From: tdsmith@oasys.dt.navy.mil (Timothy Smith)
Newsgroups: rec.bicycles.tech
Subject: Re: Wheel Wonderings
Date: 7 Jan 93 00:49:18 GMT
Reply-To: tdsmith@oasys.dt.navy.mil (Timothy Smith)
Organization: Carderock Division, NSWC, Bethesda, MD

In rec.bicycles.tech, quixote@cix.compulink.co.uk (Aurelio Barranco) writes:
>In-Reply-To: <1460053@hplred.HPL.HP.COM> jbrandt@hplred.HPL.HP.COM (Jobst Brand
>t)
>
>> Optimal trnsion for a wheel is when the spokes are about as tight as
>> the rim can withstand because the wheel can support loads only until
>> the bottom spokes become slack.  Of course being a little low on
>
>Correct me if I am wrong but I thought that the hub at the centre of
>the wheel is suspended from the top of the rim by the spoke under
>tension. In other words the spoke under the hub (between the hub and
>the ground side of the rim) plays no part in the process. Spokes at
>the side stop the wheel from buckling.

    In short, you are wrong.

    Just sitting there on your bike the wheel is a seething mass of 
tension just ready to explode... every spoke pulls on the hub with hundreds
of pounds of tension. The hub doesn't move because the pull from all 
36 (32,28,40, take your pick) spokes EXACTLY cancels out.  Disturb that 
equilibrium and a "leftover" force appears at the hub. This leftover 
force is what holds you up.
    Jobst has done a finite element model of a bicycle wheel, and found
that when you push a bicycle wheel into the ground by the hub the "disturbance 
in the spoke tension equilibrium" which supports your weight comes from 
the bottom one or two spokes pulling less hard on the hub. A few inches 
of the bottom of the rim flex in, and the spokes in that region lose 
a few tens of pounds of tension, throwing off the equilibrium at the 
hub by the same few tens of pounds.
    The alternate model, that the force on the bottom of the wheel is
somehow transfered around the rim to the top where it increases the
tension in the top spoke(s), makes the fatal (and unnessesary) assumption
that the rim is a perfectly rigid body.  It isn't. 
    You are correct in assuming that the tension in the top spokes supports 
your weight, but incorrect in assuming that the tension has to increase.
The simpler, and correct, answer is that the tension in the bottom one 
or two spokes decreases. In short, the bottom spokes "compress."

    Please, before posting the inevitable "spokes can't support compression" 
diatribe, read the following:

    This thread comes up every winter, and ususaly flares up into a
major flame war. Jobst Brandt has done the definative study on bycycle
wheels in his book "The Bicycle Wheel."  Jobst is strongly motivated
to dispell myths about bicycling technology (as anyone who has read
this newsgroup for a while knows), and myths about bicycle wheels are
sure to provoke a response.
    The flame war seems to come from basic misunderstanding between 
the language an engineer (like Jobst or myself) uses to be technically 
clear and unambiguous, and the commonly held meanings of the word compression.
    In talking about stresses (tesnsions, compressions and shears), an
engineer will ususaly ignore the original state of an object and only 
talk about the differences.

    The inital state of stress in a bicycle spoke is already known to 
be high levels of tension. There is no need to state the obvious between 
engineers. So, one engineer would say to another that the bottom spoke 
supports the weight by compressing.

    Everyone old enough to stand up knows that spokes, like ropes, will 
collapse under compression.  If they are unaware of the (clearly and
unambiguously defined) engineering use of the word compression they
may lurch into action and post a "correction."  (And Rec.bikes.tech gets
a few dozen exchanges of mortar rounds to liven up the winter season...)

    LESS TENSION == COMPRESSION

    To Jobst, the bottom spoke on the wheel experiences compression.
He is very clear and unambiguous in his use of the word. Technicaly
speaking, he is right.

Jobst continues:

>> gradual loss of wheel alighment.  A spoke that becomes momentarily
>> loose, will allow the nipple to unscrew a little at a time.  This is
>> the cause of the classic wheel that won't stay true even after
>> several trips to the shop.  It is most often too loose.
>
>This would make sense.
>
>Aurelio

    Jobst usualy does.

Tim Smith

PS:
    Jobst may use the 'C' word again this winter...which is why I used 
it first as a form of firebreak. (light a little fire first to use up 
the fuel so the big one can't get going...)