crossmember update (more pics..)

[treed2@u.washington.edu (T. Reed)]

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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>
>
>