Syncro Driveshaft Decouper

What Benefit Does a Decoupler Provide:   It depends on who you ask...
Steyr Puch incorporated a drive shaft "decoupler" into its design of the syncro, which allowed the driver to engage and disengage power to the front differential.  With a few exceptions, it never made it into production.   The basic design and parts are depicted in ETKA, although only some of them are available from VW.  The critical parts are not available.  The reason that the decoupler never made it into production appears to be that it was designed primarily for the few very early production syncros that did not have a viscous coupling.  On those syncros, the drive shaft needed to be disconnected when four wheel drive was not being used or was not needed in order to reduce stress on the transmission and improve drivability on paved roads.

The incorporation of a viscous coupling rendered the decoupler redundant.  The VC acts as an automatic decoupler or disengager.  In theory, it transmits power to the front wheels only when there is a differential in speed between the front and rear wheels.  If there is no slippage or differential in speed, the main drive shaft is in effect disconnected from the front wheels since the VC is spinning freely and not transmitting any power.  Thus there is, in theory at least, no need for a drive shaft decoupler.

In reality, however, the VC does not function flawlessly under all driving conditions.  Even on straight freeway, the VC transmits some torque to the front differential.  On twisty roads, the VC gets a good workout and in turn transmits additional stresses to the entire drive train.  And if the VC is on the aggressive side, or is aging, it can engage on hot days after high speed traveling to an extent that does cause wind up in the drive train, and hence undue strain on the components.  For this reason, many believe that a decoupler saves wear and tear on the drive train (particularly during high-speed freeway driving in hot temperatures) and thus  prolongs its life. Others claim that the decoupler does not save on wear and tear, and note that the benefits of AWD are lost with it.

If the VC is removed and a straight shaft is put in its place, a decoupler becomes essential.  This converts the syncro back to the pre-VC early production stages.  Why would anyone do that?  At Syncro de Mayo this year, a couple of vans had the straight shaft-decoupler set up.  It was obvious to the casual observer that the straight shaft syncros out-performed the VC equipped syncros in the toughest off-road conditions.  The syncros with VCs were not as sure-footed and not as smooth over the really rough and steep terrain.  While it appears, therefore, that a straight-shaft - decoupler combo is a sure thing when it comes to enhancing the syncro's off-road 4WD performance, the straight shaft precludes use of 4WD on paved roads, except on snowy and slick roads that allow some wheel slippage.

Decouplers are available from enterprising inventors in South Africa, Germany and the US.  There are two designs.  The South African and German designs are depicted in the photo on the left below and track the VW design depicted in the drawing, above.  The US design uses a different approach for mating two shafts together, but otherwise functions the same.   The South African kit required assembly and also machining of the nosecone casing to accept the actuator rod, the switch and mounting plate, which is a job that requires machine tools.  The German and US kits come pre-assembled.  Installation takes about 4 hours and is not a complex job, nor does it require special tools (see photos linked below).

South African & German Design

US design

South African Decoupler Kit

US Decoupler Kit

Decoupler Installation Photo

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