|   Question RE Jobst Brandt | Matt
Jun 14, 2002 8:21 AM | | I know Brandt's book is the definitive text on wheel building, and I've been reading it, but there is something in there that I just don't get.
There's a heading in one of the early chapters that says "A Wheel Stands on its Spokes". It talks about the wheel being supported by its bottom spokes, and suggests checking the tension in the bottom spokes (by plucking) with the wheel unloaded and then loaded. It seems perfecly logical that the bottom spokes would have lower tension when the wheel is loaded. The part I don't get is the conclusion that this constitutes "standing." In my mind, a reduction in tension does not equal compression. He also says that the concept of the hub hanging from the upper part of the rim is incorrect. It seems to me that "hanging" from the upper part of the rim is the only logical way of thinking about it.
Can anyone shed any light on this? I'm neither wheel expert nor physicist, but I am curious about this question.
Thanks.
Matt |
| I agree with you | shiggy
Jun 14, 2002 8:55 AM | | As the bottom spokes lose tension, the load is transfered to the rest of the spokes. The increase in tension most likely can not be heard when plucked because of the number of spokes taking the load makes the tension increase fairly small for each spoke. |
| Consider this. . . | czardonic
Jun 14, 2002 10:07 AM | | If spoke is tensioned by stretching it, a spoke that can be demonstrated to have lost tension must be under some kind of compression. If, by way of testing the tension of the spoke you can find those that have the greatest difference in tension under load, then it seems logical that those spokes are carrying the most weight. In order for Jobt's argument to stand, he would have to prove that the loss in tension in the bottom spoke was not eclipsed by an inrease in tension in the opposing top spokes. (I would assume that he has sprung for a tensionometer.)
I admit,though, that concept is very counter-intuitive. I think that this is becuse of the odd engineering that make a wheel work in the first place. The idea that stretching something makes it more resistant to being compressed wasn't the most obvious to me when I first heard it. |
| I also content that... | shiggy
Jun 14, 2002 3:08 PM | | ...no one really knows why and how a tensioned wire wheel works. It is a dynamic structure in which the forces are constantly changing. Until someone is able to measure the forces at every part of a wheel in actual use we may never know the truth. |
| I didn't read that in the book... | Metroid
Jun 14, 2002 12:12 PM | | and it's been a while but I don't get it either. A bottom spoke has nothing to stand on. The nipple isn't supporting the spoke in the rim from the bottom, only at the hub. I'm sure Jobst knows what he is talking about here and we are just missing something in the explaination. He pops in the message boards every now and then. Maybe he'll chime in. |
| re: Question RE Jobst Brandt | CSB
Jun 14, 2002 3:16 PM | | Believe me, you are not the only one who finds the concept difficult to visualize the first time they hear it. There have been many arguments in rec.bicycles.tech (where Jobst Brandt frequently participates) where even engineers argue that Jobst's model is wrong, including:
http://groups.google.com/groups?q=g:thl1692055102d&dq=&hl=en&lr=&ie=UTF8&oe=UTF8&selm=84F23C04620FC332.C9AEB1619E8549DF.206A4D9FD796E6D3%40library-proxy.airnews.net&rnum=28
(You will have to cut and paste the link)
The point is that the spoked wheel behaves the same way as a 'wagon wheel' with rigid spokes which support compression. Because slack spokes cannot support a compressive load, they must be prestressed so that they never go slack when 'compressed' (tension is reduced). The link I posted has some very good examples of how a tension member can support a compressive load. You should also consult the illustration of how the rim deflects under a radial load so you can visualize the whole effect properly. |
| I don't like it either. | Dougal
Jun 15, 2002 8:49 PM | | There's the wheel spoke story and the prestressed concrete beam analogy he uses to support it.
His concrete beam theory is wrong, the steel in a prestressed beam is under lots of tension, when the beam is cast the ends of the steel rods are released, this creates compressive stress in the beam, but overall, when you sum the steel and the concrete stresses the underside of the beam is still under lots of tension when in use.
His wheel method I don't like. Lets have a wheel with 4 spokes, (12,3,6 & 9 o'clock).
Load the wheel and the top spoke increases in tension, the lower one decreases. The load is the sum of the change in tension of each spoke. The sideways spokes don't contribute at all.
Increase the spoke number and the behaviour doesn't change, it just gets spread over more spokes.
When the load on the hub gets high enough the bottom spokes will go slack. This makes the wheel squirrely (while the impact or load remains) to ride and lets the nipples on the bottom spokes unwind a little. The vicious cycle starts until the wheel loosens itself it can't be ridden.
Dougal.8m.com |
| What's to like/dislike? | CSB
Jun 15, 2002 10:49 PM | | Where does he mention concrete? Have you even read his book? If you don't believe that a tension member can support a compressive load by a reduction in tension then why do you say:
"Load the wheel and the top spoke increases in tension, the lower one decreases. The load is the sum of the change in tension of each spoke. The sideways spokes don't contribute at all."
Also, it sounds as if the rim is infinitely rigid in your '4-spoke' (?) model. Are you sure that this is the case?
