[python] Re: New Jetrike Rev B Plans

  • From: "Carl Punton" <carlpunton@xxxxxxxxx>
  • To: python@xxxxxxxxxxxxx
  • Date: Sun, 14 Jan 2007 20:01:29 +0100

I think its probably safe to say that round section is the best compromise
of wight versus  strength ,  its pretty much standard for bicycle frames &
various other vehicles that I'm not allowed to mention !  There is a reason
for this , its not just coincidence.
The fact is , our homebuilts are easier to build with square ,  yes , round
would be lighter for equal strength ,  but much harder to align & join ,
hence the slight weight gain is usually accepted.

On 14/01/07, Henry Thomas <whpthomas@xxxxxxxxx> wrote:

25hz wrote:
> 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.
>
The other thing to consider is how pipe, tube or box section is made.
Most mild steel pipe is made out of sheet metal that is folded and
welded. Box is generally made out of pipe that is drawn through a die
and mandrel to form its shape. Some box is drawn through several dies.
With each die that the box is drawn through, the crystal lattice of the
steel is elongated, which increases its tensile strength. So box is
generally stronger than pipe, box that is twice drawn can be as much as
twice as strong (according to Australian standards at least). Some tube
is actually drawn right from the start, and drawn many times before its
final shape so it can also be very strong when compared to regular pipe,
but also more expensive. I have some software on my work computer from
Smorgan Steel (one of our local manufacturers) that has all the
engineering data, so I will compare two standard profiles of pipe and
box that have the same cross sectional area and see what the actual
difference in strength from that. It won't be authoritative for
non-Australian steel, but it will give you some idea at least.

The other thing about elongated crystal lattices in steel, is that when
you weld them, often the heat effected are is somewhat normalized. So
you have to be careful how you weld this stuff together, sometimes you
need gussets etc., otherwise you welds become the weakest link in your
frame.

-h

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