Re: Hey, Jobst, on p39 of The Bicycle Wheel the graph appears to show the ?impossibility of...

On Sat, 13 Sep 2008 18:05:39 -0500, Tim McNamara
<timmcn@xxxxxxxxxxxxx> wrote:

In article <4gTyk.37$MX3.133@xxxxxxxxxxxxxxxxx>,
"Philip Holman" <pholman@xxxxxxxxx> wrote:

"Ben C" <spamspam@xxxxxxxxx> wrote in message
news:slrngcnp9i.5lv.spamspam@xxxxxxxxxxxxxxxxxxxx
On 2008-09-13, Peter Cole <peter_cole@xxxxxxxxxxx> wrote:
Ben C wrote:

I assumed the FEA that generated the plot included the boundary
condition of the spoke slacking. I didn't think that the
"hypothetical compression" implied by the graph was carried into
the FEA, that would be a pretty obvious gaffe.

In that case I'm confused. The lateral force curve on the left
hand side of the diagram (not actually labelled) seems to be
basically a straight line through the -4mm position at which the
left side goes slack. Shouldn't there be a more abrupt change in
stiffness at that point?

Here's the diagram again: http://i33.tinypic.com/2ykj9rq.jpg

If you can do an FEA in your head & eyeball that graph & declare
it inaccurate, you're a better man than I, Gunga Din.

I'm not declaring it inaccurate, just hoping for some explanation.

I would have expected that once the spokes on one side went slack,
the stiffness of the whole structure would go down by about a half.
Perhaps by more, since the left side has the better bracing angle
and it's the side that's going slack.

If not, why not?

You are correct. One would see a discontinuity in the line as the
spoke(s) went slack which isn't there. Having said that, to model
such an event is more difficult and may be beyond the scope or
purpose of the model. As the lateral load increases, the left side
spoke closest to the point of load application will become slack
first followed by the spokes adjacent and so on. Modeling such a
sequence is more complicated.

And of course if the lateral displacement continues, spoke tension would
rise as the graph shows.

Instead of throwing the whole book out as flawed (which it isn't), it
probably just needs a few refinements to improve it. That's the
reason for editing and publishing a revised edition :-)

I know it's been asked previously and I must have missed the answer, but
is this graph reproduced "as is" in all the editions of the book?

Dear Tim,

The same drawing appears in all editions.

The third edition changes the vertical force scale from +400 kg to
+4,000 N and adds the explanatory text below the caption "Lateral
force and spoke tension graph."

All versions show the same minor error of an incorrectly centered rim,
which Tom Ace noted.

In the drawings with the mistake, the rim sits about 5 mm to the right
of center, which Jobst has said is due to confusing 5 and 6 speed hubs
in the drawing process.

The same centering error occurs in some other drawings, but was never
present in others.

Curiously, the centering error appears on the cover of the first
edition, but is corrected on the cover of the second edition.

The drawing error is worth noting, but it's hardly earthshaking. In
the First Folio, "Hamlet" skips from page 156 to page 257, but few
critics believe that a hundred pages are missing from the middle of
the 31-page play.

Anyone who thinks that he has a substantial disagreement with Jobst's
diagram can simply sketch what he thinks it should look like, upload
it to www.tinypic.com, and post the link.

Cheers,

Carl Fogel
.

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