Posi rear end was a serious performance benefit. It allowed more of the power to reach the pavement, which helped to get the two to three tons of steel moving quickly.
These days, there are a number of different styles of LSDs that are used in a variety of applications. They all have the same principle, but they go about it in different ways. Many of them rely on mechanical systems to limit the torque differential. Like Subaru’s transfer cases, some limited slip differentials rely on a viscous coupling. When the fluid heats up, due to the rapid rotation of one side of the differential, it locks the components to distribute the power. There are cone-type LSDs, which involve pushing a cone-shaped clutch against the differential housing.
In addition to the mechanical styles, there are electronic systems that typically use a clutch pack, much like a Positraction rear end, that is controlled by the chassis management software. There are even some systems that use the brakes to simulate a LSD without needing to change the differential itself. By applying the brakes on the wheel that is slipping, it allows for a greater amount of torque to be sent to the wheel that still has traction.
One interesting modern concept is the use of multiple LSDs on the same vehicle. For some cars, such as the Subaru WRX STI, there are four different LSDs on the same car. One is used for the front wheels, one for the rear wheels, and two to control the torque split between the front and rear axles. The Driver Controlled Center Differential (DCCD) on the latest STI allowed the driver to have significant level of control over the torque split between the front and rear of the car. Depending on the conditions and the choices of the driver, it can provide a 35/65 front-rear split of torque up to a 50/50 front-rear split. This changes the dynamics of the car and allows for customization to fit varying situations. The DCCD is a combination of an electromagnetically controlled LSD with a standard planetary gear LSD, which reacts more quickly then the electronic unit. Furthermore, the car uses its brakes and brake-biasing to enhance the torque dynamics.
As you can see, there are a variety of choices and technologies, far too many to cover in this introduction. Regardless of the technology, the goal of the LSD remains the same: To provide the ability to evenly split torque between the wheels across a wide variety of traction situations. Unless you really like peg leg burnouts, I think this is a concept we can all support.
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