Need some help on spokes

[Johan Bornman]

So at least you agree that the above mentioned finite element analysis is flawed.  But if you agree that the all the spokes are ALWAYS in tension and no compression force can be transferred how does the hub press on the road with out effecting the tension of the top spokes? <?:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

On the "twang"' date=' I haven?t heard of the unit before nor have I seen any standard (SABS, DIN , ASME etc) on it nor instruments that measure it.  What is the conversion factor between "twang" and Force? As such I have difficulty excepting any ?experiment? where the "twang" is used as the only measurable to prove a theory that is not supported by engineering theory.

Lastly why on earth would a wheel collapse if it loses tension in ?the? bottom spoke or even the bottom 3 or 4 spokes? The rim may be damages but it would not collapse.

[/quote']

Please don't put words in my mouth. I don't agree. You still haven't done the experiment, have you? Why not? Are you scared of the outcome? As for the "twang" comments, I'll consider them argumentative and as something you should take up with the OP, if you don't like that term.

 

As for the wheel that collapses: If I said bottom spoke, I should have said "load affected zone" instead, to be precise.  This would constitute a few spokes, depending on the strength of the rim.

 

If this were to be the case, the slightest lateral force will collapse the wheel since it has no bracing at that moment.

 

Lets try a final, imaginary experiment. Make a sketch as I go along, it'll help you visualise what I'm doing.

 

Cut yourself two (imaginary) rectangles of steel plate, say 3mm thick. Their dimensions will be 300mm x 150mm or slightly bigger than your foot, if you have big feet.

 

Now drill a 10mm hole in each corner of each plate.

 

Now find yourself four sturdy pieces of rope, say 9mm thick and 200mm long. Put a big fat know in the one side of each of your four ropelets.

 

Also find yourself four car valve springs of approximately 80mm in length.

 

Fit the four rokpes through the 4 holes in one plate, place the valve springs over the four ropes and the other plate on top of that with the ropes now coming right through from plate to spring to plate.

 

Full the ropes very tight by compressing the springs a little and put a knot in the top of each unknotted end so that the sandwich becomes a sturdy block with no play between the springs and plate and you have a nice sturdy sandwich of plate with four springs inbetween.

 

Now place the box onto a piece of 50mm thick wood cut so that it is just smaller than the area between the knots. In other words, the four bottom knots are not touching the floor but the bottom plate is fimly supported nevertheless.

 

Stand on the box with one leg and rest all your weight on the box. Notice that the springs are still keeping the top and bottom plate firmly pressed against the insides of the knots.  You'll notice that the springs are still keeping the plate firmly against its restraints in spite of the fact that you're standing on the box.

 

Convince yourself that the bottom plate is transferring all your weight to the piece of wood. If you can't see that, get someone else to stand on the box but not before you've first put your finger or something that rhymes with block between the bottom plate and wooden block. See? Weight is transferred or if you like, compression exerted. Also notice that the box is rigid. If you wiggle, it feels just like you're standing on a brick. Nothing moves.

 

Now get Fat H to swap places with you and let him stand on the box. You'll notice that the springs have now sagged considerably more and the plates are no longer pressed tight against the inside of the four restraining knots.

 

Get Fat H to wiggle his arse and you'll see that the box is now unstable. The bottom plate remains firm on the wooden block but the top plate starts to sway.

 

Get it?

 

Now, plug this analogy into a wheel. The plates are the hub and rim respectively. The spokes are the springs and ropes.

 

You can compress up to a point before the nipples lift and the wheel becomes unstable.

 

How much load can it take? 4 x the load you trapped behind each knot before you loaded the experimental block.

 

 

[Ox_Wagon]

It is a bit unclear with out a pic but sounds promising.  Again, not to put words in your mouth, is ?sound if? you agree with me that BigH weight is SUSPENDED from the top in the same way that the hub is suspended from the top spokes.  

