If you’ve ever found yourself in a car conversation with one of your nerdier car-person friends, they’ve probably mentioned how a certain performance car does or doesn’t have an “LSD.” No, they are not talking about hallucinogenic drugs.
In car-guy speak, LSD is short for limited-slip differential, and judging from the name, you might assume that limited-slip differentials, well, limit slip. And you’d be right. But how does it do this? What is an LSD exactly, and how does it actually affect the way a car drives?
Wonder no more because Motor101 is here to explain.
What Is A Differential?
ACD (Active Center Differential)
To understand limited-slip diffs, it’s probably a good idea to understand the regular differential. When a car is turning, the outside wheels (that is, say, the ones on the left when making a right turn) have to spin slightly faster than the inside wheels since they are covering more ground.
Cars solve this problem using a differential. A differential is a device that lets two wheels on opposing sides of an axle spin at different speeds. Picture the engine at the front of a car having to send power to the rear wheels. The engine spins a driveshaft running under the passengers toward the rear axle. The metal lump-looking device that exists at the intersection of the driveshaft and axle is what’s called a differential.
A differential is a device that lets two wheels on opposing sides of an axle spin at different speeds.
This is where it gets capital-T technical, so feel free to skip this paragraph if you only need the simple explanation. Inside a regular, “open” differential, the driveshaft spins a small pinion gear which turns a ring gear. A cylindrical casing inside the ring gear holds a set of bevel-style spider gears as well as side gears that are attached to output shafts, which are ultimately attached to the wheels. When a car turns left, for example, more road resistance on the inside (left) wheel causes the left side gear to slow down relative to the outside (right) side gear, and the spider gears inside the casing, rotating on their own axes accommodate the speed difference.
Too many gears, didn’t read: the normal, open diff uses a bunch of cleverly positioned gears to distribute the torque of a spinning driveshaft to two wheels on opposite sides of the car in a way which allows the outside wheels to spin faster when the car is turning. We should note that differentials are only necessary for a car’s driven wheels (e.g., a rear-wheel-drive car only needs a diff in the rear, not the front), as non-driven wheels can just… roll around at whatever speed they need to freely.
Open differentials are ubiquitous and relatively inexpensive, but there’s one big downside: if or when a driven wheel on one side loses grip as a result of a patch of ice or, say, a driver cooking a little too hard coming out of a corner, the open differential’s, er, open nature means torque will go to that side and result in slip. Slip is not good.
What Is A Limited-Slip Differential?
Photo by: Cadillac
How do we limit slip? You guessed it: with the limited-slip differential (LSD). First commissioned by Porsche in the 1930s to put in its race cars, the key function of a limited-slip differential is to maintain control by shuffling power to the other, grippier wheel when one wheel starts to lose traction and spin wildly out of control.
There are several types of LSDs, but in the most common clutch-based LSD, a set of clutch packs sits behind each wheel’s side gear. Half of the clutch plates on each side are connected to the side gears (they move with the wheels), while the other half is fixed to the diff casing (they move with the upstream drivetrain). Under normal cornering, the clutch packs are lightly loaded (thanks to a spring in the middle of the two side gears) so that both wheels spin together, but can do so at mildly different speeds.
The key function of a limited-slip differential is to maintain control by shuffling power to the other, grippier wheel when one wheel starts to lose traction and spin wildly out of control.
When you accelerate while one wheel loses grip, internal mechanisms push the side gears outward, causing the clutch packs to clamp tighter together. When the clutch packs are tightly engaged, the speed difference is neutralized, allowing more torque to reach the grippier wheel. Hence, slip has been limited, and, hence, we have a limited-slip differential.
These are a few common signs that indicate your car doesn’t have a limited-slip differential:
- One tire spins when accelerating hard
- Trouble getting moving on snow or ice
- Wheelspin exiting corners
- Reduced traction on uneven surfaces
Many Flavors, Same Goal
A common alternative type of limited-slip differential is the Torsen, which uses a set of worm gears to mechanically force torque to the wheel with more grip; no clutches needed. There are also viscous LSDs which keep the two wheels spinning at a reasonably similar rate via connecting the output shafts in a chamber filled with thick silicone fluid.
Whereas most performance car LSDs do their thing in the background passively, off-road vehicles with a locking open diff let the driver manually tell the car to lock the wheels so they spin at the same speed. Useful for when one wheel is stuck in mud or hanging in the air.
A lot of regular, modern open diff cars, meanwhile, can electronically replicate the effect of an LSD with sensors, computers, and brakes—when the car’s sensors detect slip in one wheel, onboard computers apply the brakes on just that wheel to, well, limit slip. These systems are effective enough for regular road driving, but since they inherently rely on slowing the car down, they aren’t as desirable as a true LSD setup for performance cars.
| Open Differential | Limited-Slip Differential |
| Cheaper | More Expensive |
| Common In Regular Cars | Common In Performance Cars |
| Sends Power To Spinning Wheel | Redirects Power To Wheel With Grip |
| Easier One-Wheel Burnout | Better Traction |
Why Would You Need An LSD?
Like a good butler, the entire goal of a good limited-slip differential is that you never really feel it working at all: the car just goes where you point it, no matter how gnarly road surfaces and corner-exit speeds get. You might want one if you:
- Drive in snow
- Enjoy backroads
- Attend track days
- Own a performance car
Instead, you feel it more when a car doesn’t have an LSD or a reasonable electronic stand-in. “One-wheel peel” is a common result of old, rear-wheel-drive muscle cars not having an LSD, while early, front-drive hot hatches were prone to spinning their inside front wheels when clawing out of a bend. Hop in something sufficiently modern and performance-oriented (like, for example, a Ford Mustang GT, Honda Civic Type R, or any new BMW M car), and things are much more manageable in large part thanks to the limited-slip differential.
They make precision driving so much easier, in fact, that an LSD (or, in some competitive arenas, a welded diff) is basically a prerequisite for pulling off sick drifts. That said, it isn’t a cheat code nor will having a car that has one turn you into the next Formula Drift champ overnight. Now, that would be hallucinating.
