Stress!! was Re: Front crossmember stress bar

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

Forgot to mention; during the test of jacking the car up by the stress
bar????
The stress on the steel will be ~171,000 psi! Considering that garden
variety tubing has a yield stress of <66,000 psi, there might be a problem.
The bar will bend and will stay bent!
Even most pretty trick steel alloys won't pass this test!
OK! Those are the engineering calculations.
Dan

----- Original Message -----
From: Dan Bubb <jdbubb@ix.netcom.com>
To: <scirocco81@vwmail.net>; T. Reed <treed2@u.washington.edu>
Cc: <scirocco-l@scirocco.org>
Sent: Tuesday, March 11, 2003 12:54 PM
Subject: Re: Front crossmember stress bar


> 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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>
>
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