- Tubing Selection for Recumbent Frames
Considerations By John Zabrieski 1995
Considerations By Mark Stonich 1995
- From: Rob Lloyd
Subject: Aluminium Tubing for a Mono-tube
I am considering building a lowracer or trike. But, I'm
still in the planning stages at this point. I'm a
mechanical engineer, I design car parts for a living,
most of it is fabrication of steel and aluminium parts.
Finally, I have a topic I can actually help out with.
For tubing stiffness equivalents, you have to consider
the section modulus.
Basically, that's just the modulus of rigidity times the
moment of inertia for the section. Aluminium has almost
exactly 1/3 the modulus rigidity of steel (~30000KPSI
steel/~10000KPSI ally), so for an equivalent tube stiffness, you would need the moment of inertia to be 3
times as high.
For a 2" OD, 0.035" wall steel tube, the moment
of inertia is about 0.101in^4. So, you need an ally tube
of about 0.303in^4. This equates to a wall thickness of
about 0.114". Of course, there is no 0.114"
wall thickness tube available from stock sources, so if
you use a 0.125" wall tube, you will get a moment of
0.325in^4. which is actually slightly stiffer (~10%) than
the steel tube.
Here's the rub: Aluminium also happens to be about 1/3
the density of steel. The cross sectional areas for the
above 2 tubes are 0.216in^2 and 0.736in^2 for steel and
ally respectively. For equal length tubes, the cross
sectional area is going to be proportional to the weight.
So, what you get by converting to ally is a frame that is
about 10% stiffer than the steel tube, but is also 13%
heavier. If you could build find 0.114" wall tube,
then the 2 designs would be almost identical. I assume
you want to build an ally frame because you want it to be
lighter. Obviously, this won't really do the job.
The way to make ally work better than steel is to use it
in a way that steel can't be used. For example, a stiffer
& lighter design, in this case, would be a 2.5"
OD, 0.020" (I=0.119in^4;A=0.155in^2) wall steel
tube. It would be both 28% lighter and 15% stiffer than
the 2" OD 0.035 wall tube.
But who the hell can weld it, right? (not to mention even
being able to find 0.02" wall tube...) So what you
do is build it out of 2.5" OD 0.065"
(I=0.369in^4;A=0.497in^2) wall ally tube, which is both
available and weldable. This would give you a tube that
was 23% stiffer and about 30% lighter than the original
2" X 0.035" wall steel tube (Although, it will
be heavier than the 2.5" OD, 0.02" wall
unobtanium steel tube in this example). If you notice,
this is exactly what bike manufacturers do. A steel frame
bike will typically be small OD, thin wall tube. Ally
frames will be large OD, thin wall tubes.
You can build a better frame in ally only because of the
manufacturing limitations of steel. If you want a better
material than either steel or ally, use titanium. It's
modulus is nearly that of steel, but it's weight is
nearly half. Of course, it's outrageously expensive to
both purchase and weld.
Thursday, 29 January 2009