Spoke tension under load (hanging vs standing)

[Johan Bornman]

 

Thanks - that Gordon book sounds like exactly what I'm looking for.

 

I actually have the Brandt book already (got it in a box of old bike stuff from a friend)' date=' but haven't read it yet.  I got the impression it wasn't going to explain the theory behind my question, but rather just present the results of his FEA model and note that this is consistent with the theory.

[/quote']

 

The Brandt book explains it all very nicely. Read it tonight and save us the hassle of posting the theory here. Buy yourself the Gordon book for Christmas. It is one of the most valuable books on my bookshelf.

 

I rank my books like this:

 

1) Boer War by Deneys Reitz

2) Die Groot Gedagte (minus the religious bits) by Gideon Joubert

3) Structures, by Gordon

4) Mad Magazine

5) Anything by JM Coetzee

6) The Bicycle Wheel

7) LBJs made Easier.

8) Lots of other stuff.

 

[fandacious]

7) LBJs made Easier. 

 

please elaborate...

[Christie]

..... to a finite element analysis that is using incorrect boundary condition.<?: prefix = o ns = "urn:schemas-microsoft-com:office:office" />

http://www.astounding.org.uk/ian/wheel/index.html

 

 

 

 

why do you say that?

Christie2008-12-10 10:24:57

[4ph3X]

It is true. At first glance it does seem counter-intuitive. Upon proper' date=' deeper, analysis the stress/force calculations show it to be true.
It's been discussed in the past on the Hub with much associated wailing and gnashing of teeth. [/quote']

 

Where can I find these calculations (and gain an understanding of the behavior of structures under tension)?

 

In my view, the best book for the layman is Structures (Or Why Things Dont Fall Down) by JE Gordon and published by Penguin. Exclusive Crooks has it from time to time. I think every engineer, architect, wheelbuilder or lawnmower repairman should read this book.

 

Secondly, if you want wheel specific, get a copy of The Bicycle Wheel by Jobst Brandt, published by Avocet Press. I stock copies of this for my wheelbuilding courses - yours for $25.

 

 

 

 

why should an engineer read that book? what does it "explain" ?

[Johan Bornman]

7) LBJs made Easier. 

 

please elaborate...

 

LBJ - Little Brown Job - a birdwatching term for those pesky little pippits, larks and cisticulas that are so difficult to identify. This book shows their flight patterns and talks about their behaviour in a way that Roberts or Newman's doesn't.

 

 

 

 

[Johan Bornman]

 

 

why should an engineer read that book? what does it "explain" ?

 

For a start, it explains how a wire-spoked wheel supports a load.

 

Secondly, it brings engineers in touch again with the everyday things that even they don't think about, such as tendons, recurve bows, parthian shots and of course, architecture.

 

After reading this book you'll enjoy a trip to Europe just so much more. You'll understand why buildings look like they do etc etc.

 

Many engineers are so stuck in what they do from day to day that they've lost touch with some of the beautiful physics in the world around us. Read it and see if you disagree.

[4ph3X]

[Ox_Wagon]

..... to a finite element analysis that is using incorrect boundary condition.<?: prefix = o ns = "urn:schemas-microsoft-com:office:office" />

http://www.astounding.org.uk/ian/wheel/index.html

 

 

 

 

why do you say that?

 

He simplified the problem by rigidly attaching the spoke to the rim to make his analysis easier. This makes it possible for the spoke to transfer compression force to the rim. <?: prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

In reality a spoke can only be in tension due to the way the spoke connects to the rim.  The moment the deflection is so much that the spoke looses tension the nipple will lift of the rim.

[TiBones]

I think the popcorn should stay -  thanx guys for interesting stuff. I got some lemonade to mix???

[Minion]

 

 

 

He simplified the problem by rigidly attaching the spoke to the rim to make his analysis easier. This makes it possible for the spoke to transfer compression force to the rim.

It is a valid assumption, based on the remainder of his method using the superposition principle.

 

Provided his final result shows that the spokes all retain some amount of tension, then his simplified boundary condition remains valid. If it shows some of the spokes entering compression (which it doesn't), then yes, the assumption will be invalid.

Edman2008-12-11 00:20:07

[Johan Bornman]

..... to a finite element analysis that is using incorrect boundary condition.<?: prefix = o ns = "urn:schemas-microsoft-com:office:office" />

http://www.astounding.org.uk/ian/wheel/index.html

 

 

 

 

why do you say that?

 

He simplified the problem by rigidly attaching the spoke to the rim to make his analysis easier. This makes it possible for the spoke to transfer compression force to the rim. <?: prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

In reality a spoke can only be in tension due to the way the spoke connects to the rim.  The moment the deflection is so much that the spoke looses tension the nipple will lift of the rim.

That's not a mistake though. He knows that the moment the spokes have zero tension, the wheel is no longer a wheel. It is a pre-stressed structure that acts in a certain way up to the point where the spokes have zero tension.

 

The same goes for analysis with any other pre-stressed structure. Clearly the values don't hold true for conditions where the structure is compromised.

 

 

[Johan Bornman]

Sorry, I responded directly from my e-mail, didn't see that Edman already explained the assumptions. Didn't mean to piss over your markings Ed.

 

 

[TNT1]

 

I think the popcorn should stay -  thanx guys for interesting stuff. I got some lemonade to mix???

 

Dammit! I already threw all my tins of pineapple at the oom...

 

[Ox_Wagon]

True, unfortunately he does show compression force therefore using the model outside it?s range of applicability.  He then builds the standing spoke argument on these compression forces. <?: prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

And if we want to do some nit picking we can further point out the he did not take buckling into account.

 

The main point remains, for which only common sense is required, that you can not transfer a compression force (push) from a spoke to a rim OR from a hub to a spoke if you ride a fancy strait PULL spoked wheelset.

[Johan Bornman]

True' date=' unfortunately he does show compression force therefore using the model outside it?s range of applicability.  He then builds the standing spoke argument on these compression forces. <?: prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

And if we want to do some nit picking we can further point out the he did not take buckling into account.

 

The main point remains, for which only common sense is required, that you can not transfer a compression force (push) from a spoke to a rim OR from a hub to a spoke if you ride a fancy strait PULL spoked wheelset.

[/quote']

 

Ox, I erroneously assumed you now understand prestressed structures and in this very same thread congratulated you on joining the club. Clearly that was premature.

 

Your understanding is wrong and all I can do to try and convince you is to re-hash stuff we've discussed here ad infinitum.

 

I suggest you first figure out how a simple concrete lintel above a door frame can take tension and not break. Once you are there, we'll take the next step.

 

 

[Ox_Wagon]

 And I was under the impression we discussing wheels not composite concrete structures.  And yes I know how a how a lintal works but a spoke is not a composite structure. <?: prefix = o ns = "urn:schemas-microsoft-com:office:office" />

You need to answer the following for your self:

Can a spoke transfer a compression force to a rim? Yes/No

Are all the spokes always (<?: prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />Normal operating conditions) in tension? Yes/No 

Ox_Wagon2008-12-11 02:10:32