[python] Re: New Jetrike Rev B Plans

  • From: "25hz" <25hz@xxxxxxxxxx>
  • To: <python@xxxxxxxxxxxxx>
  • Date: Sun, 14 Jan 2007 01:28:08 -0500

When you talk about strength, it can get sort of complicated due to physics
and geometry.  I'll try to put it in laymans' terms, becuase those are what
I understand :)

- Ultimately, regardless of shape, wall thickness and diameter/external
dimensions determine the "stiffness" or what we're calling strength here.
- for a given weight of steel, say 1kg, a round tube will be stiffer than a
square tube of the same wall thickness.  Why?  Well, if you imagine a 1"
circle and a 1 square, the square has small sections of each side that stick
out past the circle and form the corners.  So, a 1" square tube has 4" of
steel going around it, and if you took that same 4" of steel and made it
into a round tube, you'd have a round tube of a little over 1 1/4" in
diameter, and remember, the bigger the diameter/external dimension, the
stiffer it is.
- a square tube is stiffer with regards to bending because it has vertical
sides to resist the forces, while round tubes have far less vertical surface
- a round tube is stiffer with regards to torsion than square.  I have seen
the effect first hand of too much torque on a square tube.  What happens is
the tube gets a twisting force, and at the point that it starts to fail, the
torque takes the corners and the "extra metal" (remember comparing a circle
and square) and starts to flatten them out, turning the quare into a circle.
Now the exact math and physics mechanism behind this, I couldn't say, but I
do know the end result.
- a round tube is also stiffer with regards to compression than square.

So, for the same weight of metal and same wall thickness, round will be
stiffer than square by about 20%.  For the same dimensional size, square
will be stiffer than round by about 40% and about 20% heavier.  What I found
though, was that by using 1 1/4" x .049" square instead of 1" x .063"
square, the bigger but thinner square tube was 60% stronger and 5% lighter.
Those strength percentages apply to both bending and torsion.

A real engineer could likely explain the modulus of elasticity for the
bending and torsion better than I, but I like square because it's easier to
jig up and join than round.

| "....I found some engineering formulae to calculate the
| bending moduli of square and round profiles....."
| Siwmae 25hz,
| The quote above from your conversation with Henry prompts a question:
| Which is stronger? Obviously for tubing of identical width and
| wall-thickness, the round will have less steel than the square, and
| therefore be lighter. But what about relative strengths?
| I too do 'seat-of-the-pants' building, designing everything in the
| drawing office of the mind's eye for a long 'brooding' period before
| ever I pick up saw or angle grinder; more than by any other method,
| for everything that I make. Time consuming, and leading to a good
| deal of trial, perceived error, and re-shaping once you do actually
| start the physical making. Often I feel embarrassingly amateur and
| ultra-slow, when I read how fast some of the professional engineers
| on the list can turn out rideable bikes, from concept to riding down
| the road. And showroom quality finish too, sometimes. But - well -
| the artist's way sketched above is just amazingly pleasing in the
| final results, both aesthetically and for engineering soundness. I do
| all the furniture that I make by this method now, and I've even built
| boats thus, up to 22 metres. Hairy at these sizes. There can be
| panics along the way: "Oh Jesus, I've blown it! There's no way out of
| this jam..." (There always is with steel, though; violent, murderous,
| filthy - but wonderful - material!) But still, I'd never revert to
| more orthodox techniques. Did you ever hear that quote from
| Michelangelo Buonarotti, that he saw himself as 'releasing' his
| sculptings from the raw block of stone, and never knew exactly what
| he'd get to in the end. The Galeria in Firenze, where they now keep
| his original 'David', also contains a number of his never-finished
| pieces, where you can actually see the rough-surfaced figures
| emerging from the blocks. At least with HPVs the amount of time,
| materials and money invested is less frightening. And you can always
| re-cannibalise.
| I'm building one of my current bike projects using a spine made of
| two round steel tubes welded in contact with stitch-welds at regular
| intervals, over and under, like a Winchester rifle. I've used this
| idea before on a much bigger scale, and it makes a beautifully
| strong, rigid beam. Just curious to know if anyone has any
| properly-competent-engineer's knowledge of the relative strengths of
| this versus rectangular tubing? All professional information humbly
| welcome.
| Cofion, Rhisiart G
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