What is a limited slip differential? A Limited slip differential helps you drive effectively and safely. At times, one of the leading wheels might get stuck when driving on gravel or sand. But if you have the differential, you can keep going and eventually get out of it. How does a limited slip differential work? Let’s take a look.
What Is a Limited Slip Differential?
The limited slip differential (LSD) distributes the wheel’s torque and gives it to the one with more traction. It is the most common differential in sports cars, trucks, and SUVs. However, more and more manufacturers install it in casual vehicles.
Like an open differential, it allows the wheels to rotate at different speeds but effectively shifts the torque to the tire with higher traction when needed. However, the amount of torque transferred to one wheel is limited – that is why it is called “limited slip”.
For instance, when one of the wheels completely loses traction, keeping the balance and moving becomes difficult. That is where LSD or positraction, the term coined by General Motors, comes in.
What’s The Function Of A Differential?
A differential is a gear assembly that transfers the torque from the engine to the wheels. It includes three shafts, and the main shaft rotation speed is the average of the others. An open or traditional differential allows the wheels to rotate at a different rate so that the car can rotate.
But when one of the wheels loses traction, a particular problem occurs: one of the wheels gets more torque as it slips while the other almost stops, and the vehicle doesn’t move. This usually happens after the rain, when the road gets wet and the wheels get into puddles, or when one wheel is on the sand and the other on the asphalt. That’s precisely where the slip differential comes in.
How Does A Limited Slip Differential Work?
The principle of the limited-slip differential is that it offers more control over power delivery than a conventional ‘open’ diff. An open diff uses gears to ensure that the wheels turn at different speeds when going round corners, but when there is lots of power being delivered, it’s easy for an open diff to be overcome by its delivery.
When power arrives at the wheels, it looks for the path of least resistance, which in this instance means the tyre with the least amount of grip. If you’re heavy with the throttle in a powerful car, this can mean all the power evaporates in a cloud of smoke as the unloaded tyre spins away while the other tyre continues to grip.
Add an LSD, and extra mechanisms – usually in the form of a clutch assembly, cams, or even a viscous fluid system that’s part of the diff – counter this natural flow of power to redistribute the engine’s torque to the wheels with the most grip.
The result is reduced wheelspin for the unloaded tyre, and the car’s power is put to the road more effectively, which will boost grip and therefore cornering and acceleration performance.
There are different types of limited-slip diff available, and which one a car uses will depend on the drive system it uses. On rear-wheel-drive and 4WD cars, a 2-way LSD might be used. This means that the LSD will have an effect when applying power and also when slowing down, meaning a consistent feel to the car.
A one-way LSD is better suited to a front-wheel-drive car, because this will only have a limiting effect when accelerating. When slowing, the LSD is inactive, which helps with cornering off the power because a 2-way diff has a tendency to introduce understeer to the drive system.
In between these two is a 1.5-way LSD. This offers an LSD effect under acceleration and when slowing, but the amount of slip isn’t the same in both directions, so there’s less effect in one direction than in the other. This can be more useful than a one-way LSD because it still enables a car to use engine braking when slowing down.
Another type of LSD is the torque-sensing differential. Otherwise known by its product name of ‘Torsen’ diff, this is a special type of differential that’s used by four-wheel-drive cars to split power between front and rear axles. One of the first production cars to use a Torsen diff was the Audi Quattro, and this system helped it to dominate in rallying in the early 1980s.
What Are The Types Of LSD
An LSD has several design options, made to smoothly balance the rotation of the wheels in different traction conditions. Generally, two main types, torque-sensitive andspeed sensitive, of positraction can be distinguished. The maximum possible difference between the torque inputs on both wheels is called a bias, and some LSDs have the bias as a fixed value.
Torque-sensitive differentials are mechanical differentials responsive to the driveshaft torque. The most common among them is a clutch pack differential. The idea is that behind each differential side gear, a clutch pack of discs is located. When one wheel starts slipping and the torque input changes, the load on the clutch pack increases and transfers the power to the other wheel smoothly.
This type of positraction is more expensive and complicated, commonly used in sports cars to balance the extra power the engine transfers to the wheels to provide a smooth start.
Speed-sensitive differentials are triggered by the difference in the speed between two outputs. The most common speed-sensitive differential is called viscous LSD. They use hydrodynamic friction of a fluid with high viscosity or silicon-based gel.
These differentials look like cylindrical chambers filled with fluid, and perforated discs stack connected to the shafts and differential with the inner and outer surfaces. The motion of the differential causes disc friction which warms up the gel. The heat makes the gel expand and pull together the discs and “block” the difference in speed on the wheels.
