When/why we need x-member bracing

[mkii_84@yahoo.com (Andrew Wong)]

Neal would these be considered stress marks on my
Scirocco.  Its an 84 Scirocco built in 12/83 so it has
the small tank and full size spare.  Even though I am
in Florida, that area has some rust frrom being a
northern car so I am unsure of what these lines are. 
My 84 is complety stock also
http://camo.flage.net/vag/view_photo.php?full=1&set_albumName=albuo48&id=P3050123
--- Neal Tovsen <nealtovsen@yahoo.com> 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.
> 
> 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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=====
-Andrew Wong 
1984 Black Scirocco
1987 White Quantum wagon

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