 

I agree with you about loss of lateral stiffness if the tension reduces too much in the bottom spoke.  This is precisely the reason that Mavic develop the new R-sys wheelset (http://www.mavic.com/r-sys/).   <?:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

My turn with a few pics

 

Let assume Shimano/Campy/Alex/Fucrum/etc brings out a new super wheelset in response to Mavic?s new R-sys uses unobtainium to make a super stiff rim that only needs 4 spokes with very little rim deflection.  Unobtainium is also used for the spokes which means there is very little elogation of the spoke under tension.  Which means one can reduce the problem to a very simplified two spoke system shown in the figure A.  Figure B shows the effect of the pre-tension on the Hub (Node) with no weight on the hub.  Figure C shows the effect of a portion of the rider weight plus the bottom spoke pre-tension on the tension on the top spoke.

 

 

 

Lastly let?s look at the collapsing wheel scenario.  Lets again look at the simplified two spoke wheel.  Firstly let?s cut the bottom spoke. The wheel(hub) is unride able but it does not collapse.  Now lets look what happens when we cut the top spoke.  The wheel(hub) collapses.

 

Ox_Wagon2007-12-19 08:35:25

[Johan Bornman]

Sorry to say Ox, but you have made no progress. You are just regurgitating the same old prejudice over and over, no matter how pretty your pictures. Please do the experiment.

[Guest Big H]

It is like cooking fondant....... you can use the droplet in the water technique and you might get proper fondant' date=' but if you use a proper thermometer you WILL get proper results, even the most inexperienced of cooks.

 

[/quote']

Now that would be a bit like using a HR monitor to measure cycling intensity, when you could instead use a device, not unlike a thermometer, that measures output directly wouldn't it ?

Big H - you make me laugh sometimes

 

Teee heeee!!!!!! yup I agree but it made a few people sit up and listen..... lively debate it was, not the senseless and boring mumbo jumbo going on around us, and who might the Fat H be the mr Bornman, the self taught wheel expert, that ignores other experts in the field be talking about, pray may I ask???????

[Ox_Wagon]

Not prejudice, I am just regurgitating engineering principals.  <?:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

I recommend any first year engineering textbook that on mechanical of materials.  The basic static force systems are usually discussed in the first few pages.  You forced me to go dig op my old textbook:

Mechanics of Engineering Materials

P. P. Benham & R.j. Crawford

ISDN : 0582286409

I hope this info may be of help.

 

[Guest Big H]

Calling Johan Bornman ...... calling JB....... the resident spoke "expert"............... He will most likely tell you' date=' you are stupid and find something wrong with your chain in the end!!!!!! Teee heeeee!!!!![/quote']

 

Was my prophetic word not true.......... long and boooooring with dear Johan proving evveriebody wrong exept himself!!!!!!! teeee heeeeee seker maar swaar om so te wees!!!!!!!

[Johan Bornman]

Not prejudice' date=' I am just regurgitating engineering principals.  <?:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

I recommend any first year engineering textbook that on mechanical of materials.  The basic static force systems are usually discussed in the first few pages.  You forced me to go dig op my old textbook:

Mechanics of Engineering Materials

P. P. Benham & R.j. Crawford

ISDN : 0582286409

I hope this info may be of help.

 [/quote']

And who may these principals of engineering be?

 

Not much help, throwing an entire book at me. Why not refer me to the page were it postulates another method of supporting a load?

 

How about me throwing a specific page of a very easily obtainable book at you. P316 of Structures or Why Things Dont Fall down by J.E. Gordon.

 

You may also want to look up the history of Sir George Cowley (1773-1857) who in the year 1808 discovered that a great deal of weight could be saved on wheels by creating spokes that take the load in tension, rather than in compression. He created the first wire spoked wheels.

 

But, let me guess, you still have not done the experiment.

[Minion]

Ox, your freebody

diagram is wrong. In the problem, as constrained there, you will get both an

increase in TS tension and a reduction in BS tension. Since the geometries of

both spokes are the same, the magnitude of these changes should both be equal to

half of the weight.

The relative values change, and the problem becomes much

more complex when you allow the supports to move, as you then also have to take into account the elasticity of the rim.

 

You should also

thoroughly read the whole FE analysis article linked previously: FEA without an

understanding of the assumptions and physics behind the problem is just an

expensive and time-consuming way of producing pretty

pictures.

Reading

through the preamble to the linked analysis, under the Linear Superposition

heading, you find this paragraph

Therefore, when the analysis shows a force in a spoke, the

real force in the spoke

is whatever the preload (the initial tension) was, plus the force calculated. If

the force was tension, we end up with a more highly stressed spoke. If the

calculated load was compression we end up with a less tensile spoke. That is, a

reference to a 'compressive' spoke could be read as a 'less tensile' spoke. To

get the true state in the wheel you need to superimpose (ie, add) the results of

this analysis on the initial state.

