Two driven wheels can share the same engine, the same gearbox and the same axle, yet they cannot always turn at the same speed. Every time a car takes a corner, the outside wheel travels farther than the inside one. Lock both wheels together and something has to give: a tyre must scrub, slip or deform as the car turns.
The differential solves that problem. It lets the driven wheels rotate at different speeds while both remain connected to the drivetrain.
That freedom is essential in a corner, but it creates another problem when traction is uneven. Put one driven tyre on a slippery surface, unload the inside wheel under hard acceleration or lift a wheel entirely, and an ordinary open differential can struggle to make useful use of the grip that remains.
A limited-slip differential, usually shortened to LSD, changes that behaviour. It still allows the wheels to rotate at different speeds, but it adds resistance to excessive differentiation between them.
How it creates that resistance depends on what is inside the differential. Clutch plates, helical gears, viscous fluid and electronically controlled clutches can all produce an LSD effect, and they do not behave in exactly the same way.
Why Your Drive Wheels Need to Turn at Different Speeds
Imagine looking down at a car as it drives through a right-hand bend.
The right wheel is on the inside of the corner. It follows a relatively short arc. The left wheel is farther from the centre of the turn, so it has to follow a larger arc in exactly the same amount of time.
The outside wheel therefore needs to rotate faster.
This difference may be small in a gentle motorway curve, but it becomes much more obvious in a tight turn or when manoeuvring in a car park. If both wheels were rigidly connected to a single shaft, they would be forced to rotate at the same speed despite travelling different distances.
That arrangement can work in machinery that mostly travels straight. It is much less satisfactory for a road car. One or both tyres would have to slip against the surface whenever the vehicle turned.
A differential sits between the driven wheels to avoid that conflict.
Power arrives from the engine through the transmission and final drive. Inside the differential, a set of gears divides the drive between the left and right axle shafts while allowing one side to rotate faster than the other.
In simple form, the path looks like this:
Engine → transmission → final drive → differential → left and right drive wheels
When the car travels straight and both tyres cover essentially the same distance, the wheels can rotate at similar speeds. As the car turns, the differential permits the speed difference required by the two paths.
That is the fundamental job of any conventional differential. Limited slip is an additional function layered on top of it.
What Happens Inside an Open Differential
An open differential is the useful starting point because it shows both why differentials exist and why an LSD can be valuable.
The final-drive gear turns the differential carrier. Inside that carrier are side gears connected to the axle shafts and smaller differential gears that allow the side gears to rotate relative to one another.
There is no need to memorise every gear to understand what happens.
When both wheels encounter similar conditions and rotate at the same speed, the internal gearset effectively carries them around together with the differential case. During a turn, relative movement inside the gearset allows one axle shaft to slow while the other speeds up.
An open differential does this extremely well. It is simple, smooth and does not try to prevent the speed difference that a car naturally needs in a corner.
Its weakness becomes apparent when the two tyres have very different amounts of available traction.
This is also where a familiar explanation causes confusion. It is often said that an open differential “sends all the power to the wheel with the least grip”. That phrase describes the visible result reasonably well in some situations, but it is a poor explanation of the underlying mechanics.
The important quantity to think about is torque.
The Problem Appears When One Wheel Loses Grip
Suppose a rear-wheel-drive car stops with its left tyre on ice and its right tyre on dry asphalt.
The right tyre has plenty of traction. The left tyre does not.
For an illustrative example, assume the icy tyre can sustain only 50 Nm of drive torque before it starts to spin. The tyre on dry asphalt could accept far more.
With an ideal open differential, the torque available at the two outputs is essentially equal. Once the low-grip side can support only about 50 Nm, the high-grip side is also limited to roughly 50 Nm.
The result looks like this:
Left wheel on ice: 50 Nm
Right wheel on asphalt: 50 Nm
Total useful axle torque: about 100 Nm
The frustrating part is that the right tyre may be capable of putting several times that amount of torque into the road. Its grip is available but cannot be fully exploited by the open differential under these conditions.
Press the accelerator harder and the low-traction tyre simply spins faster. The car may make little progress despite having one tyre sitting on a perfectly good surface.
A less dramatic version of the same problem can occur in performance driving.
As a car accelerates out of a corner, load does not remain equally distributed across its tyres. The inside driven tyre can become relatively lightly loaded. If enough drive torque reaches it, it may begin to spin even though the outside tyre still has substantial traction available.
This is the problem a limited-slip differential is designed to manage.
What a Limited-Slip Differential Actually Changes
An LSD does not eliminate the differential action that allows a car to turn. If it did, it would simply be a locked axle.
