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Re: [Fot] Fw: Stainless Steel Roll Bars

To: "Robert M. Lang" <lang@isis.mit.edu>
Subject: Re: [Fot] Fw: Stainless Steel Roll Bars
From: Michael Porter <mdporter@dfn.com>
Date: Thu, 26 Aug 2010 13:17:49 -0600
  On 8/26/2010 10:05 AM, Robert M. Lang wrote:
> Not trying to be funny here, but SS roll bars seem like a bad idea. 
> Plus I'm relatively certain that they are not allowed by some 
> sanctioning bodies (SCCA comes to mind).
>


Likely true about not complying with the rules, for much the same reason 
as high-tensile steels are not recommended or sometimes not allowed.  
Most commonly available grades of stainless (with the possible exception 
of 409, which has just enough chromium to qualify as a stainless, less 
than 5%) have enough chromium to make them more brittle than malleable 
steels.    In similar thicknesses, they don't form as well, either.  
More ductile steels stretch quite a bit up to the yield point and then 
go on stretching until failure, absorbing energy as they do so (pretty 
much the same principle of energy-absorbing deformation in unit bodies 
today).


More brittle materials stretch less under load, and have a smaller range 
of deformation under load between the yield point and failure.  
Therefore, a cage made of 1020 steel bends, while a cage made with a 
high-chrome stainless or, say, 4130, will resist deformation better, but 
once loaded much past the yield point will break.  Once it breaks, it is 
no longer absorbing energy, and becomes a potential threat to safety 
(who wants a loose spear in the cockpit as the car is rolling over?).


Nothing's really gained by stainless, anyway, except for some additional 
corrosion protection.  Stiffness is determined by section modulus and 
Young's modulus.    Bars made with, say, 1020 and 304 with the exact 
same tube cross-section will be equally stiff, because the section 
modulus is identical, and the difference in Young's modulus between mild 
steel and stainless is negligible.


Cheers.

-- 


Michael Porter
Roswell, NM


Never let anyone drive you crazy when you know it's within walking distance....
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