So the presence of a compressive load on a spoke does not mean

that there is compression in the spoke.

A graph to illustrate this:

 

And an expensive pretty picture just for the sake of

it:

 

[Christie]

I have enjoyed some of the discussion so far. The last few rounds of going around in circles got a bit much.

Edman, I see the pic is not linked to the web, but an upload. Did you make the pic yourself? Agee totally about te expense of it

One of my fav quotes on fea is by J Box (dont kow anything else about him): "All models are wrong. Some are usefull"

I light of the quote, I agree with you, although the model in question does not include pre-load, it is still a usefull model.

All the smugness and throwing around of engineering text book names is not cool, because stuff written in books can be misinterpreted, is not always correct, means nothing if used wrongly.

[Minion]

 

Edman' date=' I see the pic is not linked to the web, but an upload. Did you make the pic yourself?[/Quote']

What can I say? Last week before Christmas and I'm starting to get a bit bored at work .

It sometimes scares me that the software I use on a day-to-day basis costs more than most of my directors' cars (and the annual maintenance costs more than mine )

 

 

One of my fav quotes on fea is by J Box (dont kow anything else about him): "All models are wrong. Some are usefull"[/Quote]

A colleague describes FEA as a "dangerous tool' date=' often wielded by fools".

 

because stuff written in books can be misinterpreted, is not always correct, means nothing if used wrongly.

Interesting fact:

If you read the Wikipedia article on bicycle wheels, in the section on reaction to load it states that all of the spokes increase in tension in reaction to load. It gives as a reference the 2nd edition of a book published by MIT in 1982.

In the next paragraph, it notes that some sources say that only a few spokes in the load-affected-zone take the load. The reference for this is the 3rd edition of the same book, published in 2004.

 

[Christie]

A colleague describes FEA as a "dangerous tool' date=' often wielded by fools".

 

[/quote']

 

Hehe, a less flattering version I heard was "as dangrerous as a baboon with a shotgun" 

[porky]

OH BOY I battled through those 8 pages, and sad to say I got lost.!

 

The reason I read through it all was because I had a sort of similar problem and I was trying to understand what was the cause.

 

My problem was a sort of "pinging" from the rear wheel when sitting on the saddle (I guess loading the wheel?) as soon as you stood up the pinging went away.

 

The bike went to three shops to sort it out, one told me the free wheel hub was faulty (replacing it made no differance) the other told me it was my bottom bracket (it wasnt) the third shop said it was caused by the spokes not been tensioned correctly (they gave me some kind of value of  the spoke tension needing to be "30" but was only "5") and the pinging was the spoke "releasing the tension" at the top of the wheel rotation.

 

Their "words not mine", before everyone jumps on me for using wrong terminology.

To be honest, I didnt understand a word of it.

 

Anyway, at a cost of R900.00 they rebuilt the wheel using some other type of spoke and nipple, (although they look the same) re-tensioned ?? the spokes etc, and to be honest, the noise (ping) is gone, and the bike seems stiffer somehow.

 

I had noticed before that cornering was becoming a bit of a hit and miss affair  and the bike sort of "wondered around" but now its rock steady.  

 

Now I am not at all technical, but I read everything here about tension etc and at the risk of sounding dumb  I STILL dont understand how the spoke "pinged" ?

 

I wondered if Johan has seen or heard of this happening before (or could say if its even possible).

 

Was I duped Johan.?

  

[straatvark]

Was my prophetic word not true.......... long and boooooring with dear Johan proving evveriebody wrong exept himself!!!!!!! teeee heeeeee seker maar swaar om so te wees!!!!!!!

 

And all of a sudden I realize that the ?kef? ?kef? ?kef? that I hear is from my PC?<?:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

Actually, I experience it the exact opposite, very informative thread, thanks to those that are willing/able to give input.

 

If I compare the wheel ?user? references (that JB and other get) with the number of issues I had in last year with my ?high tech? LBS wheel building skills ? I know how (tools, science & ?art?) my next wheels will be build.

