A limited-slip differential (LSD) is a mechanical device that's attached to a vehicle's drive axle. It prevents traction loss and optimizes power distribution when needed. During normal operation, the wheels are allowed to spin at different rates. But when one wheel requires extra power or traction, it can transfer torque to that wheel.
How Does a Limited-Slip Differential Work?
To understand how an LSD works, it helps to compare it to other differential types. An open differential allows wheels to spin independently of each other. A locked differential keeps them spinning together at the same rate. A limited-slip differential is like a marriage of the two.
In normal driving conditions, an LSD allows the vehicle to operate like an open differential. When more grip for one wheel is needed, it functions more like a locked differential.
By adding pressure to the side of the axle with less grip, the wheels are closer to being locked together (though typically not 100% locked). This means they won’t be able to spin at drastically different rates. In turn, this allows the wheel with more traction to put torque down to the ground.
The limited-slip differential changes the balance of power to the two drive wheels, limiting wheel slip. When driving, this will allow for cornering without the loss of control and extra wear caused by a slipping tire.
Types of Limited-Slip Differentials
There are many varieties of limited-slip differentials. But for the sake of simplicity, we can divide them into four primary types.
Clutch Limited-Slip Differential
A clutch differential (also called a mechanical clutch differential or clutch-type differential) is the most common type of LSD. They use clutch plates to transfer torque as needed by providing axle lock during a loss of traction.
Depending on how it operates, these can be a one-way, two-way, or one-and-a-half-way LSD. One-way LSDs work only under acceleration. Two-way LSDs also work during deceleration. A one-and-a-half-way LSD works under acceleration and, to a lesser degree, during deceleration.
Viscous Coupling Limited-Slip Differential
A viscous coupling LSD (also called a viscous LSD or VLSD) uses gear fluid to achieve the same effects as a clutch-based LSD. However, they tend to be smoother than other LSDs at low speeds. They’re also simpler in design and usually more efficient overall. A downside is that the gear fluid will heat up with extended use, lowering its effectiveness.
Geared and Torsen Differentials
Instead of using clutches or fluid, geared differentials use worm gears and spur gears to distribute torque to the wheels. The Torsen type is the most common and well-known geared differential. They don’t require as much maintenance as clutch-type LSDs. They also offer better efficiency. However, they will typically be noisier and have more vibrations.
Electronic Limited-Slip Differential
Electronic LSDs (or torque-vectoring differentials) are the most expensive and complex. The benefits are that it can be proactive rather than reactive. They’re programmed to do exactly what they need to at a given time.
This means they can give power to the outside wheel when exiting a corner, which is the wheel with less grip. This helps with sharper cornering and reducing understeer. Electronic LSDs are perhaps most prominently seen in Lexus vehicles.
Limited-Slip Differential vs Open Differential
An open differential is the lowest-cost when it comes to options for the drive axle. It’s also the most common for everyday cars that won’t ever see a track or serious off-road conditions. The wheels are allowed to spin at different speeds, meaning that they won’t slip while cornering. Another benefit is that there's less overall power loss.
An open differential does not have the ability to transfer power during a loss of traction
There are some downsides, however. Despite having “open” in the name, torque split is actually fixed at 50/50 between the two wheels. This is usually fine for normal driving on paved roads. However, it’s terrible for off-roading.
As the most extreme example, think of a situation where one of the drive wheels is lifted off the ground. Only the wheel on the ground is putting torque down, and it’s still stuck at just 50% of potential output. The other wheel, up in the air, is putting its 50% share of the torque completely to waste. Accounting for friction, the power making it to the “ground” follows the path of least resistance: to the wheel with no traction. This means you’re not getting close to maximum power.
A limited-slip differential helps solve some of these issues.
Limited-Slip Differential Benefits and Drawbacks
A limited-slip differential has many advantages compared to an open differential, with just a few downsides.
Limited-Slip Differential Benefits
Limited-slip differentials allow drivers to put down as much power as possible without breaking traction. This means the car can corner faster, without the unnerving feeling of tires losing grip. It also means less wear on tires due to loss of traction.
A limited-slip differential effectively transfer torque to the wheel with more traction.
In a RWD car, an LSD can prevent oversteer by mitigating a loss of grip during hard cornering.
For FWD cars, LSDs are great for mitigating torque steer while cornering. With so much weight above the wheels in a FWD car, they can have a tendency to get squirrelly under acceleration. An LSD reduces this, preventing the driver from feeling the fun but unsettling shake of the steering wheel.
Limited-Slip Differential Drawbacks
A major downside of a limited-slip differential is that it adds cost. They’re expensive to design and manufacture, and that cost gets passed on to the consumer.
Another drawback is that LSDs aren’t always ideal for getting moving in snowy conditions. It sounds counterintuitive, but sometimes wheel spin can get you out of slushy snow. Turning off your traction control can help with this.
Finally, a limited-slip differential will require maintenance over time, such as oil changes. Clutch-based systems will eventually need their clutches replaced. While this isn’t as difficult or time-consuming as replacing your transmission’s clutch, it’s still something to consider in the long-term.
Limited-Slip Differential vs Torsen
A Torsen differential, which is a type of helical differential, does not use clutches as a traditional LSD does. Instead, it uses gears that can provide instant response to changes in traction. Another benefit is that the gears don’t require the maintenance that clutch packs do.
A Torsen differential uses gears instead of clutches.
Performance drivers that will be tracking their cars prefer Torsen differentials. That’s why Ford includes one in its Performance Packages for the Mustang.
For drivers that may autocross their cars, a traditional clutch-based LSD will do fine. It will feel smoother and less noisy than a Torsen, especially during tight parking lot turns. Plus, you’ll be saving money from the get-go.
Which Vehicles Have Limited-Slip Differentials?
Limited-slip differentials are not common for the automotive market as a whole. They are typically only used on performance vehicles, and even then, not all have an LSD. In fact, even the McLaren P1 supercar doesn't have an LSD.
Limited-slip differentials are commonly found on performance cars from BMW, Audi, Lexus, Dodge, and Cadillac, among many others. They’re also used in off-road applications for some Jeeps. Relatively inexpensive performance cars like the Subaru BR-Z and Ford Mustang also have LSDs.
Other cars that don’t have a limited-slip differential often have software-based alternatives. Many modern systems use the car’s brakes to mimic the effects of an LSD, such as Ford’s torque vectoring control. The majority of drivers will never be able to tell whether their car has an LSD or not.
Sources: Car and Driver | Car Throttle | Engineering Explained | It Still Runs | CarFax | Mat Foundry Group
Images used under Creative Commons License.