When/why we need x-member bracing

[Dan Bubb" <jdbubb@ix.netcom.com (Dan Bubb)]

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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