Vanagon EuroVan
Previous messageNext messagePrevious in topicNext in topicPrevious by same authorNext by same authorPrevious page (April 1996)Back to main VANAGON pageJoin or leave VANAGON (or change settings)ReplyPost a new messageSearchProportional fontNon-proportional font
Date:         Sat, 13 Apr 96 00:42 CDT
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         khooper@wsp1.wspice.com (Ken Hooper)
Subject:      Locked rear axles...

Jim Davis posted a very lucid article on locking differentials. I'll try not to re-explain anything he explained, but there's a little more. Safety notes first:

Locking diffs are intended for *off-road* use. That means low-speed manuevers on low-traction surfaces where there is the opportunity for wheel spin. There is a reason for the distinction:

Do NOT install an automatic locking diff and go tearing off on a slick surfaced road at speed. If you enter an iced-over curve at high speed and goose the accelerator, you will find yourself spinning like a propellor. And here you were thinking the locker would increase your traction...you might have time to think, "isn't this ironic?" before the oncoming tractor-trailer squashes you flat.

Auto manufacturers don't install open diffs to be mean, they install them because they're the best way to maintain control of the vehicle at speed. If you lose traction in one wheel, yes, you can't accelerate--but the other three wheels still have traction and you're much less likely to spin. This is plain physics. If you don't intend to go offroad, leave the locker alone. It won't help you with commuting, it'll likely hurt you.

Now, the original comparison was Bus w/locker vs. Syncro. I said that installing the locker in the Bus would give you 90% of the traction of the Syncro, and several people have taken issue.

HOW THE SYNCRO WORKS, or what the f*ck is a viscous coupling?

The Syncro sends power to its front wheels by means of a viscous coupling. This is a differential between the front wheels and the rear wheels, enabling power to be transmitted to both ends of the vehicle. The rear-to-front viscous coupling operates full-time, like the side-to-side differential in your Bus or Vanagon. But unlike the transaxle on your Bus or Vanagon, it uses a fluid coupling. The adjective "viscous" describes the fluid in the coupling.

All fluids have some viscosity, but they vary. In order to understand how the viscous coupling works, here's how to brew your own (this really is a gas and kids love it, too. Tell them it's a scientific experiment):

Take about two cups of ordinary corn starch and mix it into a paste with water. (For those of you who don't cook, corn starch is next to the baking powder and flour at the supermarket. It's cheap.)

Got it mixed up? It wasn't easy, was it? Keep working until you get it mixed, then play with it. It's a lot of fun.

The paste is a fluid, officially. You can pour it. If you hold it in your hand it will drip through your fingers. It will conform to the shape of any container.

But wait: You can roll it into a ball like clay, and if you throw the ball onto the floor, it'll land with a hard "thud." Then the ball will spread out into a puddle. If you punch it, it barely makes a dent--and the harder you hit it the more solid it feels. If it drips onto the floor, you can pick the drips up like they were solid if you move fast.

It's friction that does it. The corn starch paste responds to friction by becoming more solid than liquid. It can't help it, that's its nature. The more you try to force it, the more resistance it puts up. This is how a highly viscous fluid behaves.

The Syncro uses a fluid like this to transmit power to the front wheels. Volswagen isn't unique in this; other vehicles use a viscous coupling, including the Audi Quattro, the Toyota FJ-80 Land Cruiser, and (I think) the various modern Land Rovers.

As long as the front wheels are turning the same speed as the rear wheels, the fluid coupling makes no difference. But if one end loses traction and starts to spin, it spins against the fluid in the coupling. That causes friction. The friction causes the fluid to become more solid, which transmits more power through the coupling to the front wheels. The more spin in the rear, the more power is transmitted to the front, gently but firmly.

This is all accomplished without any mechanical wear at all in and of itself. No metal touches metal, yet it transmits a vast amount of force exactly as you need it as conditions dictate. Engineering-wise, this system is a confection. It is delectable. And I never meant to imply anything but.

Discerning readers will have noted, though, that the viscous coupling cannot lock. It transmits power through friction; the more wheel-spin the more power transfer. If it were to lock, both front and rear wheels would be turning at the same rate, hence there would be no friction in it, hence there would be no power transferred at all, which is the opposite of locking. Which is absurb. _Quod erat demonstratum_.

