I saw this article in a magazine and thought you guys might like to take a look so I typed it up. (yes its a slow day at work). Its a bit long but pretty informative. Sorry for any typos.
Not the sexiest part of your car but, nonetheless the differential is vital to making it perform. We look at how they work, why break and how to improve them. By: Peter Knivett.
It’s no use spending thousands of pounds on a superbly tuned engine if you simply cannot make best use of the power available. If you’re spinning the wheels with that monster engine, then that precious horsepower and torque is being wasted – it’s simply going up in tyre smoke rather than being translated into acceleration. Spectacular though the sight of burning rubber ism it doesn’t actually so anything for your car’s performance, other than hamper it.
Put simply, beyond a certain torque and horsepower level, improving traction I sa must, particularly on a tuned front-wheel- or rear-wheel-drive car. Even some standard cars will benefit from the improvements in this department, as they’ll be able to use more of the available horsepower and convert it into forward motion. Which begs the question – what’s the answer to this problem? Well, in some cases it lies in improving traction by fitting a performance differential.
And don’t think that four-wheel-drive cars are immune from traction improvements in this vein, either. Significant changes can be made to the handling balance and traction levels on some four-wheel-drive vehicles by paying attention to this area, particularly as there are three diff units to worry about: front, centre and rear.
But, before we examine how a differential unit can be improved, it’s worth examining what a diff actually does and how and why a standard production diff can swiftly be overwhelmed by a torrent of torque and horsepower.
So what exactly is a differential?
When a car travels around a corner, the outside wheel travels further than the inside one. If both wheels were driven at the same speed, the inside wheel would skid as it attempted to make the same number of revolutions as the outer one. To this end, each of the driven wheels is rotated by its own drive-shaft and the two shafts are driven by a differential connected to the crown-wheel which, in turn, is driven from the gearbox.
The differential consists of a cage driven by the crown-wheel, containing two side (sun) gears, each of which is splined to accept the end of each drive-shaft. These mesh at 90 degrees with a pair of smaller (planet) gears that are pinned into the differential to the other.
When you’re going along a straight road, the doff cage rotates, being driven by the crown-wheel. Because both wheel are turning at the same seed, the smaller diff (planet) gears remain stationary, until the car meets a corner. Then, because the inside wheel has less distance to travel, its unwillingness to turn at the same speed as its outside partner is transmitted along the drive-shaft into the differential cage. That causes the small (planet) pinions to rotate, allowing the inside wheel to slow down and the outside wheel to speed up. Meanwhile, the crown-wheel turns at the average of the drive-shafts speeds.
To you, the driver, the whole process goes totally unnoticed. All of which is fine and dandy, until you start putting serious power through the differential, which is when problems can start to occur.
Problems, Problems
Space limitations dictate that the diff gears are small components, so subjecting them to excessive torque or a number of standing starts can cause them to break up. That’s especially true with the very powerful cars.
Spirited driving can also bring unwanted characteristics of production differentials to the fore. Most basic production-car differentials are designed so that the power and torque will be transmitted to the wheel with the least loading on it – which is precisely what you don’t want. For example, at a standing start, if you break traction on one wheel (as often happens) and it starts spinning, most production differentials (unless equipped with a limited-slip diff) will channel the power and torque to the spinning wheel, where it’s wasted and you go nowhere.
Similarly, during hard driving, particularly around bends, centrifugal force causes weight transfer away from the inside wheel, effectively making it go ‘light’. In these circumstances, a production diff will attempt to transfer more power to the unweighted wheel, possibly causing it to lose traction and spin.
That’s frustrating enough on a rear-wheel-drive car but, on a front-wheel-drive vehicle, it can cause confidence-sapping torque steer, where the car pulls and tugs one way or the other through the steering. In either circumstance, engine performance is being wasted, either because the total potential driving force of the powerplant is being spun away in a cloud of burnt rubber, or because you cannot maintain control of the car without easing off the power.
