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


"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.

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 :-)

Phil H


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