[DeTomaso] Mangusta Box Frame

[Richard Chandler]

> zumzum at cox.net zumzum at cox.net wrote:

> Question for you Mangusta guys. With my center consol out, the famous center 
> spin box is in plain view. This car is a flexi flyer because this spine allows 
> too much twist/bending action which does distort handling, shifting and braking.  
> What if I were to engineer a steel, alloy or carbon fibre cover that would be 
> welded or bolted to the spine/box from front to rear walls with also wall overlap?
> No doubt torsional rigidity would be greatly increased since now the center floor
> would have a very low twist or bend action. This over plate would not be seen, 
> so no originality issues should come up from its use. This should have been 
> recommended from the factory, if they would have had more time and money for 
> research. This would only make the car better. I'd like to hear your views. 
> Jeff Cobb - 1196.


The backbone of the Mangusta is a 7.5 in. W  X 9 in. H rectangular tube of 13 gage
(.09 inch) material. The top and sides of the backbone are flat, but the bottom
surface of the backbone has numerous 90 degree bends to accommodate the coolant and
vacuum tubes, etc.  The numerous 90 degree bends make this lower surface similar to a
corrugated sheet where the corrugations run lengthwise along the backbone. The 90
deg. corrugated bends make the lower panel flexible when the backbone is loaded in
torsion and this severely compromises the backbone’s ability to resist torsion loads.
IMHO, the compromise is quite severe and it would not be tooooo unreasonable to
completely disregard the contribution of the lower 90 degree corrugated panel and
think of the backbone as an inverted U channel with an open bottom – i.e. just a top
and two sides. Admittedly, this is somewhat overstated, but not by a lot.

In reacting torsional loads, the integrity of the four sides of rectangular structure
is essential. The result of a flexible or compromised side in a rectangular tube is a
little hard to characterize, but a common engineering textbook example of compromised
torsion structure involves a round tube. If a round tube of 2 inch diameter X 1/8
inch wall tube were slit along it’s length (making the “closed section” an “open
section”) the torsional stiffness is lowered by a factor of 50 times!   

Adding material to the outside of the slit 2 inch tube will enhance it’s torsional
stiffness, but the return is pretty poor as massive amounts of material would be
added for very little gain in torsional stiffness - since an “open section” is
fatally weak in torsion. Similarly, adding material only to the top and sides of the
Mangusta’s backbone will help, but massive amounts would be needed for a
comparatively small gain in torsional stiffness.  

The greatest reward would come from welding a 13 ga. flat panel along the bottom of
the backbone. The flat panel will complete the backbone structure and the backbone
will then realize the torsional rigidity of a “closed section” rectangular tube.  By
simply welding a sheet on the bottom, the torsional rigidity would increase by a
large factor. It would not surprise me if it were 4 or 5 times stiffer!

The optimum solution would be to also incorporate diagonals or X members inside the
backbone. A nice example of the use of internal diagonals to enhance torsional
stiffness of a Lambo Miura rear chassis can be seen here: 
http://www.lambomiura.com/rest10.htm
An excellent analysis piece showing the benefits of adding diagonal ribs for
torsional stiffness is here:
http://plastics.dupont.com/plastics/pdflit/americas/design/DCI388.pdf
In the latter URL, an open section U channel – with a height to width ratio not
unlike the Mangusta’s - had 5.62 degrees of twist under the test torque. When the U
channel was modified to become a closed section (forming a rectangular tube), the
closed section had 0.63 degrees of twist. The closed section was 9 times stiffer!

Sadly, the torsion situation is exacerbated by suspension loads that try to bend the
backbone. (During cornering, a portion of the suspension forces is acting
perpendicular to the side backbone.) Normally these loads would not be of concern
because the floor pans would provide enormous stiffness to the backbone and easily
resist these bending loads. However, the ability of the floor pans to stiffen the
backbone means that the floor pan must push against the corrugated panel.
Unfortunately, the corrugated panel has no ability to resist loads from the side as
the corrugations just flex and allow the panel to compress. The compression of the
corrugated panel allows the side of the backbone to flex inward. This flexing has two
effects – one is just bending the side of the backbone, and the other is an
introduction of a secondary torsion load into the backbone. So when the backbone is
bending from suspension loads, the torsional deflection gets even worse. 

My Goose is a few days from being soda blasted to bare metal, and I’m looking for
ideas on upgrades and enhancements (additional to the normal restoration stuff) that
I should implement while it is in the raw state. 

Dick Chandler 8MA-708

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