Small­est Audi keeps things sim­ple to de­liver its thrills

Evo - - 4WD MEGATEST -

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.

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