Four-wheel-drive differentials
So far we’ve only mentioned two-wheel-drive cars in this feature, yet differential action is just as important on four-wheel-drive machines – and four-wheel-drive cars have three of them. There’s one in the front axle, transmitting torque to the front pair of wheels, one in the rear axle doing the same for the rear wheels and one in the centre of the car, splitting the torque front and rear (usually within the gearbox casing). In essence, that means that, within a four-wheel-drive system, not only is torque split between each side of the car, it’s also split end to end.
The centre diff unit is important on an all-wheel-drive car because it governs and controls the torque split front to rear using exactly the same method as a conventional transverse axle differential. Unfortunately, this can give you the same limitations, too, because, if a wheel on one axle is spinning then an unsophisticated centre differential will merely transfer power to the spinning wheels, thus exaggerating the problem. Fortunately, performance cars always have a more advanced system that solves this problem, but we’ll come back to this later.
As well as affected traction qualities, centre differentials have a massive effect on handling balance on a four-wheel-drive car. Early non-Japanese four-wheel-drive rally cars, such as the Audi Quattro, suffered because they ran a heavy engine up front but were limited to a 50/50 torque split front to rear. This combination produced excessive understeer when pushed hard, and this hobbled the competitiveness of the Quattro in later years. But, soon enough, this problem would be overcome by a quantum leap forward in differential design.
When the Japanese started to design and build four-wheel-drive cars in the 1980s, they swiftly overcame this limitation by designing centre differentials with either a greater rear-wheel bias or an adjustable torque split front to rear. This could be set up to allow power oversteer – an essential for rally driving.
Although the rally cars could run with sophisticated adjustable centre differentials, their production car relatives usually made so with a fixed torque split front to rear. Today, things have become far more complex. (see ‘Active diffs’) and advanced. Contemporary competition-oriented Imprezas, such as the Japan-only Spec C derivative (which is the basis for the Group N Rally car) or the new WR1, use a driver adjustable centre diff, which can bias all the power to the rear axle, if required. And that’s the march of progress, because what was cutting edge 20 years ago on rally exotica is now within reach of the enthusiast.
Performance differentials: the different types
Now we get onto the interesting part. Many Japanese cars now come with sophisticated limited-slip differentials (LSDs) as standard others don’t, but can have a performance unit fitted as an aftermarket item. Automatic torque biasing differentials aside one thing they all have in common is that they aim to limit the amount of spin or ‘slip’ that occurs when a wheel spins, with the goal of switching the torque to the driven wheel that still enjoys traction, thus maximizing performance. As you might expect, there is more than just one way of skinning this particular engineering cat, so here’s a breakdown of the more popular types and where to expect to find them.
Plate-type LSD
Of all the performance differential designs out there this is by far the most ‘traditional’ in its design and you can purchase them from OS Giken, TRD,Cusco, Kaaz and many others. The ‘plate’ diff gets its name from the pack of clutch plates that sit behind the large side gears in the differential cage. These are designed with a certain amount of ‘pre-load’ torque and, below this level of torque, the unit functions normally. As load is applied to the differential, the separation forces between the planet gears and the sun gears increase this clutch loading, providing a positive flow of power to the sun gears. When one of the wheels breaks traction, the clutch unites transfer the power directly to the other sun gear allowing the wheel with grip to take more of the load and thus decreasing the slip on the on the spinning wheel. Hence the term ‘limited-slip differential’.
The amount of torque transfer between each side of the differential is governed by the clutch pre-load. Excessive pre-load will offer good toque transfer, but can produce noise during cornering (particularly at low speed), but set the pre-load too low and the transfer will be too low as to be useless. And there’s another factor to take into account. The clutch packs are worked by a system using metal rods that work in angled slots. This angle is called a ‘ramp’ angle, rather like a gradient on the road, and it governs how quickly the diff ‘locks’ to transfer the torque. The steeper the ‘ramp’ angle the more resistance the diff has to locking, and vice versa. It’s a crucial factor to get right, because, if you fit a plate diff with too shallow a ‘ramp’ angle to any two-wheel-drive car, it will be a real handful to drive, switching torque from one side of the axle to the other in an alarming fashion.
