Technical Part 2 - Limited-Slip Differential Types

This is Part 2 of our technical articles - Part 1 can be found here.

Limited-slip differentials vary in design, and since their invention in the 1930’s they have become increasingly complex. Motorsport has been the leading cause of better, and more advanced limited-slip differentials. In a desire to shave down lap times LSD manufacturers added to tuning options to certain limited-slip differentials for optimization. Modern LSD’s offer the traction advantages of a locked rear diff, whilst allowing for the wheels to spin independently when needed. This gives excellent grip, and a huge improvement in handling.

Viscous LSDs

Viscous LSD’s have been fitted to road cars from the factory since the early 1980’s. They work on the principle of speed sensing, and are quite primitive in design. Speed sensing works by slowing the wheel that has started spinning, and diverting torque to the wheel that still has grip. Viscous LSD’s work in a very similar way to an open differential, with the difference being a clutch pack attached to one of the drive shafts. This clutch pack is inside the differential carrier and contains a thick viscous fluid. Under normal turning conditions when both wheels have grip the clutch plates are able to move through the fluid and there is only a mild torque biasing effect. When one wheel spins the viscous fluid heats up and gives extra resistance, this has the effect of slowing the spinning wheel down, and diverting torque to the wheel with grip.

In principle this works fairly well, however over time the fluid degrades leaving the viscous differential to act like an open one. As the viscous differentials were designed not to be repaired, replacing the fluid is usually a costly job. There is a delay in the torque transfer as one wheel has to start spinning enough to heat the fluid before the mild locking action takes place. In certain motorsport applications this delay is unacceptable, and will impact on lap times.

However in spite of this many road cars had them fitted from the factory as they were cheap to make, and reliable in the 3 year manufacturer's warranty. The design allows for small speed differences between the wheels, and because of this they are smooth when turning.

Whilst viscous LSD’s have become obsolete because modern ATB’s and plated differentials can achieve better performance with more reliability, they are still effective as centre differentials. Under normal conditions the fluid doesn’t degrade as much as the front and rear axles are usually at the same speed. This also allows for a near 50/50 torque split front to rear. When a whole axle starts spinning the viscous centre LSD distributes torque unevenly. This only happens in extremely low grip situations making it a fairly un-intrusive LSD when used for normal driving.

ATBs

Automatic torque biasing differentials (ATB’s) are a standard fit in performance vehicles because they are smooth and offer increased traction over an open diff. They are composed of multiple helical gears, which distribute the torque according to a bias ratio whilst still allowing different wheel speeds. An open differential has a bias ratio of 1:1, whereas an ATB can have a bias ratio of 4:1. For example a 4:1 bias ratio means that the wheel with the most amount of grip can transfer 4 times the torque to the ground than the wheel with the least amount of grip. Unfortunately this means that in ultra low grip situations (wheel lift) that if one wheel has zero grip, the other wheel also has zero grip. This is because 4 multiplied by zero, is still zero.

Both of the ATB manufacturers we supply (Quaife & Wavetrac) differ in torque bias and design, but they both are very reliable. They both offer excellent strength and both have a lifetime warranty provided they have been supplied and fitted by an authorised dealer.

Quaife ATB - Internal View

Quaife ATB - Internal View

Quaife supply differentials to vehicle manufacturers, this includes Ford, GM, Lotus and Chrysler, they are valued for their increased performance, longevity and excellent value. Quaife ATB’s vary the torque bias between vehicles as different vehicles suit different bias ratios. For example a 2000kg rear wheel drive vehicle will require a different bias ratio to a 1000kg front wheel drive. This ensures the ATB will work as effectively as possible for your vehicle. Quaife ATB’s never fully lock to provide smoothness at all times.

Wavetrac differentials offer more modest torque bias ratios of 2.5:1 for their complete range. This ensures that the bias ratio is not too extreme and will suit every application without being overly aggressive. Wavetrac gets its name from the unique ‘wave’ device in the centre of the differential. When a wheel starts to spin, the wave device creates internal load across the differential. This internal load stops the wheel from spinning, and diverts torque to the wheel that still has grip. Having the ability to prevent wheel spin and divert power whilst still allowing the wheels to spin independently makes them an excellent choice for high powered vehicles such as BMW’s and Mercedes.

