Front crossmember stress bar

[Dan Bubb" <jdbubb@ix.netcom.com (Dan Bubb)]

I just wanted to throw some rough numbers at this so we have something
alittle bit better than a WAG.
First off rotating the bar 45 deg does nothing for the stiffness of the bar
in bending. I would have thought it would, but the equation for the moment
of inertia is the same whether the bar has a flat surface down or is rotated
45deg.
Second, the deflection of a beam, whether it is a bar or a flat sheet, is
proportional to its moment of inertia. So, a simple comparison of moment of
inertia (designated as "I") will tell us comparative stiffness.
For the front crossmember I assumed a simple plate 5" high X .03" thick.
Then I = .31 in^4.
This does not take into account the contribution of the top or front of the
crossmember. Nor does it account for the various holes in the rear face of
the cross member. My engineering estimate is the actual number is somewhat
higher than the simplified calculation.
For the tube I assumed a 1" square tube with 3/16" wall. (I believe these
are the dimensions I stated for the stress bar) In this case I=.07 in^4.
This is a pretty accurate number since it's just a simple tube.
So, the tube is ~22% of the stiffness of the crossmember. A contribution,
but only carrying <1/5 the load.
If the tube was increased to 1.5" square with a .25" wall , then it would
approximate the stiffness of the crossmember. Course the tube would also
weigh 15# and the rigidity (or lack thereof) of the rest of the parts of the
assembly would need to be accounted for.
A test was proposed to jack the car up at the center of the stress bar.
Assuming the weight of the front of the car is 1200#, the length of the bar
is 40" and the bar dimensions are 1" sq, X .187" wall, then the deflection
should be on the order of ~3/4". This is just deflection of the bar and
assumes every other part of the stress bar is prefectly rigid.
Personally, I like the "ultimate solution" proposed below of doubling up on
the rear skin of the crossmember.
Dan



----- Original Message -----
From: Eric S <scirocco81@vwmail.net>
To: T. Reed <treed2@u.washington.edu>; Eric S <scirocco81@vwmail.net>
Cc: <scirocco-l@scirocco.org>
Sent: Sunday, March 09, 2003 3:55 AM
Subject: Re: Front crossmember stress bar


>
>
> --- "T. Reed" <treed2@u.washington.edu> wrote:
> >Eric,
> >
> >The V-bend looks like an interesting idea, let us know how it works. How
> >do the brackets at the side attach? They don't appear to be connected to
> >anything in the pictures..
>
> Like I said, rough prototype, no holes yet...should have a boltable
version this week.  It will bolt on through the bumper bolts and the front
cup bolts.
>
> As for the V, its very shallow...and with the square tubing at 45 degrees,
its like having a double V.  I have a feeling people are underestimating the
strength of the square tubing.
>
> >I'm still trying to re-evaluate my bar now that my brain has been
> >flipped upside down as far as which motor mount goes which direction.
> >
> >Personally, I think the only way to convince the "doubters" on the list
> >would be to build a bar strong enough that you can jack the car up from
> >it. (I have some ideas about how to do this but the bar would weigh more
> >than twice as much..) Fortunately, what other people think isn't too big
> >of a concern for me, unless they can show that they're right.
>
> That will be one of our tests, to jack up the car by the bar with the
front cup bolts out...measure the deflection (if any) of the bar under
serious load.
>
> >My favorite solution to the cracked crossmember problem is to drill out
> >the spot welds holding in the crossmember, remove the crossmember, get
two
> >uncracked ones from junkyard cars (pre-84 would be preferable, apparently
> >the metal used was superior), seam weld them together and weld them back
> >in to the car. That is the "ultimate" repair, in my book. Stock-looking,
> >using stock parts, very strong, and very expensive and very painstaking.
>
> Right, well, it is after all just stamped sheetmetal...so you are
effectivly doubling its thickness...which still isn't much.  It surprises me
how flimsy these cars are!
>
> >But bolt in bars are "easy", and you can't have it all.
>
> Exactly...our design is not just for the Scirocco list, its for all A1
chassis, and to be able to make a bar that bolts up, and fits all A1, alot
of design considerations had to be taken into account.  Not alot of people
have the money, time or patience to weld plates or replace crossmembers
entirely on thier car, but an effective bolt on stress bar...yes, that is
"marketable".
>
> >Anyway, good luck w/your bar..
>
> Thanks Toby....let me know how your tests turn out.
>
> --
> Eric
> Northglenn, CO
>
> 79 VW Iltis
> 81 Scirocco S (*TDI* swap and complete restoration in holding pattern)
> 81 Rabbit Truck LX Diesel (Waiting for BAE Turbo Install)
> 83 Scirocco Wolfsburg
> 91 Cabriolet (2.0 Crossflow 8v swap and mechanical rebuild in progress)
>
>
>
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