On Tue, 28 Nov 2006 22:02:41 GMT T i m wrote :
Ok, on the geometry then (and this is probably a stupid thought) what
about a 'X' section inside a tube (I'm guessing it would still be a
function of the overall x sectional area)? This probably sounds like a
perpetual motion question ;-)

So, failing a 'yes' to the above, is there a simple formula (rule of
thumb) for how much 'stiffer' any particular steel would be for a
percentage increase in diameter please?

Say I go from 3/8 to 1/2" for example, would it be 25% or 33% or
something else stiffer?

For a solid bar, stiffness is proportional to D^4, bending strength (in
simple terms) to D^3 and weight to D^2. For a tube substitute D^x -d^x
where D is the OD and d the ID.

So if your tubes are of 1/16" material the numbers are (work in 1/16" for
ease of calculation)

3/8: Stiffness 6^4-4^4: 1040 Weight 20 Stiffness/weight 50.2
1/2: Stiffness 8^4-6^4: 2800 Weight 28 Stiffness/weight 100.0

But it's not quite as simple as just making it as big and thin as
possible as you reach a point where the tube buckles before its
theoretical strength is reached.

Note that no reliance should be placed on any calculations done at this
time of night g

--
Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk