A differential of some sort is required in the drive axle(s) on all vehicles. It is there so that when one negotiates a turn—and the drive wheel on the inside of the curve begins to rotate at a different rate than the outside drive wheel—no binding occurs. Most vehicles, 2WD Vanagons included, have what is called a “fully open” differential. That means absolutely no binding on turns. It also means that when one drive wheel loses traction, no torque is delivered to the ground, and the vehicle stops moving forward. Many people refer to this as “one wheel drive."
The first solution for better traction was the clutch-type limited slip or posi-traction differential, which came into use in the ‘40s and is still in use today. The basic premise is that the two wheels are joined to each other through a series of clutch discs loaded with springs. When one wheel tries to spin independent of the other, the friction between the clutch discs transmits some torque to both wheels. The amount of “slip” that is allowed is predetermined by the spring load on the discs and the design of the friction material. These types of limited slip differentials vary in degree. The ones with more spring/friction work better to keep you from getting stuck. However, the downside of all friction type limited slip differentials is that they are not totally smooth on tight turns in high traction conditions. They can groan, chatter, hiss, chirp the tires—all kinds of unwanted behavior, especially on vehicles with a tight turning radius. This type of limited slip is found mostly on American made trucks. No such device is available for any Vanagon.
There is a fully mechanical, low budget “poor man’s posi-traction” that is available for the VW Vanagon. It is basically a couple of gears and shafts that replace the “spider gears” inside the existing fully open differential. When these new shafts and gears sense that one wheel is turning at a different rate than the other, movement is induced within the assembly, thereby locking the two wheels together. I had one of these on my 2WD Vanagon for about two weeks. They do work, but man are they clunky on (not even so) tight turns on (not even so) high traction situations. I had people in the back seat asking me if the rear axle was coming apart. I guess you get what you pay for….
Then there is the torque-biasing differential invented in the ‘60s by a British guy by the name of Quaife for use in Formula One racing. Instead of using clutches and springs and friction, this design employs a clever series of intricate worm gears. It somehow (I don’t really know exactly how it works, to be honest) biases torque back and forth between wheels based on demand by using these gears and the inertia of the assembly. Quaife’s patent on the design eventually ran out about ten years ago, and other companies started reproducing his design. GoWesty offers one made by the Peloquin company here in the USA. The benefit of this design over conventional friction type posi-traction units is that this design is totally smooth and does not bind, vibrate, or chatter at all on tight turns in high or low traction situations. This design works dynamically, and is therefore very difficult to actually “check," other than raising one wheel off the ground and spinning it by hand
—and feeling that there is some resistance, as opposed to no resistance with a completely open differential. (Note: This test needs to be done with the transaxle in neutral
—and with the drive shaft removed on Syncro models.) That is, they work WHILE IN MOTION and delivering torque to both wheels. The design senses if a wheel is slipping and BIASES torque delivery to the wheel with more traction, while in motion. Torque biasing differentials keep you from getting stuck before you get stuck and keep you moving forward. Once you’re stuck and one wheel is totally free, you are not MOVING and no TORQUE is being delivered, it does not work at all. It is not an “ultimate” solution, just an affordable compromise. Worked GREAT on Formula One cars that never got totally stuck, and in fact were rarely moving under 180 mph! I have driven thousands of miles without the posi-traction unit
—and now thousands of miles with the posis
—and I can tell you, unequivocally: They work exactly as designed, for sure.
The ultimate solution for getting out of stuck situations or maneuvering over extreme terrain, is what is commonly referred to as an “air locker” differential. This type was made famous by ARB company of Australia, which offers one that is controlled via compressed air (they don’t offer one for a Vanagon). These types of locking differentials are “ultimate” in that they are fully controllable. The 4WD SYNCRO models were commonly available with a fully controllable locking differential installed at the VW factory. Instead of compressed air, however, it is operated using vacuum. With one of these remotely controllable fully locking differentials, you have the best of both worlds. The differential is totally OPEN for normal driving. Then, when you're off-road or in slippery conditions or when you are totally stuck, you just pull a knob and the rear axle locks up 100%—and stays locked until you push the knob to disengage it. The only downside to this system (besides cost and complexity) is that if you inadvertently switch it to LOCKED while on dry pavement and make a sharp turn for a long enough time, you will break something, most commonly an axle or CV joint. As far as cost is concerned: whereas the passive torque biasing Peloquin units are only $1175, and do not require complete transaxle tear down, parts needed to add an OEM Syncro locker to an otherwise unequipped 2WD or 4WD Vanagon are rare, difficult to install, and expensive. It requires complete disassembly of the transaxle, and the operating system is expensive and difficult to install. The system installed costs around three times that of a torque biasing unit installed.
And finally... here's the ultimate, ULTIMATE option: A Syncro full locking differential and posi rolled into one. Not only does Peloquin offer a torque biasing differential for the manual and AT Vanagon transaxles—there is also one that works WITH the Syncro full locker system. So, when the knob is IN, you have a posi, and you can go about the business of driving without worry—having maximum stability and traction—automatically. Then, when you really need a full lock up, you pull the knob and voila! The downside, of course, is cost. Not only for the cost of the rare and costly-to-install Syncro full locker set up, but ALSO for a posi. But, hey, ultimate is ultimate in more ways than one!