Both Quaife and Wavetrac are excellent ATB’s and work very well on vehicles seeking an improvement in traction and handling. It isn’t a question of Quaife vs Wavetrac as they each have traits better suited to certain vehicles and one isn’t better than the other. Either ATB makes an excellent addition to modern electronic brake based LSD’s, and will dramatically improve performance. We would recommend ATB’s for track cars seeking a less aggressive differential, but still requiring increased traction and improved handling. Due to the smoothness, quietness, and reliability they are an excellent choice for a road car seeking more performance. They are also a good choice for road vehicles driven in low traction conditions such as snow, ice, gravel, or mud, providing increased grip and therefore safety, to the driver & occupants.

Plate LSDs

Plated LSD’s have been used for decades, after being developed initially for motorsport. They all have a similar design, but different manufacturers have developed their own differentials and some are radically different. They all share a common idea of using a series of clutch plates and internal ramps to apply pressure to those clutch plates, causing the differential to lock like a solid axle. This locking is what allows torque to be distributed to the wheel with grip, and how much lock can be controlled with the throttle.

A number of manufacturers make plated LSD’s, some are far more effective than others. We only supply plate LSD’s from the top manufacturers (Cusco, Drexler, Kaaz, Titan & Tran-X) which we trust. These have proven to be strong, reliable, controllable, and highly effective. Regardless of the manufacturer, there are two main things that determine how an LSD works, preload and ramp angles.

Preload is the static load across the differential, this a base level of resistance between the two wheels. It is determined by measuring the torque taken to turn one output shaft whilst the other is still stationary. Preload determines how difficult it is for the wheels to turn independently whilst no power or braking is being applied. The lower this figure the more the LSD acts like an open differential when no force is applied to it. A higher preload reduces the locking aggressiveness when accelerating or decelerating. Preload is applied by spring washers in the differential and adjusted by shims.

Ramp angles primarily determine the behaviour of the LSD under acceleration and braking. They are often referred to as 1 way, 1.5 way, and 2 way, these terms refer to when the differential locks. Contrary to popular belief a 1.5 or 2 way is not necessarily better than a 1 way. Different vehicles suit different combinations of ramp angles. The British system of measuring ramp angles starts with 0, and ends at 90. 0 being the most aggressive, and 90 being the least aggressive (no additional load over preload). Ramp angles of less than 20 degrees are not common, and ramp angles over 80 degrees are ineffective as they work like a 90 degree ramp.

We have found the best way to visualize preloads and ramp angles is to think of the ramp angles as a hill, and the preload as the force that pushes it back down the hill (gravity). The aggressiveness is how quickly the car will rise up the hill. The steeper the hill and the more gravity, the less quickly the car will rise up the hill. This equates to a less aggressive locking action.

Plated LSD mode of operation.

Plated LSD mode of operation.

A number of other factors influence the locking characteristics of plate LSDs. For example plate friction materials, belleville washer characteristics, and the number of mating plate surfaces. External factors such as the weight of the vehicle, weight distribution, traction of the tyres, and the power of the engine also influence how well the differential locks. For example a low power, lightweight, front wheel drive car on gravel will not lock an LSD designed for a heavy, rear wheel drive drag vehicle as efficiently.

Some LSDs refer to percentage lock, this is generally anything from 100% to 25% lock. Unfortunately this figure is highly confusing and largely irrelevant to modern LSDs. ZF started this trend as they didn’t want to release the ramp angles in vehicles as this was a trade secret. Instead they released a % figure to show how aggressive they were. This percentage refers to both acceleration and deceleration as the ramps are equal. Modern LSDs are capable of locking up completely if needed, and usually feature uneven ramps. This renders the % lock figure useless as a comparative tool.

Plated LSD’s offer huge flexibility and can be set up to suit your vehicle, giving the aggressiveness required on power, and the stability when braking. Higher powered vehicles, and vehicles prone to lifting wheels benefit hugely from plate LSDs as all the torque can be transferred to the wheel with grip. This locking on demand gives the driver the ultimate control, increasing confidence and decreasing lap times. For motorsports, plated LSD’s provide a huge increase in performance and handling. Road cars also benefit from plated LSDs as they allow more torque to be transferred to the road and improve control of the vehicle.

Electronic Torque Vectoring Differentials

Torque vectoring differentials are arguably the most complex type of differential, they rely on using the sensors from a modern vehicle to calculate exactly how much torque should go to each wheel or each axle. Using information such as : Individual wheel speed, throttle / brake position, steering angle, lateral / longitudinal G force, a computer calculates how much torque to send to each wheel in order to aid cornering speed.

