http://www.brick-yard.co.uk/forum/westie-pop-top-conversion_topic30479_post227668.html
Alistair
----- Original Message -----
From: mark drillock <mdrillock@COX.NET>
Date: Tuesday, April 21, 2009 16:49
Subject: Re: Westfakia, Poptop, Access, Design
To: vanagon@GERRY.VANAGON.COM
> I agree, but no one suggested removing the entire roof skin.
> Only a
> section for standing in the main floor area is trimmed out, and not
> anywhere near all the way to the gutters. Cut 1/3 of the center
> of the
> shoebox lid out and not all the way to the edge and see the minimal
> impact on rigidity.
>
> Mark
>
>
> Mike wrote:
> > In a 'unibody' (also referred to as 'monocoque')
> design structure, any
> > skin section is a structurally-stressed component of the entire
> > structure. A Vanagon is considered unibody, even though
> it does
> > have frame strengthening members underneath.
> > Those roof support arches are more for rollover protection than
> > torsional rigidity of the body. The roof skin,
> however, is an
> > engineered, diagonal bracing that contributes greatly to torsional
> > rigidity of the entire body, in addition to providing added metal
> > structure to add crash or rollover protection.
> > Take a shoe box, remove the lid and grab the ends
> and attempt to twist
> > it; it will easily deform.
> > Now, put the lid on and repeat; you'll have a hard
> time deforming the
> > box from it's basic shape.
> > And, now, for the final proof of the body weakness
> created by a
> > large, unreinforced sunroof opening; cut a good -sized
> 'sunroof' into
> > your shoebox lid and repeat your twist test. What
> happens? My bet is
> > that a great amount of overall rigidity is lost due the
> 'missing' area.
> > You've cut the center out of your 'diagonal bracing!
> > VW made up for it by adding 'doublers' inside the
> factory sunroof, not
> > just a couple of cross ribs. The probably also added
> outboard, lower
> > frame cover plates (I know they did on the earlier
> buses). Side impact
> > crash protection, must also be taken into consideration in this
> > modification. (Smack the side of the shoebox to
> demonstrate this aspect.)
> >
> > Mike B.
> >
> >
> >
> >
> > As long as you don't cut the 2 main
> roof arches, one at the front edge
> > of the slider door and one at the rear
> edge, the roof area between is
> > just roof skin and not of much
> importance structurally, IMHO.
>
