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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


'87 16v Scirocco - had US bumpers, had crack starting by 145k mi...now
has Euro's and the Saltyweazle bar (crack stopped, handling stiffer)

'87 16v    -occo (? car-trailer) - no crossmember :)

'82 DIESEL Scirocco - US bumpers, no cracks (yet!!!), plan on installing
bar prior to any developing

'87 16v Scirocco (back-up shell) - US bumpers, no cracks, but, the
front-end was replaced at some point




Hope that helps,

Dan (^^^^ that was my sig....)



-----Original Message-----
From: scirocco-l-bounces+danws69=earthlink.net@scirocco.org
[mailto:scirocco-l-bounces+danws69=earthlink.net@scirocco.org] On Behalf
Of Larry Fry
Sent: Tuesday, June 14, 2005 20:30
To: Brian McGarvey
Cc: scirocco-l@scirocco.org
Subject: Re: Crossmember Bars



 Meaning some sort of stress was put on them... Like not changing your
motormounts for 20yrs..., getting in wrecks, taking the strong a$$ front
bumper off and replacing it with a decrotive peice of eurotrash. :-)...
Brian



Hmmmm. Very interesting point.  The Euro bumpers, I mean.  Brian may
have
something

How about a nice, constructive poll?

How many have Eurobumper?
How many have US-spec bumpers?
How many have cracked crossmembers?
How many don't?

I'll start:

US-spec.......no cracks......220,000miles.  (But I just put Eurobumpers
ON!!!...this may get interesting.)

Larry
sandiego16v


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