As this is a very complicated type of differential, it is only usually available directly from the manufacturer. Each manufacturer differs slightly in their implementation, but when wheel slip is detected, torque is reduced to that wheel, allowing for more torque to go to the wheels with grip. The same process happens when the computer feels that the car should be cornering more or less aggressively, resulting in you being able to make a faster turn. In a straight line they allow for more precise torque transfer to each wheel, increasing the amount of power you are able to put down.

Modern cars commonly use torque vectoring between the front and rear axles. On certain 4x4’s they are programmed to be front wheel drive for fuel economy, and to only engage the rear axle if wheel spin is detected. The computer reacts so quickly the driver doesn’t realise that the wheels have started to spin. Other vehicles are distinctly more rear biased, for example the Nissan GT-R can send 98% of the power to the rear, but only a maximum of 50% to the front.

There aren’t many downsides to electronic torque vectoring systems, however their biggest issue is the cost to repair when something goes wrong. Complicated computer chips, sensors, hydraulic actuators, and so on, are expensive to repair. When heavily modifying a vehicle the factory fitted torque vectoring may not respond in the way you want as it can’t take into account modifications, this has the potential to hinder performance. The only other reason you might want to remove the torque vectoring system and install a traditional LSD is due to personal preference. Some people simply do not like the way the torque vectoring works, and may find it too aggressive, or not aggressive enough.

Brake Based Electronic Differentials

Brake based electronic differentials are not true limited-slip differentials, however they rely on similar technology to torque vectoring differentials. Instead of reducing torque to the spinning wheel, the brake is applied to slow it down. As the differential is a standard open differential, when a brake is applied to the spinning wheel it creates a false sense of grip, allowing equal amounts of torque to be transferred to the wheel with grip. Whilst this does remove some of the downsides to an open differential, the wheel being braked still does not transfer that torque to the ground.

When compared to a torque vectoring differential, brake based differentials sap power, and aren’t as efficient. However they are better than open differentials, and might be the difference between getting stuck in mud or not. Fortunately upgrading them with an ATB is an excellent choice, and greatly improves their performance. ATB’s work very well with brake based LSDs as only a small braking force has to be applied to the spinning wheel in order for lots of torque to be transferred to the one with grip.

Other LSDs

There are other types of limited-slip differentials that aren’t used often due to their antiquated design. These include the cam and pawl, the detroit locker, and the fixed value differential.

AP Suretrac view of internals

AP Suretrac view of internals

Cam and Pawl differentials such as the AP Suretrac rely on a system of interconnected blocks. The design is quite primitive, and due to the fact that there are no gears it isn’t the smoothest of differentials. A difference in wheel speed is achieved by these blocks sliding up and down allowing the output shafts to rotate. The locking action is achieved by blocking rapid wheel spin as the blocks can only move at a set rate. Under conditions where both wheels have the potential to spin, such as standing starts, the torque gets transferred to one wheel then back to the other. This continues until both wheels have enough grip not to spin. During hard use the blocks wear and round off, reducing the effectiveness. As other styles of differential have advanced the cam and pawl has become redundant.

The detroit locker is a type of differential designed primarily for drag use. It is another gearless design and differs from other limited-slip differentials as its default state is locked. Heavy duty springs push the ratcheting device so they are locked solid until there is enough traction and a wheel speed difference to overcome the spring pressure. This momentary release in pressure send all the torque to one wheel as the other one is free floating. This type of differential is hugely noisy, and very unpredictable when cornering. For these reasons its highly unsuitable for road vehicles, and the same level of lock can be achieved from plate style limited-slip differentials.

The fixed value differential is a very simple, its an open differential with clutch plates providing a fixed level of resistance. This resistance doesn’t change under wheel spin, or any other conditions. By providing resistance across the differential some additional torque can be distributed to the wheel with grip, this varies depending on how high the resistance is. This style of differential was used in some older American road cars to combat wheel spin in high power vehicles. The level of resistance varies the handling, with lower resistance acting like an open differential, and higher levels acting like solid rear axle. This is another style of differential that has been largely phased out and replaced with other superior types.

Other styles of differential do exist, however these are not effective enough for mainstream use. Plate style differentials, and ATB’s have largely replaced the more unusual types of differential mentioned as they have been proven to be more effective in all types of use.

To buy Cusco LSDs click here.
To buy Drexler LSDs click here.
To buy Kaaz LSDs click here.
To buy Quaife LSDs click here.
To buy Titan LSDs click here.
To buy Tran-X LSDs click here.
To buy Wavetrac LSDs click here.