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Re: [T3] Seat rails guides and carbon fiber mockups


Hello-

> What does an unstressed body panel mean? Is that where it doesn't form
part
> of the structure of the car?

A good analogy is to look at biology - endoskeleton vs. exoskeleton.
Endoskeleton is like human beings - our main structure (bones) is seperate
from our outer covering (skin).  Exoskeleton is like insects - their main
structure IS their outer covering.  It would be incorrect to say that our
skin is _entirely_ unstressed, but it's really the bones that do the huge
majority of the job.

When you think of a stressed-skin vehicle (car, airplane, etc.), think
exoskeleton.  Just like in nature, they can be simpler and lighter, but are
fairly limited as far as design goes.  When you think of an unstressed skin,
think endoskeleton.  Again just like nature, they are more complicated and
heavier, but allow much greater flexibility in design.

Which is better?  Well, it all depends on what you want to do, how much
money you have for design and engineering, and how much money you have for
manufacturing.  It's an open-ended problem with no "correct" solution... ah,
design is fun!  :-)

Our little VWs are stressed-skin vehicles.  That's one reason why they're
simple, light, and cheap.  However, they are (admittingly) also quite
limited... they aren't that stiff and there's no easy way to add stiffness
without changing the entire design.  Most modern production cars have an
unstressed skin, which is one reason why the bodies crumple up so much
easier in collisions yet the passengers have a greater chance of living.
Without dramatically increasing cost, they offer more rigidity and more
flexibility for design changes, at the cost of having more parts and
weighing a little more.  Many race cars also have an unstressed skin.  When
your manufacturing quantity is one, it's a hell of a lot easier to design a
tube frame or equivalent that does the job well, then place a lightweight
skin on the top of it.  The results aren't bad at all.

To make a rigid, lightweight, stressed-skin race car is a daunting
engineering task.  It takes budgets like F1 to make it possible.  And, if
they want to make any changes to the car, the whole thing has to go out the
window.  Considering that Schumacher makes over $80 million a year, it's
possible to understand that this probably isn't a big deal for them :-).

Take care,
Shad Laws
LN Engineering - Aircooled Precision Performance
http://www.lnengineering.com

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