That said, plate diffs are very, very effective pieces of a kit that are used to extensively in both front- and rear-wheel-drive cars. Transmission specialists are well versed in specifying differential unit that suit the particular driving environment, whether fast road, track day or drifting. There are down sides to this type of diff design, including the requirement to overhaul the unit at intervals to replace the clutches and check the ramps for wear and tear. Also, repeated shock loadings, such as drag racing starts, don’t suit plate diffs at all well, as they cause the clutch packs to work excessively hard in a very short space of time. That said, this type of differential is widely used and nestles in the front and rear of the Impreza STi under the name ‘Suretrac’.
Viscous coupling
As with the plate diff unit, this style of LSD again uses a clutch pack but, in this case, its suspended in a silicon fluid with thixotropic properties. As its name suggests, this style of differential is dependent on viscosity to make it work which, in simple terms, is the measure of how ‘runny’ the fluid is that’s stored within. The hard part to get your head around in that this silicon-based fluid has a variable viscosity, dependant on how agitated it is. Imagine a bowl of this fluid on your kitchen table, complete with a wooden spoon. Gentle movements of the spoon would reveal a near liquid-style feel but, try to stir the fluid fast and it wont let you, it feels thicker and thicker.
Now imagine this fluid inside a differential cage, complete with clutches, sun and planet gears. As one sun gear starts to spin faster than the other, the action of this fluid increases its turning resistance, as well as biasing the drive to the wheel that’s not spinning and is therefore encountering less resistance from the fluid. Because it’s cased around a fluid system, this style of extremely smooth in its operation, as well as being lower maintenance than the plate-style unit. That’s a major plus to any car manufacturer, so its no surprise to discover that both Subaru and Mitsubishi use viscous coupling center diffs (VCUs) on both the Impreza and the Evo.
Automatic torque biasing/helical ’Torsen’ differential
The automatic torque biasing (ATB) diff is manufactured in the UK by Quaife, and is unique because it doesn’t actually ‘lock’ to transfer power away from the spinning wheel, or limit slip. What is does do is to bias the torque to the wheel that’s enjoying the most traction – and it does it purely my mechanical action.
Look inside the ATB diff and you’ll not see any plates, just sets of floating helical gear pinions in pockets that mesh to provide the normal running action. Should a wheel start spinning, then torque bias is generated by the axial and radial thrusts of the pinions in the pockets. This causes friction and enables the sun gear and driving wheel enjoying the most traction to transmit a greater proportion of the torque.
Because it’s progressive in action, this style of differential is ideally suited to two-wheel-drive cars, especially front drivers. Strength is a major plus point too – there are no plates or ramps to wear out – and the longevity is proven by drag racing cars in the USA, which run over 800bhp through ATB units without problems, Regular rebuilds are not necessary with this style of differential, so they’re a real ‘fit and forget’ item.
Similarly, the helical-geared ‘Torsen’ differential senses the wheel that can handle the most torque and switches it to this drive shaft. This type of unit is fitted to the front of some Mitsubishi Evo RSIIs and RS variants, whereas the GSR model uses a standard ‘open’ type diff.
‘Active’ diffs
Pioneered by the Japanese on production cars, an ‘active’ differential is basically a mechanical centre diff unit governed by a computer to greatly speed up and enhance its efficiency and operation.
Nissan pioneered this technology back in the late 1980s with the Skyline R32 GT-R, debuting a centre-diff system dubbed ‘ATTESSA-ETS’. This switches the torque between the front and rear axles in response to signals from wheel-speed sensors. The diff’s action is controlled using its own ECU computer. Years later, Subaru followed suit with its DCCD ‘Driver Controlled Centre Diff’ unit, which runs in two modes – manual and automatic. In automatic mode, the system maximizes traction by switching the torque to either end of the car via a viscous coupling centre differential. Switch to manual override and you can adjust the balance to run up to 65 per cent of the torque to the rear axle, altering the handling balance to reduce understeer or even to provide power-oversteer. Next step – drift club!
Another pioneer of ‘active’ diffs was Mitsubishi, which introduced AYC (Active Yaw Control) way back with the Evo IV of nearly a decade ago. This system uses a sophisticated set-up of sensors linked to a separate ECU to sense excessive yaw switch the torque through the rear diff to maximize stability. But even this system has its limits and can benefit from upgrading if the ultimate in performance is your goal.