Instead, a limited-slip differential introduces a mechanism that resists relative movement between its two outputs under particular conditions.
That resistance is the key.
In an open differential, very little internal resistance exists to prevent one output from rotating much faster than the other. An LSD deliberately introduces a degree of internal friction or coupling. This allows the differential to support an imbalance in torque between the two axle shafts rather than remaining constrained to the behaviour of an open unit.
The tyre with better traction can therefore make use of more drive torque.
It is tempting to describe this as the LSD “finding” the wheel with grip and sending power there. Mechanical limited-slip differentials do not need to think, measure the road surface or make a decision in that sense. Their internal mechanisms react to forces, speed differences or torque loads according to their design.
That distinction becomes important because not every LSD responds to the same thing.
A viscous limited-slip differential is fundamentally sensitive to a difference in rotational speed. A helical or Torsen-type differential develops its bias through forces and friction generated under torque. A clutch-type differential can use preload and load-dependent clamping of friction plates. An electronically controlled LSD can actively change clutch pressure according to commands from the vehicle’s control system.
They share an objective, not one universal internal mechanism.
The name describes the compromise neatly. The wheels are still allowed to slip relative to each other in the sense of rotating at different speeds. The amount or effect of that differentiation is limited when the mechanism is active.
Clutch-Type LSD: Using Friction to Tie the Wheels Together
One of the traditional ways to create limited-slip action is to put friction clutches inside the differential.
A clutch-type LSD contains packs of friction plates associated with the side gears and differential case. Depending on the design, spring preload and forces generated when torque passes through the differential press those plates together.
Friction between them resists the tendency of the two outputs to rotate independently.
That resistance allows a clutch LSD to maintain a greater difference in torque between the drive wheels than an open differential can. If one tyre begins to lose traction, the other side can continue to receive a more useful share of the available torque.
Clutch-type units are particularly attractive in performance and competition applications because their behaviour can be tuned. Preload, friction materials, plate arrangement and ramp geometry can all influence how aggressively the differential responds under acceleration and deceleration.
The same feature brings compromises.
Friction plates wear. The differential may require a particular lubricant or friction modifier. Aggressive setups can produce chatter, noise or a noticeable resistance to differentiation during tight, low-speed manoeuvres. A setup that works beautifully on a racing circuit may be unnecessarily intrusive in a road car that spends most of its life commuting.
This is an early hint that an LSD cannot simply be rated as “better” because it locks more strongly. The useful amount of limiting action depends on what the car is supposed to do.
How a Torsen Differential Biases Torque Without Clutch Packs
A Torsen or similar helical-gear limited-slip differential takes a very different route to the same general objective.
Torsen is derived from torque sensing. Instead of relying on a conventional friction clutch pack, the differential uses carefully arranged gearing to generate internal forces and friction as torque passes through it.
Helical gears naturally generate thrust forces when they are loaded. Inside a torque-biasing differential, those forces push components against the differential casing and other reaction surfaces. The resulting friction creates resistance to differentiation.
The useful consequence is that the amount of limiting action can rise with applied torque. Under light load, the differential can behave smoothly and allow the speed difference needed for ordinary cornering. Apply substantial drive torque with unequal traction available and the internal forces increase, allowing more torque to be supported on the high-traction side.
This behaviour is often described using a torque bias ratio, or TBR.
Suppose a particular differential has a theoretical TBR of 4:1. If the low-traction wheel can sustain 50 Nm, the differential may be capable of supporting up to four times that torque on the high-traction side:
Low-traction side: 50 Nm
High-traction side: up to 200 Nm
50 × 4 = 200 Nm
Compare that with the simplified open-differential example, where the same 50 Nm traction limit on one side restricts the other side to approximately 50 Nm as well.
TBR does not mean that the differential always splits torque 80:20 or that it manufactures extra torque. It describes the maximum bias the mechanism can support under the relevant conditions. Torsen itself defines the ratio in terms of how much more torque can be supported by the high-traction output relative to the low-traction one.
There is an important consequence hidden in the calculation.
If one tyre is completely off the ground and can provide essentially zero reaction torque, then:
0 × 4 = 0
A conventional torque-biasing differential needs some reaction torque at both outputs to create its useful bias. This is why a Torsen-style LSD is not the same thing as a fully locking differential. Brake intervention can sometimes provide the missing resistance at a spinning or unloaded wheel, allowing the torque-biasing differential to work against that artificial load.
That limitation is not a defect in the concept. It is part of the trade-off that lets a helical LSD remain smooth and unobtrusive during normal driving.
