The Ultimate Guide to Limited Slip Differentials (LSDs)
Ask any experienced Ford enthusiast what the single best handling upgrade is for a front-wheel-drive performance car, and you’ll hear the same answer time and time again:
A Limited Slip Differential.
While bigger brakes, stickier tyres and suspension upgrades all improve how your car performs, an LSD changes how effectively your engine puts power onto the road.
Whether you’re driving a Fiesta ST150, Focus ST225, Mk7 Fiesta ST, Focus ST250 or Focus RS, understanding how a limited slip differential works could completely change the way you approach your next modification.
In this guide, we’ll explain exactly what an LSD does, why it makes such a dramatic difference and whether it’s the right upgrade for your Ford.
What Is a Differential?
Every driven axle has a differential.
Its job is simple:
It allows the driven wheels to rotate at different speeds while cornering.
Think about driving around a roundabout.
The outside wheel travels further than the inside wheel, meaning it needs to rotate faster.
Without a differential, both wheels would be forced to turn at the same speed, making steering difficult and causing tyre scrub.
What Is an Open Differential?
Most standard Ford models leave the factory with an open differential.
An open differential sends power to the wheel with the least resistance.
That works perfectly during everyday driving.
However, when you’re accelerating hard out of a corner, it creates a major problem.
If one wheel begins to lose grip, the differential sends most of the engine’s torque to that wheel.
Instead of accelerating forward, one tyre spins while the other does very little.
The result?
- Wheelspin
- Lost acceleration
- Reduced corner exit speed
- Increased understeer
- Wasted power
What Does a Limited Slip Differential Do?
A Limited Slip Differential is designed to reduce this problem.
Instead of allowing one wheel to spin freely, an LSD transfers more torque to the wheel that still has grip.
The result is dramatically improved traction.
Rather than wasting power through wheelspin, the car drives itself out of the corner with far greater confidence.
Why Does an LSD Make Such a Big Difference?
Imagine standing on wet grass wearing trainers.
If one foot slips, it’s difficult to push yourself forward.
Now imagine both feet maintaining grip.
You move much more efficiently.
That’s exactly what an LSD does for your car.
Instead of allowing one tyre to waste power, both driven wheels work together to move the vehicle forward.
Benefits of an LSD
A quality limited slip differential offers several advantages:
- Improved traction
- Reduced inside wheelspin
- Faster corner exits
- Better acceleration
- More predictable handling
- Increased driver confidence
- Better wet-weather performance
- Improved performance on track
Many drivers describe an LSD as making the car feel like it has gained extra horsepower—even though the engine produces exactly the same power.
That’s because more of the available power actually reaches the road.
Helical vs Plated LSDs
Not all limited slip differentials work in the same way.
The two most common designs are helical and plated.
Helical LSD
Helical differentials use gears to transfer torque between the driven wheels.
Advantages
- Smooth operation
- Low maintenance
- Excellent for fast road driving
- Quiet in everyday use
- Progressive engagement
Perfect for:
- Daily drivers
- Fast-road cars
- B-road enthusiasts
Plated LSD
Plated differentials use clutch packs to lock both wheels together under acceleration.
Advantages
- Extremely aggressive locking action
- Maximum traction
- Highly adjustable
- Favoured in motorsport
Perfect for:
- Dedicated track cars
- Competition use
- Rally applications
The trade-off is increased maintenance and a more noticeable effect during low-speed manoeuvres.
How Does an LSD Affect Cornering?
This is where the magic happens.
With an open differential, accelerating out of a tight corner often results in the inside front wheel spinning.
With an LSD installed, torque is directed towards the tyre with more grip.
Instead of fighting for traction, the car pulls itself cleanly through the corner.
The steering feels more connected, throttle application becomes more confident and you can get back on the power earlier.
Does an LSD Reduce Torque Steer?
Not completely.
Torque steer is influenced by several factors, including suspension geometry, tyre grip and driveshaft design.
However, a quality LSD can make power delivery feel more controlled and predictable by distributing torque more effectively across both driven wheels.
Is an LSD Worth It on a Standard Car?
Yes—but it’s especially beneficial once power increases.
For standard cars, the improvement is mostly felt during spirited driving, especially in wet conditions or when accelerating out of bends.
For remapped or heavily modified cars, an LSD quickly becomes one of the most worthwhile upgrades available.
The more power you make, the more important traction becomes.
Supporting Modifications
To get the most from an LSD, consider combining it with:
- Performance tyres
- Quality coilovers
- Uprated suspension bushes
- Professional wheel alignment
- Lightweight wheels
These upgrades work together to maximise grip and handling.
🔧 Retro Fast Fords Workshop Tip
An LSD is only as effective as the tyres connected to it.
Even the best differential can’t create grip where none exists, so investing in quality tyres should always be part of your upgrade plan.
Installation Considerations
Fitting an LSD isn’t a simple bolt-on modification.
In most cases, the gearbox must be removed and dismantled to replace the factory differential.
Because of the labour involved, many enthusiasts choose to replace:
- Clutch
- Release bearing
- Gearbox oil
- Driveshaft seals
at the same time, helping to save on labour costs in the future.
Common Myths
“An LSD adds horsepower.”
No.
It doesn’t increase engine power—it helps you use the power you already have more effectively.
“It’s only for race cars.”
Not true.
Many fast-road cars benefit enormously from an LSD, particularly front-wheel-drive performance models.
“It’s pointless unless you’re making big power.”
Even a standard or lightly modified Ford can feel more composed and confidence-inspiring with a quality LSD fitted.
Frequently Asked Questions
Is an LSD worth fitting to a Fiesta ST150?
Absolutely. It’s one of the most transformative handling upgrades available, particularly for enthusiastic road driving.
Will it make the car faster?
It won’t increase horsepower, but it often improves acceleration out of corners by reducing wheelspin and improving traction.
Which type is best for road use?
A helical LSD is generally the preferred choice for fast-road and daily-driven cars thanks to its smooth and maintenance-friendly operation.
Does an LSD work in the wet?
Yes. By helping distribute torque to the wheel with more grip, an LSD can improve confidence and traction in poor weather conditions.
Is it an easy DIY upgrade?
Not usually. Because the gearbox needs to be removed and stripped down, installation is best left to an experienced specialist unless you have the appropriate tools and expertise.
Final Thoughts
A Limited Slip Differential doesn’t make the loudest noise, doesn’t change the look of your car and won’t add horsepower on a rolling road.
What it does is arguably even more valuable.
It transforms how your Ford uses its power.
Better traction, cleaner corner exits, greater confidence and a more rewarding driving experience make an LSD one of the smartest upgrades you can invest in.
If you’re building a Ford that’s designed to be driven—not just admired—a quality limited slip differential deserves a place near the top of your modification list.
Related Articles
- The Ultimate Guide to Coilovers
- The Complete Guide to Brake Upgrades
- The Ultimate Guide to Intercoolers
- Engine Mapping Explained
- Powerflex vs GodFlex Bushes
Focus Keywords
- Limited Slip Differential
- Ford LSD upgrade
- Fiesta ST150 LSD
- Focus ST LSD
- Helical differential
- Plated differential
- Open differential vs LSD
- Ford performance upgrades
- Front-wheel-drive traction
- Ford handling upgrades
