For some, the locking of an inter-wheel differential seems like a miracle that can instantly turn a regular two-wheel-drive car into either an off-road SUV or a rally car. But is that really the case? And why don’t manufacturers install self-locking differentials on every car right from the factory?
As usual, let’s start with some theory: what exactly is a self-locking differential, and how does it work? Revisiting a few details is important, because these devices come in different designs. That means they differ in their operating principles, cost, and reliability—the last two parameters being especially critical for mass-production vehicles.
All self-locking differentials are generally referred to as limited-slip differentials (LSDs). Alongside them, there are also fully locking differentials with manual activation, but let’s leave those to hardcore off-roaders. No normal driver today wants to push an extra button or—heaven forbid!—pull a mechanical lever. LSDs are far more convenient: when needed, they lock on their own. That’s why they’re called self-locking. But the way they are built can vary greatly.
Open Differentials and Their Drawbacks
An open differential allows the wheels on the same axle to rotate at different speeds. This is crucial when cornering because the outer wheel covers a longer distance and therefore must rotate faster than the inner wheel. Without a differential, cornering would be much worse, tire wear would increase, and on front-wheel-drive cars the steering feel would also suffer—returning the wheel more aggressively to center while creating unwanted reactions in certain conditions.
In short, the open differential is a very useful invention. But it comes with a serious drawback: if one wheel loses traction, the other wheel also loses drive. For torque to reach the wheel, some resistance must exist.
This very feature of open differentials is behind an old trick familiar to drivers of classic rear-wheel-drive cars: lightly pulling the handbrake (which acts on the rear wheels) creates resistance on the slipping wheel. That resistance forces the differential to transmit torque to the other wheel. Sometimes this trick could save the day.
But let’s get back to LSDs.
How LSDs Work?
With an open differential, wheels on the same axle may differ in rotational speed and in torque. Accordingly, there are two main ways to achieve locking:
- Based on differences in speed (clutch-type LSD).
- Based on differences in torque (helical/worm-gear LSDs, e.g., Torsen).
Clutch-Type LSD
In a clutch-type LSD, a set of friction clutches connects the axle gears with the differential housing. Some clutches are linked to the axle gears, others to the differential housing. Even when “unlocked,” a small torque is still transmitted to the slipping wheel due to the preloading of the clutches by springs. This preloading is not true locking—it only ensures some torque transfer. Actual locking occurs when the axle gears shift relative to each other, which happens when one wheel spins faster than the other.
Helical/Worm-gear LSDs
A helical LSD is better known under the name Torsen (from “Torque-Sensing”). Instead of simple gears, it uses worm gears. The unique property of worm gears—the ability to transmit torque in one direction but not the reverse—is the foundation of the design.
There are two main types: Torsen T1 and the simpler Torsen T2 (also known as Quaife). Despite their differences, both rely on worm gears for their locking function.
Why Manufacturers Avoid LSDs?
So why aren’t clutch or helical LSDs commonly installed at the factory?
First and foremost: cost. No manufacturer will put an LSD in a budget car where every cent matters. In such cars, price is everything, and you can live without an LSD. After all, aftermarket LSDs are widely available. But on the production line, keeping costs low is key.
Second: necessity. An LSD can be useful, but it’s not equally valuable on all drivetrains. On front-wheel-drive cars, its benefits are limited, while its drawbacks are significant. Rear-wheel-drive cars—where LSDs are very helpful—are nearly extinct today.
Manufacturers know this, which is why LSDs are still offered (standard or optional) on light commercial vehicles, where rear-wheel drive is still common and where the benefits outweigh the costs.
The Downsides of LSDs on Front-Wheel Drive
On a front-wheel-drive car, combining power delivery and steering on one axle makes things tricky. A locked differential drastically changes handling.
Every driver knows that FWD cars tend toward understeer, which is usually easy to control—ease off the throttle, adjust the steering, and you’re fine. But with an LSD, reactions become sharper, and the car can switch unpredictably between understeer, oversteer, or sideways slides.
Professional drivers love this because they know how to exploit or counter these dynamics. But for ordinary drivers, it makes the car harder to control and less predictable. In fact, some situations may even require advanced techniques like left-foot braking, a skill used by rally legend Rauno Aaltonen to control oversteer in tight corners. Needless to say, very few everyday drivers are ready for this.
On top of that, torque steer becomes more pronounced: the steering wheel may tug violently in ruts or when traction suddenly changes. Most people aren’t prepared for that either.
In short: while trained drivers can benefit from an LSD, for daily family driving it’s far more comfortable to go without.
Maintenance and Durability
Clutch-type LSDs are loved by racers because they can be tuned for different conditions. But will everyday drivers bother? Certainly not. Clutch-type LSDs also engage harshly and can surprise the unprepared driver. Worse, friction clutches wear out over time, changing the LSD’s characteristics. Eventually, they must be replaced—requiring a full teardown of the differential.
They also need frequent oil changes, since the fluid gets contaminated with clutch debris. All this makes them impractical for most people, so clutch-type LSDs remain the realm of rally and motorsport.
Helical LSDs are more practical. They’re often found in rear axles of commercial vehicles, where they provide real benefits. They’re also a reasonable upgrade for FWD cars in snowy regions—provided the locking strength is moderate (up to ~85%). Stronger helical LSDs wear faster, complicate handling, and may even require stronger drivetrain components, which becomes unnecessarily expensive for everyday use.
Conclusion
LSDs are not for everyone. Except for certain rear-wheel-drive commercial vehicles, they’re best left to those who understand their downsides: more complex handling, higher costs, and more maintenance.
If you frequently drive in difficult conditions, a moderate helical LSD like a Torsen T2 can be a good personal upgrade. But manufacturers will never mass-install them on modern front-wheel-drive cars. Why make a car more expensive and harder to handle if there’s no real need?
For most buyers, comfort and predictability matter more. It’s far easier and more appealing to offer extra features like heated seats or a larger infotainment screen—these cost less, add more everyday value, and keep drivers safer.
XJXPARTS, as a professional Torsen limited-slip differential manufacturer, we specialize in providing reliable and customizable LSD solutions for various applications. Whether you need high-performance products for commercial use or custom designs for specific vehicles, we are ready to support you. Feel free to contact us for technical consultation and purchasing cooperation.