Why Viscous LSDs React Differently
A viscous limited-slip system does not need friction plates to clamp together or helical gears to create torque-dependent thrust forces.
Instead, it uses sets of plates operating in a highly viscous fluid. Alternate plates are connected to different sides of the driveline. When both sides rotate at similar speeds, there is relatively little relative motion between the plates.
When the speed difference increases, the fluid between them is sheared more aggressively. That creates drag, coupling the two sides more strongly and resisting the growing difference in rotational speed.
This makes a viscous system fundamentally speed-sensitive.
That distinction is useful because it shows why saying “an LSD senses wheelspin” can be misleading. A viscous coupling really does rely heavily on a speed difference developing before its coupling effect increases. A Torsen-type differential, by contrast, is torque-sensitive and can generate bias through load before a large wheel-speed difference develops.
Viscous systems have the advantage of smooth, progressive operation and relatively simple control because the behaviour emerges from the physical properties of the coupling itself.
They also have limitations. Their response is not as immediate as a torque-sensitive geared unit, the fluid’s properties can change with temperature and age, and the system has less scope for active control than a modern electronically managed clutch differential.
| Type of LSD | What creates the limiting action? | Basic response | Main strength | Main compromise |
|---|---|---|---|---|
| Clutch-type | Friction plates | Preload and torque/load dependent | Strong and highly tunable locking effect | Wear, maintenance and possible low-speed harshness |
| Torsen / helical | Gear forces and internal friction | Torque sensitive | Smooth, immediate mechanical torque bias | Needs reaction torque at both outputs |
| Viscous | Shearing of viscous fluid | Speed sensitive | Smooth and mechanically simple | Less immediate response and fluid ageing |
| Electronically controlled clutch LSD | Actuated clutch pack | Actively controlled | Can adapt locking action to driving conditions | Greater cost and complexity |
What an LSD Changes When You Accelerate Out of a Corner
The benefit of an LSD becomes much easier to appreciate from the driver’s seat than on a workbench.
Consider a rear-wheel-drive performance car approaching a tight corner. During the turn, the inside rear tyre is typically less heavily loaded than the outside rear tyre. As the driver begins to accelerate, the engine sends increasing torque towards the rear axle.
With an open differential, the lightly loaded inside tyre may reach its traction limit first.
Once it starts spinning, the amount of useful torque available at the other rear wheel is constrained. More accelerator does not necessarily produce proportionally more forward acceleration. It can simply produce more wheelspin from the inside tyre.
A limited-slip differential resists that situation.
By supporting a torque difference between the two axle shafts, it allows the more heavily loaded outside tyre to make better use of the grip it still has. The car can convert a greater proportion of engine torque into acceleration rather than spinning one tyre.
This is why limited-slip differentials are so strongly associated with performance cars. The advantage is not that the differential somehow creates additional tyre grip. It helps the drivetrain make better use of grip that already exists.
For the driver, that can mean being able to apply meaningful throttle earlier on the exit of a corner, with less wasteful inside-wheel spin.
The same principle is useful away from a circuit. A wet road, snow, mud, a steep uneven driveway or any surface that gives the two driven tyres significantly different traction can expose the weakness of an open differential.
Exactly how much an LSD helps depends on its design. A helical unit, a heavily preloaded clutch differential and an actively controlled eLSD will not react identically to the same situation.
More Lock Isn’t Always Better
If limiting wheelspin improves traction, it is easy to assume that increasing the locking effect must improve the car further.
That logic eventually runs into the original reason a differential exists.
The wheels need to rotate at different speeds in a corner.
Every time an LSD resists differentiation, it is balancing two competing requirements: allow enough speed difference for the car to turn naturally, but resist enough of it to prevent one tyre from wasting the available drive torque.
A very aggressive differential can therefore change how a car enters, travels through and exits a corner. The effect depends on which axle is driven, whether the driver is on or off the throttle, the type of LSD and the rest of the chassis setup.
In some circumstances, additional resistance to differentiation can encourage the car to push towards a wider line. In others, particularly under power in a rear-wheel-drive car, the extra ability to use both rear tyres can make throttle a much stronger tool for influencing the car’s attitude.
This is why competition clutch differentials may offer adjustments such as preload and different ramp configurations, and why terms such as 1-way, 1.5-way and 2-way LSD exist. Those settings determine how the differential behaves under different torque directions rather than simply giving it a single “more grip” setting.
They are a subject of their own. For understanding an ordinary limited-slip differential, the important point is simpler: a useful LSD must still permit the differential action the chassis needs.
