Crossmember Bars POLL

[juliemac57 at hotmail.com (Julie Macfarlane)]

It appears to me that Erics bar is a V config. with attachment points at 
each corner of the triangle. Torque applied to the center of the V woulds 
act like an inverted suspension bridge (Like the Sydney Harbour Bridge) 
where the road "hangs" under the load bearing arch. Pressure is displaced to 
the two base mounting points of the arch..

The vertically applied torque (at the triangles apex) would be transferred 
to the bar, length wise, to the mounting points of the front bumpers (2 
points of the triangle).
In a perfect application the applied tourque would be split evenly to these 
remote points, thus reducing the energy applied to the apex.

Sound good? (and on only one cup of coffee!)

Julie Macfarlane
"Its not just a car! Its an adventure!"
1981 MKI 2L 16v w 2Y
Amsterdam NY




>From: "David Utley" <fahrvegnugen@cox.net>
>To: "Larry Fry" <rocco16@sbcglobal.net>,<danws69@earthlink.net>
>CC: scirocco-l@scirocco.org
>Subject: RE: Crossmember Bars POLL
>Date: Wed, 15 Jun 2005 22:45:25 -0500
>
>
>
>-----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
>
>
>
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