[greatwhite]

because stuff written in books can be misinterpreted' date=' is not always correct, means nothing if used wrongly.[/quote']
Interesting fact:
If you read the Wikipedia article on bicycle wheels, in the section on reaction to load it states that all of the spokes increase in tension in reaction to load. It gives as a reference the 2nd edition of a book published by MIT in 1982.
In the next paragraph, it notes that some sources say that only a few spokes in the load-affected-zone take the load. The reference for this is the 3rd edition of the same book, published in 2004.

 

Thats not quite what it said.

[Johan Bornman]

because stuff written in books can be misinterpreted' date=' is not always correct, means nothing if used wrongly.[/quote']
Interesting fact:
If you read the Wikipedia article on bicycle wheels, in the section on reaction to load it states that all of the spokes increase in tension in reaction to load. It gives as a reference the 2nd edition of a book published by MIT in 1982.
In the next paragraph, it notes that some sources say that only a few spokes in the load-affected-zone take the load. The reference for this is the 3rd edition of the same book, published in 2004.

 

Thats not quite what it said.

I can't quite figure out who quoted what, so I'll respond to whom ever the shoe fits. I'm referring to the Wikipedia citing.

 

You carefully omitted the world "slightly" before "increase in tension" and there's the key. I earlier on urged some of you to do the experiment and then report back to us. Apparently finding a bicycle, a buddy and a table is not possible.

 

If you perform the experiment I mentioned, you'll notice that the lower spokes resonate at a signficicant lower tone than the upper spokes. If you do a comparative pluck before and after the bike is loaded, you'll notice that the top spokes resonate slightly higher after load than before. And this is the "aha" moment I hinted at.

 

The reason for this apparent anomoly is because the rim is no longer the same shape. Because it is squashed in (as per the picture by Edman), the rest of the circumference increases in size, pulling the spokes a little tighter. The key to distinguishing between what Ox wants to see in this experiment (namely that hubs hang from the top spokes) and what really happens, is to pluck at the 9 and 3 O'clock positions as well and you'll see that they all go slightly up in frequency.

 

The FEA reveals this nicely.

 

The experiment shows this clearly. I can also demonstrate it in a wheelstand by simulating a load. I can increase the tension in three adjacent spokes and the wheel will develop a dent only there, not at the opposite side, which remains the same.

 

In fact, wheelbuilders unwittingly demonstrate this to themselves when they remove hops and dents from wheels. They know they only have to work at the hop zone and not on the opposite side of the wheel.

 

 

 

Johan Bornman2007-12-20 03:30:00

[Ox_Wagon]

Ok more specific: Page 1: Verbatim quoted:<?:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

?A particle is in a state of equilibrium if the resultant forces and moment acting on it are zero, and hence according to Newton?s law of motion it will not have no acceleration and will be at rest ?

 

Page 1, Formula 1.1

Σ Fx = 0

Σ Fy = 0

Σ Fz= 0

 

So the sum of the forces equals zero if the object(hub) is stationary. Which means that the forces experience in the top spoke is opposite and equal to the force in the bottom spoke plus the weight of the rider.  In short the weight is supported by the top spoke as show in my previous pics

 

Edman, I been trawling the internet to try to understand the problem better and it seams everybody is revering to the same FEM model.  As previously stated the model is flawed in that it assumes that the bottom spoke can transfer a compression force.  In the expensive and pretty picture you can see the bottom spoke buckling, this is only possible if they are in compression. They should have absolutely no load on them exept if they are pressing against the rim liner or inner tube. The only spokes wheels that can handle compression is Mavic?s new R-sys wheels http://www.mavic.com/r-sys/  If you don?t want to believe me go check out their website. Please note the design changes that they had to make to get a spoke(slender beam) to take an compression force.

 

 I agree with you that the elongation of the spokes and the deformation of the wheel make it more complex. I think that is precisely what causing the confusion. 

 

I would like to know why you say I am constraining the bottom spoke wrongly. Here is a pic I posted in a previous thread that shows the internals of a campy aero rim.  There is no way that you can put a compression force on it.

 

 

I would be interested to see how the FEM looks like if it is properly constrained. If it is your model would you be so kind and humor me and run it with the spokes constained that they can only take a tension force. Pleaseeeeee

Ox_Wagon2007-12-20 03:52:34