OT tech question: pressurised chassis
Brafield
pressurised ostensibly "to detect cracks".
Somebody years back, (and maybe more than one) built bicycle frames
that were pressurised specifically to increase rigidity. Is there a
real and significant gain in stiffness via pumping a hollow component
to high pressure with, say, nitrogen? Or was the nitrogen to prevent
oxidation?
mower man
I don't know anything about pressurised bike frames, though there seem
to be several patent applications intended to aid manufacture of
composite frames.
AFAIK the 917 space frame was of aluminium, and permanently pressurised.
There was a gauge which, if the pressure dropped to zero, alerted the
driver that he had a crack in the frame. It didn't add any stiffness.
--
Chris
I am not young enough to know everything.
Oscar Wilde (1854 - 1900)
News
Pressurization does indeed add to structural rigidity, while pressure
monitoring would indicate cracking and incipient failure.
Imagine a thin-wall foil cylinder/tube capped at each end.
At ambient pressure, it would have no structural rigidity (and no
strength, except in tension) and would easily crush.
As the cylinder/tube was pressurized, it would have increasing bending
and torsional rigidity (as well as strength in tension), up to the point
that the thin wall or foil's hoop or shear strength limit was exceeded
and it ruptured.
The choice of pressurization agent could relate to oxidation. Lighter
is better. Helium beats nitrogen, oil or water.
Some chassis components were used as oil and water return lines, which
would also involve pressurization.
Doc Knutsen
"Brafield" <spra...@hotmail.com> skrev i melding
news:e2760c13-17ce-4e9c...@u16g2000pru.googlegroups.com...
> The Porsche 917's tubular chassis (was it mag or steel tubing?) was
> pressurised ostensibly "to detect cracks".
The frame was welded from aluminium tubing. It was, of course, much lighter
than a steel aquivalent, and stiffness was comparable.(see what Frank
Gardner had to say about Porsche frame stiffness :o) ) The downside was that
it was not possible, due to the size of the complete frame, to heat-treat
after welding. Thus, the zones next to the welds would tend do become
brittle, and cracks were liable to occur. For this to be detectable, Porsche
filled the chassis tubes with gas, and attached a big GPG for the
driver...the order of the day was, if the pressure should suddenly zero, the
car was to be driven gently back to the pits, and parked.
Doc
Ian Rawlings
> For this to be detectable, Porsche
> filled the chassis tubes with gas, and attached a big GPG for the
> driver...the order of the day was, if the pressure should suddenly zero, the
> car was to be driven gently back to the pits, and parked.
Whereas Lotus on the other hand used to pump fuel through theirs ;-)
--
Blast off and strike the evil Bydo empire!
http://youtube.com/tarcus69
http://www.flickr.com/photos/tarcus/sets/
~misfit~
> Brafield wrote:
>> The Porsche 917's tubular chassis (was it mag or steel tubing?) was
>> pressurised ostensibly "to detect cracks".
>> Somebody years back, (and maybe more than one) built bicycle frames
>> that were pressurised specifically to increase rigidity. Is there a
>> real and significant gain in stiffness via pumping a hollow component
>> to high pressure with, say, nitrogen? Or was the nitrogen to prevent
>> oxidation?
>>
>
>
> Pressurization does indeed add to structural rigidity, while pressure
> monitoring would indicate cracking and incipient failure.
It sure does. Anyone who's tried bending heat pipes (which are essentially
evacuated) knows how prone they are to 'kinking'. I have an engineer friend
who works with all sorts of odd projects and I've known him to pressurise
pipes when he's trying for good bends.
LOL, mind you, I've also seen him fabricate expansion chambers for two
strokes by cutting all the bits (according to his convoluted calculations),
welding them together into roughly the shape he wants (but they're dented
and tend to have 'folds' in them). He then welds an adjustable
pressure-release valve to one end and a fitting to the other which he hooks
up to a water blaster. After turning on the water blaster and adjusting the
valve he ends up with about 50% of his creations looking all neat and tidy
first time out. The other 50% usually blow and either get scrapped or need
re-welding....
--
Shaun.
"Give a man a fire and he's warm for the day. But set fire to him and he's
warm for the rest of his life." Terry Pratchet, 'Jingo'.