Unbelievably quick reactions - the quattro

It's interesting to note how a car's fundamental concept can often influence the decision in favour of a host of individual components. This is demonstrated most effectively by the permanent four-wheel drive featured on the quattro versions of the Audi TT Coupé, with either 132 kW (180 bhp) or 165 kW (225 bhp) engine.

The highly desirable compact character of this sports car, with its exceptionally short body overhangs, calls for a transverse engine layout. This in turn means that the system used to distribute torque between the front and rear axles must match the engine layout.

For the first time in the Audi model programme, therefore, the TT Coupé quattro incorporates an electro-hydraulically controlled torque distribution system. It has the same advantages as previous quattro models, is extremely compact and can thus be said to be ideal for installation in the TT. The ability to respond rapidly which matches the car's sporting character so well is achieved by exceptional sensitivity: before the difference in the angles of rotation of the shafts connected to the front and rear axles has changed by more than 45 degrees, the system is able to respond to changes in road conditions.

For example, if slip occurs at the front wheels so that traction is no longer available in full, the torque distribution system takes effect and redirects an increasing amount of engine torque to the rear axle in an accurately controlled process. By the time this change in torque distribution takes effect, the wheel has rotated a mere 25 centimetres further.

The hydraulic multi-plate clutch is controlled electronically and installed between the propeller shaft and the rear-axle differential. Inside its housing is a plate cluster running in an oilbath. This can be forced together by a controlled increase in hydraulic pressure, so that it is capable of transmitting an increasing amount of torque to the rear axle. An electric pre-load pump maintains pressure to ensure that the clutch responds rapidly. The greater the pressure applied to it, the greater the torque redistributed from the front axle to the rear.

In other words, front/rear torque distribution does not take place according to a fixed characteristic; this is a dynamic system in which torque levels are controlled by a variety of parameters. Two axial-piston pumps supply the pressure to the pistons which operate the clutch plates.

These pumps are driven by a swashplate, which turns when a difference in rotating speed occurs between the unit's input and output shafts. An electronically controlled valve is capable of varying this pressure continuously. The associated control unit makes use of a wide variety of information, including wheel rotating speeds, engine speed and engine torque.

A software program overrides the system sufficiently to take the driver's requirements into account. For example, the clutch is released when the anti-lock braking system (ABS) takes effect, and no further redistribution of torque front to rear takes place, so that the ABS regulating system is able to operate without outside influences.

The system remains active when the car is reversed, so that maximum traction is available in this situation too. When coasting, the Audi TT Coupé quattro exhibits a consistently mild handling characteristic, which is of considerable benefit, for example, if the driver is forced to lift off at the accelerator pedal during the actual cornering process.

Positioning the engine at the front and the multi-plate clutch for the four-wheel driveline at the rear, adjacent to the rear axle, improves the car's axle-load distribution and is a major factor in the TT's outstanding dynamic stability.

In practice, the driver of an Audi TT Coupé quattro benefits at all times from the proven advantages of this permanent four-wheel driveline: best possible traction on all road surfaces, directional stability when accelerating, high active safety since every scrap of pulling power can be ˛put down on the road˛ if necessary, greater stability when cornering ­ none of these being merely "poor weather benefits".

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