Smallest Audi keeps things simple to deliver its thrills
THE S1 HAS THE SIMPLEST OF AUDI’S QUATTRO systems. In format, it’s very similar to previous on-demand, all-wheel-drive set-ups fitted to essentially front-wheel-drive Audis, and is commonly referred to as a ‘Haldex’ system. This is the name of the device that engages drive to the rear wheels. It is a compact, multi-plate wet clutch incorporated into the nose of the rear differential housing, and is activated by hydraulic pressure.
Early iterations of the system were reactive – slip had to be detected for them to operate. Also, the hydraulic pump was mechanically driven and low pressure, so the units were slow to respond. The S1 uses the fourth-generation Haldex, as seen in the previous-generation TT. Gen four and five Haldex use an electric pump that builds pressure from the moment the car is started, and run much higher pressure too, for a faster, finer response.
Unlike other versions, the S1’s system always sends some drive to the rear wheels. Also, fuller activation does not depend on seeing slip at the front wheels; there’s a predictive element, the controller analysing inputs from various sensors – throttle position, steering angle, etc – to recognise situations when all-wheel drive may be required and pre-emptively send more torque to the rear wheels. The amount of pressure applied to the clutch determines how much torque goes to the rear, and a maximum of 50 per cent of the engine’s available output can be sent there.
At both ends, torque is fed to the wheels through an open differential. However, the S1 has a stability control function called ELD – Electronic Locking Differential – which is a virtual limited-slip differential. Conventionally, if one wheel on an axle is slipping, the torque will bleed away through that wheel. ELD applies the brake on the slipping wheel, which results in torque being sent to the opposite wheel.
It’s not sophisticated, but it is a simple and effective solution. Indeed, that’s a description that applies to the Haldex all-wheel-drive system, too: it’s a relatively simple, satisfactory system for delivering enhanced traction and stability.