This LDS is simple to produce, works softer and more proportional to the slip, but requires more careful treatment. The gel loses its quality over time and needs to be changed every 60,000 miles. Also, it is sensitive to overheating and becomes less effective whenever a car gets stuck in snow or mud.
Another speed-sensitive differential type, which is becoming increasingly popular, is the gerotor pump. The pump is installed in the differential and transfers the torque to a “slower” wheel with hydraulic pressure. The pump could be controlled electronically and “obey” the car computer stabilization system.
What’s The Disadvantage Of An Open Differential?
On a low traction surface, an Open Differential will send power to the wheel of least resistance, resulting in the tyre spinning or skipping. Let’s put this into practice…
Imagine you’re parked at the bottom of a hill. Your car is half on the road and half on a wet, muddy verge. For you to move off, you’ll need to go up the hill so you take your foot off the clutch and accelerate away. Only that’s what you hoped to do.
The two wheels ‘tasked’ with pushing your vehicle forward have very different conditions to deal with ‘underfoot’. One has slippery mud which will allow the wheel to turn on the spot with minimal resistance, and without pushing the vehicle forward.
The other has grippy tarmac. If this wheel is to turn, it will need to propel the car forward up the hill and that will require a significant amount of force.
As an open differential chooses the path of least resistance, the majority, if not all of the torque will be directed to the wheel in the mud causing this to spin without delivering any forward momentum to the vehicle itself.
Being an open differential has no limitation in the rotational speed differential between the two wheels, the further you press down on the accelerator, the more the wheel in the mud will spin without any additional power being transferred to the wheel on grippy tarmac. To get out of this tricky situation, you’ll either need to reverse or ask your unlucky passenger to get out and push.
Why An Open Differential Doesn’t Work On Track
Let’s imagine you want to take a corner at speed. As you approach the corner, you make the necessary speed adjustments with a view to accelerating through the corner and optimising your exit speed.
As you’ll know if you’ve spent time with us on one of our performance driving courses, using your accelerator intelligently through the corner allows you to bring better balance and grip to the car.
However, if we think of the forces applied to a car through a corner at speed, we can see that the outer wheels have far greater downforce applied to them and this makes them more difficult to turn.
In contrast, the inner wheels have a much lighter downward force. Depending on the severity of the corner, the suspension may be partially or even completely uncompressed (known as ‘in droop’) and the tyres may only just be in contact with the ground beneath them.
If the surface is uneven, the tyres may even temporarily leave the tarmac. The result is that the lion’s share of the power from the driveshaft gets directed to the inner wheel as this is the path of least resistance.
Your inner wheel is likely to spin as it struggles to gain traction. Meanwhile, your outer wheel, which should be balancing the vehicle and powering you out of the corner, is struggling to receive any drive at all resulting in greater body roll, less control and a slower exit speed.
Enter The Limited Slip Differential
Well, as the name suggests, a limited slip differential limits the speed differential between the two (or four) driven wheels.
This magical component was first developed all the way back in 1935 to minimise excessive wheel spin in Grand Prix racing cars.
The first LSDs connected the two half axles together with a clutch pack allowing a limited amount of clutch slip between each side of the axle.
This ensured that the torque was more equally distributed between the two sides and when cornering at speed, Grand Prix cars were able to gain greater balance through a corner and great power out of it.
Different Types Of Limited Slip Differentials
As the years have gone on, technology has progressed and today there are a variety of different types of LSD on the market. These include viscous, mechanical, hydraulic and electronic systems. Systems can also be classed as:
- 1-way – only functioning when accelerating in a forward motion
- 2-way – functioning equally when accelerating, decelerating and travelling in reverse
- 1.5-way – functioning when accelerating, decelerating and in reverse but with different behavioural characteristics for each
With so many variances between them all, it’s simply not possible to go into depth on each one in this article but it’s safe to say…
If you’re serious about your track days and want to truly optimise your performance, a limited slip differential is an essential addition to your track day car.
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.
What is the benefit of a limited slip differential?
Limited slip differentials compensate for a loss of traction in one wheel, offering you more control even when driving on slippery or rough roads. A limited slip differential increases a car’s power and speed by utilising engine power more efficiently, thus allowing for a smoother and more enjoyable drive.
What is the difference between limited slip differential and a regular differential?
The main difference encountered in limited slip differential vs open differential is that the latter transfers more power to the less resisting wheel. Meanwhile, the limited-slip differential transmits power to the wheel with the most traction. The differences between these differential systems don’t end here.
Is limited slip differential good in snow?
The primary performance goal of a limited-slip differential is to improve the traction of the vehicle during adverse traction conditions. The advantage offered on ice- and snow-covered roads is fairly obvious.
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