When/why we need x-member bracing

[nealtovsen@yahoo.com (Neal Tovsen)]

> > 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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