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Crossmember Bars POLL




-----Original Message-----
From: scirocco-l-bounces+fahrvegnugen=cox.net@scirocco.org
[mailto:scirocco-l-bounces+fahrvegnugen=cox.net@scirocco.org]On Behalf Of
Larry Fry
Sent: Tuesday, June 14, 2005 9:53 PM
To: danws69@earthlink.net
Cc: scirocco-l@scirocco.org
Subject: Re: Crossmember Bars POLL


So far it looks like the cracking is fairly random.
Since the front engine mount concentrates it's stress and, more importantly,
its particular TYPE of stress, in an area where the US bumpers have no
effect, I was afraid this might be the case.  Oh, well,...

 The problem is that the front motor mount applies a TORSIONAL movement to
the front crossmember.  The crossmember, because of it's large "diameter"
and short length has a very high resistance to torsion.  So, the very thin
sheet that the crossmember is fabricated of has to flex with the motor
mount.
 Given enough (torsional) stress, from bad motor mounts, torquey engine,
etc., the sheet steel is flexed beyond it's ultimate strength. It has no
recourse but to fail (crack).
  If the torsional stresses were purely lateral (side to side) instead, then
a reinforcing bar such as Eric's would be effective.  But they aren't, they
are vertical, which actually creates a combination of torsion and bending of
the crossmember's sheet steel around the motor mount....which is where a
straight, side-to-side bar (Eric's) is the weakest.
 Imagine no crossmember...just Eric's (don't mean to keep picking on your
bar, Eric, but it is the only one I know about/of...) bar holding the
engine's front mount...no crossmember at all; you could move that bar's
center up and down a significant amount by rocking the engine by hand; a
three foot long, 1" square tubing just has too little bending strength.
 A much better design would be a trussed bar which would have several orders
of magnitude more resistance to vertical bending.  This type of bar was
suggested by a lister who contacted me off list about this issue. The
problem is there is precious little room for a trussed bar of sufficient
size in the engine compartment.
 This is the reason, I believe, (you like that?) that the US-spec bumpers
are not a factor in the cracking of the crossmember; they don't add to the
crossmember's material strength.
  Actually, if the crossmember had less torsional strength, we would
experience less cracking, since the front motor mounts energy would be
absorbed by the twisting of the whole crossmember and not by the bending of
the thin sheet in the mount area.

 How's that, David?
:)

Larry
sandiego16v


I am impressed, sir.  We are well on our way to a bon-a-fide discussion
here...  ;-)
However, I have one idea.  All of this presupposes that the front motor
mount is the cause, or at least the only cause...  I submit the idea that
the front crossmember cracking may actually be due to the sides of the car
are stronger than the front?  I wonder if this is not possible, as I have
yet to see this damage on a Rabbit or at least on one that was not bent
badly.  If indeed these bars help slow the cracking, and if indeed they help
with steering response, is it -possible- that the real culprit could be that
the body is twisting in the turns, and that it tends to break in the center
where there are coincidentally holes for the front motor mount?

I have no stake in this idea at all, as I do not drive a 'roc at all.  Feel
free to tear on my idea, as it is just that.
  However, I am interested in a couple of Erics' bars.  I am attempting to
raise funds at the moment.  Eric, you sure you don't want the engine we
discussed as some sort of trade for one of these?

David