A differential that resists every speed difference as strongly as possible would cease to behave like a well-mannered limited-slip unit and move towards the behaviour of a locker.
Mechanical LSD, Electronic LSD and Brake-Based Imitations
Modern cars complicate the terminology because electronics can produce similar results in several very different ways.
A traditional mechanical LSD generates its limiting action internally through components such as clutches or helical gears. Its behaviour is primarily a consequence of the mechanical forces acting inside the differential.
An electronically controlled limited-slip differential, or eLSD, can still contain a genuine mechanical clutch mechanism. The difference is that an actuator controls the clutch pressure. Instead of relying only on fixed preload or passive mechanical reactions, the car can vary the amount of coupling according to operating conditions.
That gives the control system far more freedom.
Wheel speeds, steering angle, accelerator position, yaw behaviour, braking and other vehicle data can all contribute to a decision about how much differential locking is appropriate. Eaton, for example, describes its eLSD technology as an electronically controlled clutch system that responds to steering, acceleration, braking and wheel-speed inputs.
A brake-based electronic differential is different again.
The car may retain an ordinary open differential. When one wheel begins to spin, the stability or traction-control system applies the brake to that wheel. The braking force creates resistance on the low-traction side. Because an open differential can now support more torque at that output, more useful torque also becomes available at the opposite wheel.
The effect can imitate one important benefit of an LSD without installing a conventional limited-slip mechanism.
It is inexpensive because modern cars already have individually controlled ABS hardware, and it can work remarkably well in normal road conditions. But it is not mechanically identical to an LSD. The system is deliberately turning some energy into heat at the brake in order to control wheelspin, whereas a true torque-biasing differential manages the torque distribution inside the driveline.
Modern vehicles sometimes combine these approaches rather than choosing only one. A mechanical or electronically controlled LSD can work alongside brake-based traction and stability systems, each dealing with the problem in a different way.
Why an LSD Is Not the Same as a Locker
The difference is contained in the names.
A limited-slip differential still permits some differentiation between its outputs. A locking differential can physically lock them together.
When fully engaged, a locker forces the two axle shafts to rotate together. This can be extremely valuable off road. If one wheel is hanging in the air with effectively no traction, a locked differential can still deliver drive torque to the tyre that remains firmly on the ground.
Remember the limitation of the torque-biasing example:
0 × TBR = 0.
A full mechanical lock avoids that particular problem because its operation does not depend on multiplying the reaction torque available at the low-traction wheel. The two outputs are physically constrained against relative rotation. Torsen itself notes this distinction when explaining why a true locker can cope with zero-traction situations that challenge a torque-biasing LSD.
But that advantage creates another problem.
On high-grip pavement, the left and right wheels still need to cover different distances through a corner. Keep them locked together and the tyres have to absorb that difference through slip and deformation. Steering behaviour suffers, driveline loads rise and the vehicle becomes less pleasant to manoeuvre.
This is why selectable lockers are so useful in off-road vehicles: they can be engaged when maximum traction is needed and disengaged when normal differential action is preferable.
An LSD occupies the middle ground. It preserves the ability to differentiate while adding enough resistance to make unequal traction far less troublesome. Eaton similarly characterises limited slip as reducing rather than completely eliminating individual wheelspin.
What the Driver Actually Feels
During an ordinary journey, a good limited-slip differential may attract no attention at all.
The car turns into a supermarket car park, the inner and outer wheels rotate at the speeds they need, and nothing dramatic happens. That lack of drama is important. A road-going LSD still has to behave like a differential when there is no reason to limit slip.
The difference appears when the tyres stop having equal opportunities.
Accelerate hard from a tight bend in a powerful car with an open differential and the lightly loaded inside drive wheel may spin while the outside tyre still has grip to spare. With an appropriate LSD, more of the available drive torque can be supported by that outside tyre. Instead of hearing one tyre flare uselessly, the driver feels more of the engine’s effort become forward acceleration.
On a split-friction surface, the effect can be even easier to understand. One tyre encounters something slippery while the other remains on good pavement. The limited-slip mechanism prevents the low-grip side from dictating the behaviour of the entire axle as freely as it would with an open differential.
That is why an LSD matters.
It does not give the tyres more grip. It does not guarantee that neither wheel will spin. It does not turn a road car into a four-wheel-drive vehicle, and it is not a substitute for suitable tyres.
What it changes is how effectively the drivetrain can use unequal amounts of grip that are already available at two driven wheels.
The open differential solves one problem brilliantly: it lets those wheels turn at different speeds. The limited-slip differential keeps that ability, then adds a carefully controlled reluctance to let one side have all the fun.