How much power the viscous coupling "dumps," or absorbs, is a matter of design. I confess I do not know how much the Syncro dumps. In most trucks I think it's around 30%, but if anybody knows better please clarify.

HOW MUCH TRACTION A SYNCRO HAS, or why are threesomes better than twosomes?

The viscous coupling is one superior feature of the Syncro; another is the locking rear differential. Mr. Davis explained how the locker works, so I won't beat a dead horse. But it's important to note that the locker is not the same as the All Wheel Drive. It's entirely independent; you might have open diffs in both the front and rear and still employ the capacity of the center diff.

What it comes down to is, both the Syncro and the Bus with the locker can lock the rear wheels together. In addition, the Syncro may employ its front wheels, to the extent allowed by the viscous coupling, and which cannot be locked (there are rumours of a locking front diff, but as far as I know nobody actually has one in North America). That is, in reality, in really hairy terrain, the Bus with the locker can count on two wheels turning under power, where the Syncro can count on three--but the third will be limited to what power the viscous coupling is capable of sending it.

WHY APPLES ARE NOT ORANGES, or why some guys like her heavy in the rear.

Note that weight distributions are entirely different. Buses are butt-heavy; they carry most of their weight over their drive wheels. So do Beetles. You never saw a Bus sagging on its front wheels, didja? They sag in the rear.

(I don't know about Vanagons.)

Syncros, in addition to weighing about forty thousand pounds, ;) carry much more of their weight in the front because of the additional front suspension and driveline. Brother Smith claims they're nearly 50/50. I'm not completely convinced, but whatever the case is, the Bus is more able to utilize a locking rear because most of its weight is in the rear; the extra traction capacity afforded by the locker is effectively compounded by the unequal weight distribution.

NOW. I told you all that so I could tell you this:

1. We aren't talking about hard-core offroad stuff here. I know Jeep guys who carry on-board welders. They crank up the welders when they crack their frames. When, not if--they think nothing of it. Thirty-eight-inch, 44-inch tires and drivelines to match. Gratuitous abuse. No VW Bus is ever going to do this or anything like it.

When I say "offroad" here, I mean it will get your Westy to fairly remote campsites. Logging roads, fairly clean trails, 6" snow, 6" mud. No deep mud. No deep water crossings. No rock climbing. The thing is to camp off the beaten tourist track.

With chains and a High-Lift jack, the rear locker will accomplish this easily for (allegedly) $500-700 dollars and a weekend for the install. Go price a Syncro, then tell me whether it's worth it.

2. The limited-slip differential is another thing entirely, and to confuse matters there was a limited-slip available for Bays, at least, from the factory. I don't know about Splits or Vanagons. It probably does cost as much to have one of these rebuilt as to have a locker installed--but for those of you who commute in snow country, it might be just the ticket. Those things require a technique, and the limited-slip is only as good as the driver.

3. Whether a Syncro can move with its rear driveline disabled, and whether a Syncro is wondrously stable on snow, is interesting but irrelevant. I'm not talking about whether a Bus with a locker can replace a Syncro. I'm talking about whether a Bus with a locker is a feasible alternative for reasonable off-road camping, *taking into account its cost*. Moreover, this is the only alternative available for those of us who prefer to stick with aircooleds.

--Ken '68 Westy, '71 Bus, '87 Jeep


Back to: Top of message | Previous page | Main VANAGON page

Please note - During the past 17 years of operation, several gigabytes of Vanagon mail messages have been archived. Searching the entire collection will take up to five minutes to complete. Please be patient!


Return to the archives @ gerry.vanagon.com


The vanagon mailing list archives are copyright (c) 1994-2011, and may not be reproduced without the express written permission of the list administrators. Posting messages to this mailing list grants a license to the mailing list administrators to reproduce the message in a compilation, either printed or electronic. All compilations will be not-for-profit, with any excess proceeds going to the Vanagon mailing list.

Any profits from list compilations go exclusively towards the management and operation of the Vanagon mailing list and vanagon mailing list web site.