What is a Syncro?

Volkswagen Type 25 Syncro

Syncro is Volkswagen’s name for any of its vehicles using their Syncro AWD (All Wheel Drive) system. It’s also fitted to some VW cars as well as the later FWD T4 Transporter, but was first introduced around 1985. Volkswagen engineering staff mooted a 4WD for their earlier generation vans, campers and derivatives, building and testing several prototypes successfully, but when finally put into production on the T25 it was Steyr Daimler-Puch in Austria who had suggested the concept of a Viscous Torque Coupling unit in the drive-line to the front wheels (a VC was first used in the British Jensen FF of the 60’s). Subsequently, most of these vehicles were produced at the SDP plant from parts supplied from VW Germany. The T25 Syncro makes maximum use of 4WD on the road and the rough, having an ideal fore/aft weight distribution – 48/52%, which gives it quite superb all-round handling and traction. The term ‘Syncro’ evidently refers to synchronisation of the front and rear wheels… yet should be distinguished from synchro-meshing gear change mechanisms.

General Concept
Utilising a Viscous Coupling (VC) in the front differential unit, Syncros are effectively permanent 4 wheel drive vehicles, where small differences in front and rear wheel speeds can be accommodated by the unique characteristics of this type of coupling. Viscous couplings have a unique nonlinear torque/slip curve, the discontinuity in the curve being termed – ‘the hump’. They are somewhat analogous to a multi-plate clutch unit, but with the permanent gaps between the plates filled with special Silicon based fluids – their torque transmission characteristics are affected by fluid viscosity, which changes rapidly with internal temperature. Dropping down a cog from such lofty engineering principles – the Syncro system works extremely well, distributing the torque forward to the front-wheels pretty much as required. Compared to a solid connection (unworkable on tarmac), a centre differential (heavier, requires full or partial locking) or Torsen style diff (preset torque split) – it’s considered by some 4WD buffs to be a compromise design. The good news is that it is – an excellent compromise for paved/off-road use offering great traction and cornering on poor, wet or snow covered metalled roads and excellent distribution of traction loads front and rear on the loose, automatically redistributing this with varying axle loads and the vagaries of tyre-surface grip.

Driving a Syncro off-road
Normal (open) axle differential units (diffs) allow inner and outer wheels to rotate at different speeds when traversing round curves, but this arrangement has a big downside when either wheel’s grip is insufficient for the torque being applied it – not only does it slip and spin, but the torque transmiited across to the other wheel is reduced to the same value – virtually nothing for the spinning wheel ! Whilst some vehicles have a device that limits the slip, serious off-road vehicles usually allow full locking of their axle-diffs and if they have transfer box diffs, that can sometimes be locked too. Syncros, with the prop-shaft transmitting drive torque forward, use the VC like a transfer box diff, but offering controlled limited slip. Elegant! Driving a Syncro around on gravel, grass, or muddy tracks with nominal gradients and tight bends doesn’t usually require diff-locking and if taken steadily, is a very impressive experience – just crusing around with impunity where most 2-wheel drive vehicles fear to go.

The problem of wheel-spin when one of its wheels eventually loses grip on poor surfaces or very steep climbs is handled by fully locking the differentials. Syncros were produced with none (mainly US), rear only (common in the US) and both front and rear axle diff-locks (the preferred configuration for serious off-road work, but not sold in the US for liability reasons). Most but not all European Syncros have both. Neither diff-lock should be activated on dry tarmac roads due to strain on the transmission, whilst locking the front one on public roads can be extremely dangerous, as the locked diff pulls the steering straight ahead. The control for this (on the left of diff-lock panel) has to be turned as well as pulled to activate the front locker, to prevent inadvertant use!

A very nice feature is the ability to control diff-locking whilst under way from the driver’s position. When losing traction, the driver will usually first lock the rear-diff. The vehicle stiffens up somewhat at this, but remains eminently driveable. Should the gradient or surface be such that the whole locked rear axle loses grip (both rear wheels spin together), transferring more torque to the front, then its likely that one or other of the fronts may soon decide their grip isn’t good enough – and just like any ‘open’ differential axle, all ability to transmit torque to the only wheel with any grip will be lost at the front as well.

