Date: Fri, 25 Jun 1999 12:32:03 +0200
Reply-To: "Dr. Rainer Woitok" <woitok@RRZE.UNI-ERLANGEN.DE>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: "Dr. Rainer Woitok" <woitok@RRZE.UNI-ERLANGEN.DE>
Organization: RRZE (Regionales Rechenzentrum Erlangen)
Subject: Re: Turning when hot
In-Reply-To: Msg <3772806C.28C6@warwick.net> of 1999- 6-24 15:01:00 -0400 from
u1031271@WARWICK.NET
Content-Type: text/plain; charset=us-ascii
John and others,
On Thu, 1999-06-24 15:01:00 -0400, John wrote:
> ...
> When performing tight turns at low speeds, the rotational speed of the
> front axle will be less than that of the rear axle which should cause
> the viscous coupling to lock. This is normnal.
No, the rotational speed of the front axle will be greater than that of
the rear axle, and no, in this situation (different rotational
velocities caused by a tight turn at low speed) the viscous coupling
should not lock.
I'm well aware that almost all Syncro drivers agree that this is normal
behaviour, simply because that's the way their Syncros are acting. But
up to now all Syncro drivers who really replaced their viscous coupling
with a new one were amazed at how easy going everything suddenly was
(been there, done that). With a properly working viscous coupling and
on a tar sealed road any slip happens within the viscous coupling, and
never between tires and road.
> ...
> The "bigger tires" and parking situation could point to a steering wheel
> rotation effort issue. However if his parking difficulties are a lack of
> power assist, that is an entirely different matter which does not
> involve the viscous coupling.
Right. But since Steward explicitly mentioned in his original posting
"differences between wheel speeds" and that "the locks are not on", I
just assumed we were talking about binding of the viscous coupling here
rather than about power steering problems.
> ...
> However when performing the same type of turn on hot, sticky asphalt,
> the vehicle drivetrain binds and the front wheels "chirp" or jump
> through the turn. I attribute this to a "trapped stress" which develops
> in the four wheel drive system due to the "unforgiving" nature of the
> tire/hot asphalt adhesion. This of course is quite normal and not a
> reason to disconnect the driveshaft and start shopping for a front
> differential (VC) assembly.
No, it's NOT normal. Though this is already in the archives (probably
multiple times) I'll include here parts of an earlier posting from me
about what I've found in the original German VW factory repair manual:
VW's original (German) repair manual doesn't say much about how to test
the viscous coupling. They only recommend placing the rear wheels in a
break testing stand. If you then switch to the G-gear (creeping gear),
the front wheels should move the van out of the test stand as soon as
the engine is revving slightly above idle. If the front wheels fail to
do so the viscous coupling is to be replaced, VW says. VW adds another
tiny sentence to this, saying that only when the engine is revving at
idle and with the G-gear switched in, the viscous coupling is able to
absorb all the torque to the front wheels and keep them from moving.
To me this last and rather ill-formulated (in the German manual)
sentence is the key to testing the viscous coupling. For in most cases
we are not dealing with viscous couplings doing less than their share,
but rather with hard-going viscous couplings which don't have a problem
at all in moving the van out of the test stand with the engine just
idling.
Thus the really important thing here is not the van successfully leaving
the test stand. On the contrary, the important thing here is the van
not moving and staying put in the test stand with the G-gear switched in
and the engine just idling. If your Syncro doesn't pass this test your
viscous coupling is probably worn out and ready for a replacement. Or
put the other way round: as long as your van's viscous coupling is
working properly you will not notice your van has got one.
End of the quote from my own posting. Some ingenious people without
access to a break test stand have modified this test using a floor jack,
but to me this seems a rather risky procedure. However, the reaction of
the Syncro has to be the same as in a break test stand: with the engine
just idling and the transaxle in G-gear the van must stay put. If it
already drags the floor jack accross your driveway when idling I'd put
my money on the shot viscous coupling (litterally :-). Your milage may
vary.
Sincerely
Rainer
'89 Caravelle GL Syncro 16"
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| Rainer M Woitok | Phone: (+49-9131) 85-27811,-27031 |
| Regionales Rechenzentrum | |
| Friedrich-Alexander-Universitaet | Fax : (+49-9131) 30 29 41 |
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| Germany | Mail: Woitok@RRZE.Uni-Erlangen.DE |
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