[python] Re: Round and square tube & the jetrike.
- From: "25hz" <25hz@xxxxxxxxxx>
- To: <python@xxxxxxxxxxxxx>
- Date: Mon, 15 Jan 2007 21:51:20 -0500
Cool the way the math works out, but the round tube is bigger so it's no
surprise it's stiffer. How did a 35mm x 2mm OD pipe compare to the square?
I think that would be a more understandable example for homebuilders. I
tracked down my little tube stiffness calculator spreadsheet too. I'm going
to put it online but I need to make soem basic instructions for it. There's
only two dimensions you need to put in, but there's a lot of output so I can
narrow down what is important to look at and what isn't.
On the python delta note, I started some 3D drawings of it with a few of the
mods that I propose. The PC2 is still hanging on the wall in my garage as I
don't have room on the ground for it yet until the tilting tadpole is done
which will hopefully be in a couple days. Using it as a baseline for CoG,
ground clearance and seat height, I at least have some numbers for what I've
got vs trying to guess. Right now the trike has the following:
Ground clearance: 5"
Seat height: 10"
CoG height: 17"
Seat angle: 35º
Now, I'll just clamp the python front end to the frame jig, and make the
backbone meet it at what ever angle seems convenient - likely around the 65
degree range (it has rod end pivots so angle adjustment is simple). I'll
bring down the ground clearance to 3", so CoG is now at 15". I plan to run
the seat at 30º so that's another 1.5" CoG drop. Due to the frame design of
the tadpole, I can lower the seat height by another 1" by adjusting the seat
mount method, so that leaves the CoG at about 11.5". By adjusting the
swingarms up 10 degrees it gets me a little over 2" lift at the swingarm
pivot so the CoG is marginally lower than the pivot at this point and that
isn't adding suspension sag. I can minimize that by tightening the adjuster
ring on the shock though. If it all goes pear shaped on me, I can still
reset everything back to the Jetrike defaults though as there are multiple
holes in the tie rod brackets. As I get a little time to finalize the
designs in 3DS, I'll post them for critique. :) I did manage to get the
three pivot tubes made along with 6 Delrin bushings and machined some
threaded adapters to weld into the end of 1.25" x .049" box tube that will
take a 1/2" bolt. I also have some made up for 1" x .063" box so I might
try that first for the other arms of the rocker and if they fail, I just
make a new one with 1.25" arms instead.
| Just thought I should round out this discussion with some real
| engineering data. So I opened up TubeComp on my work computer and found
| two profiles that have the same cross sectional area and compared them
| for (A) canter levered and (B) load bearing deflection. Canter lever is
| attached on one side with the load on the other, load bearing, is
| attached on both ends with the weight in the middle.
|
| The profiles were 35 x 35 x 2.0mm box and 42.4 OD x 2.0 mm pipe. A bit
| thicker than what you would typically use in a frame, but both have a
| cross sectional area of 245mm squared.
|
|
| Box
| Pipe
| Yield
| 450Mpa
| 350Mpa
| Tensile
| 500Mpa
| 430Mpa
| Mx & My
| 1.25KN
| 1.03KN
| Mt (torsion)
| .945KN
| .926KN
| Deflection A
| 37mm
| 32.1mm
| Deflection B
| 2.31mm
| 2.01
|
|
| So what does this mean? The box is stronger, and can withstand about 20%
| higher loads before permanently bending. The box can also withstand the
| same torsional loads as the pipe, but this is because it has the
| elongated lattice and has the higher yield strength and ultimate tensile
| strength.
|
| However, and this is an interesting point of difference, despite being
| weaker, the pipe is stiffer, and experienced less deflection when the
| force is applied. Now the pipe here because it has a thinner wall, has
| been drawn twice and so is stronger than regular thicker walled pipes
| which have 250Mpa yield strength. From this I image that seamless tube,
| which as good or better than the box in strength for the same cross
| sectional area so would be much stiffer.
|
| Never the less, I agree entirely with 25mhz, if you are a home builder
| stick with box. If you are building a recumbent to get active and loose
| a few pounds, you have more chance of shedding weight from your body
| than from a frame by using exotic materials. Even Aluminum can be a
| trap. I notice that the Hipparion
| http://personal.eunet.fi/pp/davinci/fill.html which has a frame made of
| Aluminum actually weighs only slightly less than the steel frame I am
| currently building.
|
| -h
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