Let's spoke it and see what happens

On May 19, 4:13 pm, Chalo wrote:
Ben wrote:
I present Chalo's new favorite crankset:

http://cgi.ebay.com/SKELETONIZED-Shi...nkset-w-BB-rin...

orhttp://tinyurl.com/5xztxkhttp://preview.tinyurl.com/5xztxk

Chalo replied:
Anyway, you know how I love welded steel cranks. This extra special
eBay seller has inspired me to make a set of geodesic space frame
cranks welded up from segments of spokes. Stay tuned to eBay where I
should be able to sell them for a fortune!

Chalo

Perhaps you scoff too quickly at your straw man, Chalo. Consider the
form of a crank: the arm is basically two round units (one with a
cutout to fit over the bottom bracket axle, one for the pedal axle
thread) connected by a long rectangle pinched in towards one end, also
rectangular in through section and side section. The spoke sections
would be about 6 to 7 inches long.

Let's, just conceptually, make up the spokes, and let's make the
crosssection square for reasons that will soon appear: four spokes for
the longitudinal corners connecting the two round end pieces, four
spokes running through the centre of the assembly each connecting a
diagonally opposite corner at the other round piece (perhaps tied or
soldered where they cross), two more spokes each running diagonally
through the length of each of the rectangles formed by the corner
spokes for a further eight spokes (also tied or soldered in the centre
of each rectangle where they cross). That's sixteen spokes, definitely
more spokes than hold up half your weight in a bicycle wheel, in a
much short section than in a bicycle wheel. I think we will find those
sixteen spokes more than adequate to the job.

Now let's solder up those spokes to the round end pieces (perhaps made
into Ds for convenience of jointing). But we won't do it in the normal
orientation. Take the conceptualized spoke centre section of the arm
and rotate it around its longitudinal axis 90 degrees so that, with
the pedal at the top or bottom of its stroke you'll be looking down at
a diamond shape through section rather than a rectangle through
section. Solder/weld up. Check that the bearing ends are correctly
aligned.

In use the crank's spokes will be completely in compression or tension
twice per rotation, once at top dead centre, once at bottom dead
centre. Every point of the crank is triagulated on at least three
sides; from the laws of space frames, it is strongest when pressed
from either end. At any other pedaling position, the forces will at
least theoretically be partly resolved in bending of the four straight
(or slightly angled) corner spokes, resisted by the triple
triangulation from whichever end of the straight spoke the force is
applied. I have rotated the centre section of the arm we just built
perpendicular to its normal orientation to place a larger number of
triangles and pyramids closer to the origin of the bending force (the
foot on the pedal). In this mixed mode, for every spoke subject to a
bending force there are now three spokes in compression, and by the
nature of the design, it is opposed by three other spokes in
compression -- better make the centre joints where the spokes cross
strong!

Since we'll get the usual kneejerk naysayers soiling themselves in
their hurry to prove that this idea cannot work, I suggest that
instead of wasting time arguing with chumps, we just do it. Let me
know by private e-mail (it's pretty logical -- my monicker is fiultra
and I write from yahoo which is a commercial entity) how long it will
take you to saw up a cheap square shank crank and weld or solder in
the spokes, and I'll pay for your time and the materials and postage,
and try the spoked cranks on one of my bikes. We can build keepers
down to weight or in stainless steel or whatever once we have tested
the protos and had a chance to decide whether the saved weight
overcomes the oddity factor.

Actually, even without doing any math, I have no problem guaranteeing
that an arm such as I describe will be stiff enough (from which it
very nearly axiomatically follows that the assembly will be strong
enough if you weld or solder it right); the problem with it is much
more likely to be aerodynamic because that potage of spokes will
really churn up the air. I imagine that the reason Shimano and others
with brains and self-respect haven't made "drillium" (bril!) cranks is
not because the things cannot be made lighter thereby but because they
will cause more drag than the weight saving will justify. I notice
that even the cranks which are notionally I-beams are pretty
halfhearted I-beams, with so much rounding that you can hardly see
that the engineering concept underlying the retail form is the I-beam.

Let me know if you fancy a little prototyping.

Andre Jute
http://members.lycos.co.uk/fiultra/B...20CYCLING.html

PS Hey, Fogel, make yourself useful, gofer Google. I bet there were
spoked cranks in Neanderthal times.