[python] Re: New Jetrike Rev B Plans

  • From: "25hz" <25hz@xxxxxxxxxx>
  • To: <python@xxxxxxxxxxxxx>
  • Date: Sun, 14 Jan 2007 21:15:25 -0500

While the crystalline structure of the metals is cool to learn about, for
homebuilders it's nothing that really needs to be worried about unless the
design is so engineered to the point of maximizing weight, wall thickness,
dimensions and loadings that we need to be concerned about such things.

While DOM (drawn over mandrel) or "seamless" as it's often called up here is
stronger than welded, the cost/strength/weight ratios do not make it worth
it, IMO.  The same goes for using 4130 or any other high strength steel.
With 1020/1030/1040 welded steel square tubing that is 1.25" x .049, I can
build a trike that can support close to 400lbs, and using 1.5" for a two
wheeler supports even more.  That is more than enough for my needs and it
can also be done with only a couple pounds of weight penalty.  When a frame
can be built for $20 worth of mild steel, anything else is wasting money.

I've welded a numebr of 4130 frames and roll bars for trucks and race cars
and while, by the book, the steel should be heat treated after welding, for
all but the big commercial outfits/companies/organizations it isn't done.
The 4130 is more than strong enough and after welding for a while and with
NDT, it's not difficult to see when you get proper penetration and the weld
is strong.  Same goes for aluminum.  It's nice to have an oven big enough to
solution heat treat a welded part, but for a homebuilder, letting the frame
sit after welding for 6 weeks or more (for 6061 anyway) will age harden it
up to 60% or more anyway.  Most homebuilt AL projects end up weighing as
much or more than steel and costing 5 times as much because the properties
aren't understood and if they are, the walls have to be thicker due to the
inability to get post weld heat treatment.  If you can actually do some kind
of engineered stress analysis of the frame you want to build, you're looking
for the razor edge balance of too light/poor construction/will fail and
over-built/heavier than it needs to be/more expensive than it should have
been.

For most homebuilders, they really have no idea how strong steel actually is
and end up going for massive overbuilding, exotic alloys, AL or CF as a
result.  Unless you are over 250lbs, I'd reccommend starting with 1.5" x
.063" box for a two wheeler, and the same for a flying cross trike - you can
use 1.25" or even 1" box if you are going for some kind of space frame
trike.  Nothing fancy - just plain old mild steel works fine.  Save the rest
of your money for beer to celebrate your first ride :)

| 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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