A limited-slip differential (LSD) is a device that assesses and redistributes power by shifting varying amounts of torque among the wheels to avert the loss of traction. In other words, as the name implies, this part limits the amount of “slip” to keep you moving, unlike an open differential which can have one wheel spinning and the other stuck. In normal driving conditions, the rotational speed of the driver-side and passenger-side wheels stays the same. On snowy, icy, or muddy roads, though, a difference in the number of rotations between the wheels on either side arises. When one wheel spins due to hitting a patch of ice or too much throttle, the unit automatically sends more power to the wheels with traction. This also happens when taking turns around corners. The outer wheels will rotate more because the turning distance is longer than that of the inner wheel.
Working as an open differential to split torque equally under most conditions and locking when slip occurs, a limited-slip differential marries the concepts of both open and locking diffs. When slippage begins, the limited-slip lock-up is made possible via a complex geartrain, a clutch pack, or a viscous fluid. Limited-slip differentials work to prevent traction loss and obtain the optimal distribution of power by adjusting torque amounts among the wheels. Such allows the wheels with traction to gain more torque and keep the car moving forward. They are incredibly effective for daily driving as well as driving in tricky weather conditions. It’s important to note, too, that LSDs cost more than open diffs and require more maintenance as the fluid doesn’t last the lifetime of the vehicle. Limited slips are also not the preferred type for drifting because those vehicles are built to slide and break traction. Therefore, in this case, a locked differential is much more useful.
Advantages of an LSD
- Performance on- and off-road: Compared to an open differential, an LSD will get you better off-road traction. This is due to the limited-slip differential transmitting power to the wheels which have traction. And, as good as a limited-slip differential is off-road, it is even better on paved surfaces, boasting excellent performance and near-perfect traction. For this reason, many of today’s high-performance vehicles feature LSDs.
- Better cornering and control: When cornering, the inside wheel spins slower and travels less far than the outside wheel. To compensate for this, an LSD will alter the amount of power going to both wheels so they match. If this didn’t occur, the inside wheel would push outward and make cornering a bit harder. With that being said, a limited-slip differential also increases the safety of a vehicle because it increases the control a driver has over their car. An LSD will help reduce tire slippage and can help you get your vehicle back under control more quickly should you start to spin.
- Less tire /axle shaft wear: Because the limited-slip differential takes power away from a wheel that is losing traction and gives more power to the others with traction, it helps to prevent excess wear on the tires. Otherwise, the tire with limited traction would simply spin in place, getting you nowhere and wearing at a faster rate. At the same time, the axle shafts won’t experience as much stress and pressure during turns due to having the ability to rotate at various speeds. This means they will not get worn down as much or as fast.
- More power efficiency: As aforementioned, LSDs are extremely popular on high-power sports cars, because these vehicles take corners at high speeds. When doing so, a limited-slip differential works to slow the car down considerably. The reduction in power can be high in situations where the driver is trying to get high performance from their car. Since the LSD also generates high levels of traction, it also increases the performance and speed at the same time.
Disadvantages of an LSD
- Lack of full power to the wheels: This is the case because the limited-slip differential will always have to transmit a small amount of power to the wheel that doesn’t have traction. Even though it has taken a lot of power away from it, if there is just one wheel with traction, the LSD will not be able to provide all of the power to a said wheel. Therefore, it cannot transmit 100 percent of the power to just one wheel and the power could be shifting.
- Difficult to manage: You will not always be able to predict exactly what the traction is going to do when you are driving on rough terrain with rocks, mud, and sand. While the limited-slip differential will send some power to the wheels losing traction, it won’t be a continuous supply of power. This means that once the other wheels begin to lose some traction, the differential will supply them with more power. A possible result of this is your vehicle being pulled to one side.
- Not all work the same: A limited-slip differential will not be the same in every single vehicle that has one. In addition to a fixed value LSD, torque-sensitive, speed-sensitive, and electronically controlled variations of limited slips do exist. Some systems can control various elements differently so do not get used to one kind of LSD and think that it will work exactly the same in another car that features one, because you may be in for a real surprise.
- Requires more maintenance: A limited-slip differential typically requires more maintenance than an open differential system because there are clutch plates that can wear out and oil that may need to be changed. An open differential does not require this type of maintenance and if you do have an LSD, this type of servicing can be crucial to your vehicle’s overall performance and lifespan of its parts.
Types of Limited-Slip Differentials
There are different types of limited-slip diff available and the one being employed will depend on the drive system a car uses. On rear-wheel-drive and 4WD cars, a 2-way LSD might be used. This means that the LSD be active when applying power and also when slowing down, delivering a more consistent feel to the car. This LSD is active during both acceleration and deceleration to maintain the same slip rate during cornering. Such allows for more stability when braking into a turn.
A one-way LSD is better suited to a front-wheel-drive car, because this will only have a limiting effect when accelerating. When slowing down, the LSD is inactive, which helps with cornering off the power because a 2-way diff has a tendency to introduce understeer to the drive system. In other words, a one-way differential will provide its limiting action in only one direction. When torque is applied in the opposite direction, it behaves like an open differential.
In between these two systems is a 1.5-way LSD, which is effective when load is applied to the differential. This offers a limited-slip effect under acceleration and when slowing, but the amount of slip is not the same in both directions. That means there is less effect in one direction than in the other. This proves to be more useful than a one-way LSD, because it still enables a car to use engine braking when slowing down. A 1.5-way offers a more forgiving balance for beginners during no-throttle conditions and can also prove to be more effective for front-drive cars that need extra stability when braking.
Advancements in LSDs
Some automakers have come up with alternatives to the limited-slip differential that delivers similar results. The first is a viscous LSD, which uses thick oil to create the limited-slip effect. This system, however, can wear faster than a mechanical LSD, and the oil can heat up and lose its effectiveness too. Mechanical diffs employ a mechanically controlled clutch to control the traction of the wheels. The device distributes power, but only operates when it recognizes a difference in the wheels’ rotational speed on each side after it reaches a certain level of power.
Advances in electronics have seen brands succeed in emulating the effects of LSDs by using sensors to achieve the same effects. Some systems feature a conventional differential with an LSD-style clutch pack, but the action is controlled by a computer. This can be tweaked to suit certain requirements, including by the driver via switchable drive modes.
Another variation is a fully electronic differential or e-diff. These systems will have a conventional open diff with no LSD components; instead, the vehicle’s electronics will rely on wheel speed sensors and its ABS system to detect the early stages of spinning and use the braking system to limit the torque going to the wheel that is losing traction. It is a very effective system that has led to torque vectoring, which actively distributes power to the wheels with the most grip.