It’s at this point that the front diff-lock is required, and whilst there is a downside to locking this diff, the overall traction available is extremely high, something like a climbing angle of 50 degrees being achievable on some surfaces. The downside is that the steering all but locks up, certainly stiffening to the point that straight ahead is about the only option! But hey, we’re out of the muck…

Because of their suspension design, T25’s sometimes run-out of articulation when crossing steep slopes, kerbs etc. This means that one rear wheel cannot drop down enough to maintain contact with the ground – a 2WD without a Limited Slip Diff may get stuck at this point, a Syncro’s front wheels will normally prevent this situation occurring, but nevertheless, locking the rear diff will always resolve it..

Syncro gearboxes (and respect!)
Syncros have a 5-speed gearbox, effectively a 4-speeder with an extra low ‘Gelande Gear’ (GG, offroad gear) for climbing or crawling – Don’t hammer it in ‘G’, 1st or reverse gear, it is not necessary. The vehicle will crawl up a good slope on idle in G gear, but to get reasonable torque to the fronts through that VC requires 2500 rpm +, so when a big ramp or rock needs climbing, slipping the clutch or just more speed may be necessary. ‘G’, 1st and 2nd are nicely spaced, 3rd is a much bigger gap in ratio, which wouldn’t normally be used much off-road.

The torque multiplication is very high in ‘G’ and Reverse gear and all VW 5-speed T25 boxes, whilst nicley designed, are not bullet-proof – Abuse it continually, and one day that G will stand for Grunchengearen!
Don’t use ‘G’ gear for rapid getaways on tarmac – it just isn’t built for that!. The standard clutch is quite sufficient for the motor’s torque but like all dry clutches, is not designed for constant slipping – use the right gear and keep your foot off the pedal. Only when confounded by an obstacle not succumbing to Gelande gear’s immediate charms should slipping the clutch be contemplated and don’t ever bump-start a Syncro in ‘G’ or ‘Reverse’ – you will likely break the gearbox (casing), the ‘R/G’ gear carrier is designed for forward loads only.

Don’t tow for extended periods and never have the vehicle towed with a front or rear end lift – this is dangerous, will overheat the VC quickly and strain transmission too. Driving with unmatched tyres (wear/pressures) can put a lot of strain on transmission parts and overheat the VC too… the Tyre Rolling Circumferences must be matched. Continual tight turns on tarmac roads with an old Viscous Coupling (gone ‘stiff’) is said to strain the whole transmission. Syncro’s invariably cover 100,000 ~ 200,00 miles without serious wear or failures when maintained well. It is now possible to have the Viscous Coupling refurbished and re-calibrated (CAV16 offer a UK service, VAG only offer new units). Most VAG Syncro specific parts are now pricey and transmission parts on all Transporters are fairly expensive – ensure the correct gear oils are used and levels checked yearly. These Volkswagens are built quite strongly as standard – treat it with respect – a 2.1 litre Wasserboxer Syncro is quite a powerful vehicle.

Serious Syncros
Syncros have a lot of other modifications to make them strong, versatile, well engineered all round vehicles for occasional off-road use. Syncros 16’s or Syncro 16″ are a special model having, yes… 16″ wheels. The standard Syncro Transporters use 14″, although a good match can be made with 15″ rims and some tyre combinations, increasing ground clearance if High Aspect Ratio tyres are used. 16″ wheels cannot be fitted successfully to non Syncro 16’s – unless you want to re-engineer the whole vehicle. Whilst Syncro springs and obviously front axle, shockers and most of the running gear are different to a 2WD drive Transporter, Syncro 16’s are even more beefed up, with bigger brakes and driveshafts. Notably, they have 1″ longer rear swinging arms. Prepared well, these are very capable safari style rally vehicles, and as standard, often used in the past as support vehicles for such events. A very popular Syncro 16 configuration was the crew-cab style vehicles (DoKa’s), ideal for the construction industry, woodsmen, the military and of course, seriously difficult terrain anywhere.

Clive Smith July 2002