When/why we need x-member bracing

[Rabbit16v@attbi.com (Rabbit16v)]

I think you'll find that cars with one of the following will be the biggest
group with the problems:

1.  Cars made after 84
2.  Cars with 150hp or more
3.  Cars with previous medium front end damage
4.  MAYBE cars for more temperate geographical locations.  I haven't heard
of many cracking situations in the NW which seems to be VW haven AND (for
the most part) only has about a 60 difference in weather temps (highs in the
summer 90ish, lows in the winter 30ish).  But, I may be wrong.  What about
the southern states?

Dave
----- Original Message -----
From: "Dan Bubb" <jdbubb@ix.netcom.com>
To: "Neal Tovsen" <nealtovsen@yahoo.com>; "T. Reed"
<treed2@u.washington.edu>
Cc: "Scirocco List" <scirocco-l@scirocco.org>
Sent: Wednesday, March 05, 2003 4:54 AM
Subject: Re: When/why we need x-member bracing


> I think Neal's thoughts are worth the test. We need to get together a list
> of who has cracks and what year their car is. Might also help to know
> general powertrain configuration to see if that is a contributing factor.
> Dan
> 84 (coincidentally!)
> 2.0 L 8V NA
> Single owner, never wrecked
>
>
>  ----- Original Message -----
> From: Neal Tovsen <nealtovsen@yahoo.com>
> To: T. Reed <treed2@u.washington.edu>
> Cc: Dan Bubb <jdbubb@ix.netcom.com>; Scirocco List
<scirocco-l@scirocco.org>
> Sent: Wednesday, March 05, 2003 1:37 AM
> Subject: When/why we need x-member bracing
>
>
> > > > I have a theory here, so y'all get out your flame throwers!
> > > > The theory is based the fact that I've had my Scirocco for 18.5
years
> and
> > > > have always driven it hard. Not slamming shifts, but alot of WOT and
> high
> > > > RPM downshifts. In other words, I've had as much opportunity to
crack
> my
> > > > chassis as anyone and to date, with 240,000 miles, NO cracks.
> >
> > I pulled this portion of the crossmember thread into its own new thread.
> > See the two relevant emails below if you haven't been following.
> >
> > I've been doing a lot of thinking about WHEN and WHY the crossmember
> > cracks appear, in combination with the people who have actually said
> > that they've experienced this problem.
> >
> > Is there ANYBODY out there who has an '84 or older Scirocco with a
> > cracked front crossmember? If there are any, I bet it's only a couple
> > people out of the bunch. I dawned on me one day that I haven't heard of
> > anyone with this problem on a Mk1 or on a pre-'85 car. Nearly all of
> > them have been 16v's, and I think the 8v's were later cars too.
> >
> > While it is obvious that improper alignment of the motor mounts could be
> > a contributing factor, there is one important thing that happened after
> > '84. In 1985, VW changed the floor pan of the car to incorporate the
> > space-saver spare tire and larger gas tank. At the same time, the design
> > of the car changed a little bit across the whole car. Most importantly,
> > the type of steel used was changed! It was supposed to be (and is, from
> > my experience) much more rust-resistant. This became clear when I was
> > welding the floor pan from an '85 into my '77, and was reinforced by an
> > autocrosser who isn't on this list who has also studied the floor pans
> > of the different year Sciroccos.
> >
> > The '85 floor pan I had was very obviously of different material, and I
> > am starting to believe that it is more prone to stress cracks.
> >
> > The only other possibility that fits this theory somewhat is that in '86
> > we started seeing higher-output 16v engines. However, that doesn't
> > explain all the souped-up 8v's and 16v transplants into earlier cars
> > that seem to all do fine.
> >
> > So let's test the theory. Anybody out there with an '84 or older
> > Scirocco with crossmember cracks? Dan Bubb, what year is your car?
> >
> > Neal
> >
> > On Tue, 2003-03-04 at 17:48, T. Reed wrote:
> > > Dan,
> > >
> > > That's a very interesting theory, and I did not know that the front
> mount
> > > was an add-on!
> > >
> > > I think you're probably right to some degree. But take note that just
> > > about every case of cracked crossmember I've seen (and maybe people
will
> > > jump out here to prove me wrong - feel free!) has been on a Mk2,
usually
> > > with a 2L 16v. And the cracks typically develop within a year or two
of
> > > upgrading the engine. Those has just been my observations, though. I'm
> > > sure I'm gonna catch flak for saying that..
> > >
> > > Personally, I think poly mounts are a big contributor. I have a poly
> front
> > > mount and a vw motorsports rear mount and when I jump on and off of
the
> > > gas very quickly the whole car jerks around like a friggin air hammer.
> > > That kind of pounding has got to do bad things to the thin sheetmetal
> > > that vw spot welded together to form a crossmember.
> > >
> > > -Toby
> > >
> > > On Tue, 4 Mar 2003, Dan Bubb wrote:
> > >
> > > > I have a theory here, so y'all get out your flame throwers!
> > > > The theory is based the fact that I've had my Scirocco for 18.5
years
> and
> > > > have always driven it hard. Not slamming shifts, but alot of WOT and
> high
> > > > RPM downshifts. In other words, I've had as much opportunity to
crack
> my
> > > > chassis as anyone and to date, with 240,000 miles, NO cracks.
> > > > The two powertrain mounts that do the most to dictate the position
of
> the
> > > > engine are the side mounts. They sag with time. The rear and front
> mounts
> > > > can be adjusted to accomodate this sag to some extent. But, if
they're
> not
> > > > adjusted they end up supporting most of the weight when their main
> function
> > > > (imho) is to resist powertrain torque. So, if you never adjust your
> front
> > > > and rear mounts and don't replace your passenger side mount
regularly,
> you
> > > > have an abnormal load on the front and rear mount. The front is the
> least
> > > > able to take the load, so cracking results.
> > > > Did you all know the front mount was an after thought. The Scirocco
> was
> > > > originally produced without a front mount. A retrofit was added to
the
> '75
> > > > Scirocco, and the current mount design started in the '76.
> > > > So, that's my theory! Adjust and replace your powertrain mounts
often.
> > > > Dan
> > > >
> > > > >If this were the case, why is
> > > > > my crossmember cracked on the bottom rather than the top? Tension
> causes
> > > > > cracking, not compression (or at least it takes a great deal more
> > > > > compression to crack sheet metal in general)
> > > > >
> > > > > Could you clarify your statement?
> > > > >
> > > > > Thanks,
> > > > >
> > > > > -Toby
> > > > >
> > > > > On Mon, 3 Mar 2003, Allyn wrote:
> > > > >
> > > > > > > First of all, the force vector acting on the crossmember
points
> > > > straight
> > > > > > > down.
> > > > > >
> > > > > > umm, during a downshift perhaps, but the major load is in the
> upward
> > > > > > direction, during acceleration. i think this makes your whole
> > > > explination
> > > > > > backwards (as far as tensile/compressive stresses), but it will
> work
> > > > just
> > > > > > the same.
> > > > > > Al
> > > > > >
> > > > > >
> > > > >
> > > > >
> > > > >
> > > > > _______________________________________________
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> > > >
> > > >
> > > >
> > >
> > >
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> >
> >
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