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