Transmission - Introduction - Part 2

Part 1 - Some History | Part 2 - Back to Basics | Part 3 - S2 Configuration

Back to Basics on Differentials

If all this talk of Torsen's is all a bit much then let's go back to basics on the need for differentials in the first place. Forget about 4WD for now... When travelling in a straight line, all wheels rotate at the same speed as they cover the same distance over time. However, going through a corner - the wheels on the inside have less distance to travel and consequently their speed needs to be different for any sort of enjoyable and safe driving experience. This is no problem for freely rotating non-driven wheel as they will just go with the flow, but for the driving axle(s) something clever is needed.

Fundamentally, some sort of device is needed to provide the required difference in speed of the wheels on the driven axle. That device is known as a differential - a wonderful bit of mechanical machinery that solves the problem with some clever gears that solve this problem. I'm not to try and explain how a basic differential works - they are fiendishly ingenious - so I'm going to send you off to this page for more information, before trying to outline the differences, advantages & disadvantages of the main types of differential below.

Open Differential

This is your plain ordinary, run of the mill differential as fitted to millions of car around the world - FWD, RWD or 4WD in various guises - as outlined on the link above. The main advantages of the open diff are its relative simplicity and low cost. It's main disadvantage is that it's design principle allows it provide what I like to think of as 'infinite slip' - this is not good for the best traction thru corners at moderate to high speeds - especially on difficult terrain. Many of you will have seen photographs or videos of cars driven at track days where the inside driven wheel (especially on cars with FWD) regularly lifts of the tarmac in high speed turns. In such a situation, the open diff will transfer torque to the unloaded wheel as it provides the path of least resistance - which as one can imagine will not provide the best traction through the corner in such a scenario.

Locked Differential

When an open differential is mechanically altered to provide a permanent 50:50 torque split, it is said to be locked. Thus under whatever differing conditions and wheel speeds, exactly half the torque is delivered to each output shaft in a locked diff. This makes for a huge improvement in traction through high speed difficult terrain in comparison to the open differential. With reference to the lifting wheel scenario described above, the locked diff would ensure that half the engine's torque would be delivered to the wheel that remained in contact with the ground.

I like to think of the locked diff as having 'zero slip'. By definition, a locked diff is rather poorly suited to normal roadgoing situations on 2WD cars because of the increased tyre wear created by both wheels being forced to rotate at the same speed. However, a locked differential can be put to very good use as the centre differential in a 4WD vehicle... as proven by umpteen rally wins around the globe by the legendary UR quattro and its three world championship titles to prove it. The rally quattros always ran with a locked centre differential because of the predictable handling on the limit - especially on mud, gravel and snow.

The biggest disadvantage of a locked differential is that the car always 'wants' to go in a straight line. This gives rise to severe tyre wear and bizarre handling on sealed surfaces - especially at low speeds when turning tight corners.

Limited Slip Differential (LSD)

The limited slip differential (or LSD), as you might now imagine is one that has a limited capability to slip. What this means, is that it has the ability to provide even less torque to the lifting wheel in the scenario above. Ultimately this means even more torque delivered to the wheel on the ground than what a 50:50 torque split from a locked diff would achieve. There are many different types of LSD - some with friction plates, some with electronic controls and others with ingenious gearing mechanisms that can provide this advantageous function of asymmetrical torque split. The down sides of LSDs are cost of course, their increased complexity - and maintenance considerations for some varieties that require adjustment and/or replacement of internal friction plates.

The Torsen differential can be considered as one type of LSD. It uses ingenious worm gears and hollow transfer shafts amongst other things to provide a very compact physical package capable of providing limited slip functions for a torque split in the range of 70:30 to 30:70. A torsen differential is maintenance free - being purely a clever collection of mechanical gears. There are no electronics or friction plates used on the Torsen design. For LOTS of more technical info on the Torsen diff, take a look at this white paper written in 1988 by none other than the the Institution of Mechanical Engineeers. Another useful site is this explanation of the workings of Torsen. That site more correctly describes Torsen as a torque multiplier.

As I understand it, the Torsen differential used on the Audi S2 (to give the 70:30 torque split capability), has a Torque Bias Ratio (TBR) by means of the physical gear size dimensions of 2.33:1. The only fly in the ointment of Torsen is how it behaves in an extreme failure scenario such as when a driveshaft shears in the hub (or a wheel is in the air). As the frictional resistance on the broken corner is zero, then the Audi’s centre Torsen gearing is unable to multiply any useful torque to the other axle (i.e. the car will not move). Similarly, if you jack up the rear (or front) of your Torsen equipped Audi, run the engine with the car in gear then only the wheels on the raised axle will turn. If you pull up the handbrake, introducing friction to the rear, then the Torsen can apply torque to the front axle !!! It is very odd and rather counter-intuitive as it is natural to imagine some torque always being transmitted to both outputs in all scenarios. In short, Torsen needs some traction at each output in order to transfer useful torque to the other side. This need not concern Torsen equipped owners as this odd behaviour is not experienced in practical driving situations – even on ice and snow.

The Automatic Torque Bias (ATB) differential from transmission heroes Quaife is another type of LSD and is actually rather similar to the Torsen design, except that it uses helical gears in deference to the worm gears used by Torsen. The claimed benefit of ATBs is that they react faster than a Torsen and can be configured for even wider torque splits such as 80:20 or more ! The major disadvantage of ATB is the cost involved.... This site gives useful background info, though its not relevant to Audi applications.

One last thing about all types of LSD is that, short of welding them shut, it is not possible to lock one !

It is my, and others, considered opinion that the Torsen centre diff provides the best compromise in safety, drivetrain, tyre wear and handling for roadgoing quattros in all weathers. However, it does not provide the same intuitive balance of traction to the drivers right foot as the locked centre diff can provide. This is because the Torsen is permanently slipping in order to provide more torque to the wheels with most grip - this sounds fine in theory, but there is always a little delay in how quickly the Torsen diff can react.


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Last Updated 3rd March 2008