Crossmember Bar design

[rocco16 at sbcglobal.net (Larry Fry)]

 The crossmember is
cracking due to fatigue.

****There is no doubt in my mind that this is the simple truth.****



>that said, it would take a much sharper 'V' in the bar to generate
enough side load under

***Actually, Allyn, the more shallow the Vee, the more "spreading-type" 
forces it can generate from the same vertical force.****.



. But if you look at the geometry of that plate, the
further down that V attaches the larger the moment you are applying to
that plate becomes. This is what I was considering when I said that
looked like the weakest part of the bar to me.

****Again, this is true.****



This is assuming the bar alone could do the job of the crossmember, I
would be surprised if that was true.

****As would I.  (I've lost track, at this point, of who said what....)****


 sure you could go nuts and make a
> triangulated mini-subframe down there, but it would be much larger,

**** The ideal solution would be to actually replace the stock crossmember 
with a triangulated sub-frame of the same external dimensions as the 
crossmember.  It would be heavier, complicated to install, and pricey, but 
it could cure this cracking business posthaste. If/when my own 'Roc cracks 
its crossmember, I WILL fabricate one and install it. :)   Then we can argue 
the merits of  THIS piece!!*****



. What I'm trying to say is it
would be nice to know that this bar isn't going to fail in 5 years. I
don't know that it will, but I also don't know that it won't.

****It won't.  It's not rigid enough to fatigue and it won't be stressed 
past its yield point because the crossmember is supporting it.  ****



 and to make sure that the bar stays a simple bolt on
product. Again, that's only IF it needs improvement.

****Therein lies the problem:  it will take more than a simple, bolt on 
device to permanetly correct this.  This motor mount thing was originally 
designed to be inexpensive, and durability was apparently not a concern in 
this late '70's design for an economy car.  Remember, this is a Rabbit 
chassis....Rabbits were designed to a price point.....it is NOT a 2005 BMW.

There is some room for improving Eric's bar, but not much.  Even optimized, 
there's not much this bar can do working with the existing VW design.  The 
good news is that it doesn't cost $300 and it doesn't do any harm.

Larry
sandiego16v  ****

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