After talking to a few people on site I decided I would try to describe in layman’s terms how the Syncro/Haldex transmission system works. Some of the basic descriptions are applicable to both 2wd and 4wd transmissions.
The Basics.
Car transmissions can be divided into 4 basic systems, Automatic (not discussed here) 2WD driving the front wheels, 2WD driving the rear wheels and 4WD driving all 4 wheels. 4WD can be subdivided into 2 further groups, “Permanent 4WD” and “Variable 4WD “
Differential
All cars almost without exception have a differential on the driven axle. This device is designed to provide power to both wheels but at the same time allow the wheels to rotate at different speeds when turning corners. This provides smoother driving, longer tyre life and less strain on the transmission system.
The disadvantage of the differential becomes apparent when one of the wheels starts to slip. The differential will push power to the slipping wheel and there is no longer any drive to the gripping wheel and the car will have no drive.
To overcome this failing of the differential there are a couple of options. Locking Differentials and Limited Slip Differentials.
Locking Differentials.
The “differential “ can be locked to stop the differential action from working, giving an axle where the wheels are locked together, so regardless of the grip on each tyre, both wheels are driven. This option is generally provided on Off Road Vehicles and is often combined with 4WD.
Limited Slip Differential
This is a system that allows a certain amount of wheel slip before automatically locking the differential action, again driving both wheels until full grip is restored then the lock is released
LSD can be found in a variety of vehicles, off roaders, performance cars and used extensively in motorsport and rallying.
VAG 4WD System.
Just quick note about Syncro and Quattro systems. Both were developed by VAG primarily Syncro by Volkswagen and Quattro by Audi. The systems are different in operation despite providing a similar function. Unfortunately over the years VAG have muddied the waters by swapping the names around on the different systems, so on the newer cars the name bears little relationship to the system underneath the vehicle.
i.e. an Audi S3 has an Haldex Syncro transmission system but is badged Quattro!!!
The Syncro Transmission System.
Fitted in a variety of vehicles, the system fitted to the Mk2 golf Syncro will be described. First used in approx. 1986, a modified version of this design is still used today (Haldex)
Essentially an extension to the front wheel drive system, (without any form of differential locking) the gearbox is modified to accept a 90 degree take off (Transfer Box) from the differential cage to provide power to the rear wheels.From the Transfer box a propshaft is connected to a Viscous Coupling on the front of the Rear Differential with shafts driving the rear wheels.
Transfer Box
This splits the available power off the gearbox via a rubber bushing to the propshaft and again via a second rubber bushing to the Viscous Coupling.
The rubber bushings offer a degree of flexibility when the engine moves as well as reducing transmission snatch
Viscous Coupling
This is the “magic” bit of the Syncro 4wd system and is the part that controls the amount of drive sent to the back wheels.
The VC consists of a series of circular plates immersed in a silicon fluid. The construction is of “in” and “out” plates interleaved together, the “in plates are connected to the main casing and form the input part of the VC unit. The “out” plates are connected to the central output shaft connecting to the rear differential.
When all 4 road wheels are turning at the same speed, the “in” and “out” plates in the VC turn together at the same speed but are isolated from each other and no power is transmitted through the VC unit.
If one or both front wheels begin to slip the car begins to slow but the drive through the transfer box, the propshaft and the VC unit continue to turn at the same speed.
The “in” discs in the VC unit are now effectively turning faster than the “out” discs and this difference causes the silicon fluid in the VC unit to “sheer “and change from a thin fluid to a thick gel. This causes the VC to “lock” the discs together allowing power to be supplied to the rear wheels. (The greater the difference in speed of the discs the faster the fluid reacts and the harder it locks the discs together).
The vehicle is now in 4WD or more realistically 3 or even rear wheel drive!!
The final part of the VC story is now the discs are locked together, there is no speed difference between them and the silicon fluid begins to thin, as it does the discs start to slip and the fluid reacts and locks the VC again. (This happens so quickly that drive is supplied continuously to the rear wheels until the front wheels stop slipping)
Rear Differential
This operates as a conventional differential with one exception it allows drive in one direction and freewheels in the other. This can be reversed by a mechanism that is controlled by an electrical solenoid and vacuum actuator to enable 4WD when reverse gear is selected.
(The operation of this device gives the characteristic “Clunk” occasionally heard when the car is moved in reverse)
The 2 deciding factors that determine how much power is sent to the rear wheels is the difference in speed between the front wheels and the difference in speed of the VC plates (or more correctly speed difference between the front and rear wheels)
This system simply but cleverly provides a fully variable 4WD system that theoretically can vary between 100:0 to 0: 100 i.e. front wheel drive, through 50:50 split 4WD to rear wheel drive.
Realistically rear wheel drive (0:100) is not possible as both front wheels would have no contact at all…. I guess the front wheels hanging over a cliff just might count!
Haldex.
This system is in essence the same as the earlier syncro system except the 4WD is no longer controlled using a viscous silicon fluid. It uses variable oil pressure to clamp clutch plates together to achieve the same result, the oil pressure is controlled by electronic sensors and Haldex ECU.
Finally the Syncro system works extremely well with perhaps one minor failing..The front and rear differentials cannot be locked. If you get in a situation where 1 front wheel and 1 rear wheel are slipping at the same time, the system cannot cope and again there is no drive.
VAG attempted to overcome this problem (at low speed) by using a system called Electronic Diff Lock (has nothing to do with the diff). VAG uses the ABS to detect wheel slip and apply the brake on that wheel to slow/stop the slip and restore drive.
I hope this goes some way in explaining how the Syncro system works, if you feel I have got something wrong please feel free to comment.
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