NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/rec/bicycles/tech/3292

Internet Message Format | 1993-01-22 | 1.8 KB

From: [e-mail] (Jobst Brandt)
Date: Thu, 21 Jan 1993 19:43:59 GMT
Subject: Re: Wheel Wonderings
Message-ID: <[e-mail]>
Organization: Hewlett-Packard Laboratories, Palo Alto, CA
Path: sparky!uunet!usc!sdd.hp.com!hpscit.sc.hp.com!hplextra!hplred!jbrandt
Newsgroups: rec.bicycles.tech
References: <[e-mail]>
Lines: 29

Aurelio Barranco writes:

> I'm not seriously considering building such a wheel, I just want
> to get my brain around the way you describe how a wheel works.
> Why is the limit set by the original spoke tension when the spoke
> itself is capable of withstanding greater loads.  Forget side to
> side forces, I'm interested in a stationary wheel or one rolling
> in a straight line. This is all theoretical of course :)

I think you are still holding on to the concept that the hub is
hanging on the top spokes.  I draw this conclusion from your comment
that "the spoke itself is capable of withstanding greater loads".
Yes, in tension, but the spokes are not being used as tension
elements.  They are prestressed and are being used as compression
elements so their carrying limit is when they go slack and can no
longer perform any structural function.  WHen the bottom spokes
become slack, the rim has no contact with the hub except by the
relatively small lateral bending stiffness.  If you don't believe
that this strength is small you need only to take a loosely spoked
wheel , hold it like a car steering wheel while pressing its axle
into the floor.

Buckling is a common phenomenon in long slender beams for which
there are equations.  The equations are largely governed by the
unbraced length of the column under compression.  The rim has no
bracing when several spokes become slack so that without even a side
load, the rim tends to bow to the side.

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