I've noticed that nearly everyone is absolutely positive that he is wrong (and they would sooner die rather than change their view) but everyone also seems to have their own theory of how the wheel actually works. Even supposed engineers casually dismiss all the data (including the FEA) he provides in the book but they offer none of their own to refute it! He even suggests the simple experiment of plucking the spokes so that the relative pitches can be compared to indicate a change in tension. I just did the experiment myself and sure enough, I hear a clear difference with the spokes loaded versus unloaded near the bottom of the wheel but not around the top or sides. That sure convinces the hell out of me! How can he possibly be bullshitting? Do people deliberately avoid doing this simple experiment so that their fantasy world does not come crashing down around them? Why is this more hotly debated than abortion, politics and religion combined? |
| What's to like/dislike? | Dougal
Jun 16, 2002 1:03 AM | | I agree that it's always going to be a debate, there is evidence on both sides but that's always the case.
The analogy used by Jobst doesn't ring true which provides a bit more evidence for the other side.
The normal loading for a wheel is pretty minimal, I believe any flex in the rim is negligable during normal (road) use, when approaching the ultimate strength of the wheel it sure isn't. But at that stage I think even Jobst will agree that the top spokes have increased in tension.
FEA is highly subjective, it's not the black and white tool which the general public imagine it to be.
The boundaries of the item under scrutiny must be defined by the user, how these are defined changes the answer directly. My experience is only with basic temperature modelling, but the basic function is the same. Propagate the boundary conditions through the material and display it visually.
More hotly contested than abortion, religion and politics?
I think you need to get out of these forums and away from your computer bit more often.
Dougal.8m.com |
| Again, have you read the book or not? | CSB
Jun 17, 2002 1:18 PM | | It seems rather strange to attempt to discredit something you don't even understand.
"The normal loading for a wheel is pretty minimal, I believe any flex in the rim is negligable during normal (road) use, when approaching the ultimate strength of the wheel it sure isn't."
Are you saying that it is different proportionally? At what point does the stiffness change? Or are you kidding?
A better analogy would be a 'debate' between Creationists and Evolutionists. A creationist is not going to be swayed by any presentation by a scientists as they do not look to the scientific method for the answer, but rather to their own faith and beliefs. |
| Bout six months ago | Dougal
Jun 17, 2002 4:10 PM | | It was about six months ago, parts are getting hazy now.
The stiffness changes when the forces on the rim get higher than the spoke preload in the rim.
This is when the ultimate strength is reached.
You do understand what we're talking about?
Dougal.8m.com |
| See the first relay about the "spoke plucking" test | shiggy©®
Jun 16, 2002 1:35 AM | | I just do not think the model he uses is valid. |
| Should be "...my first <i>reply</i>..." (nm) | shiggy©®
Jun 16, 2002 2:06 AM | | |
| I agree with Shiggy | Matt
Jun 17, 2002 6:17 AM | | I don't doubt the results of the spoke plucking test, but I do doubt the conclusion Brandt draws from it. Like I said, I'm no expert, but I do find this debate interesting and worth discussing.
It could be semantics, or I could be missing something, but the fact that the spokes are stretched doesn't mean they can support compression. The fact that they lose some tension doesn't mean they are being compressed.
Another thing that bothers me is that Bicycling Magazine parroted Brandt's explanation without giving it much thought (I guess that should be no surprise, I cancelled my subscription). |
| Semantics, deep rooted beliefs, etc. | CSB
Jun 17, 2002 1:41 PM | | It could be semantics, or I could be missing something, but the fact that the spokes are stretched doesn't mean they can support compression. The fact that they lose some tension doesn't mean they are being compressed.
First off, in the book, Jobst deliberatly avoids suggesting that the spokes are compressed.
The semantic argument is because tension and compression are being used interchangably as relative and absolute terms to cloud the issue. If you were visibly reducing the tension in a tension member then you would probably state that you are compressing it even though it is always in a state of net tension.
I don't read Buycycling (when was this anyway?) but perhaps they latched onto Jobst to gain credibility in the technical area, where they have a reputation for making frequent blunders.
The funny thing is that everybody who doesn't buy Jobst's explanation has their own slightly different theory of how the wheel actually works! The only thing people seem to share is a lack of understanding of Jobst's explanation. Any scientific debate quickly turns into a battle of personalities and inner beliefs. You may as well ask if you should grease bottom bracket tapers. |
| So have you read the book or not? | CSB
Jun 17, 2002 1:26 PM | | It doesn't sound to me like you even understand his 'model'.
The rim deflection inside the 'load affected zone' is of a magnitude of about 25 times greater than outside of it. There are figures in the book which were derived from the FEA which illustrate this clearly.
Rims are not perfectly rigid as some people like to imagine. |
| Yes, I have read the book | shiggy©®
Jun 17, 2002 10:17 PM | | And I just looked at it again tonight.
I still say his model is flawed. Claiming that a compression spoke wheel (wagon wheel) and a tension spoke wheel (bicycle wheel) support loads the same way is just wrong.
Also I took the "pluck test" "proof" one step further.
Using a wheelsmith tensiometer I measured the spokes with and without my wife sitting on the bike.
Yes, the tension on the bottom spokes does drop. And yes, the tension to the top spokes does increase - very slightly but it is higher.
Jobst himself admits that he was not able to get reproducable results in his testing so he came up with a mathematical model to "test" the forces. If the assumptions used for the model are wrong, so are the results.
The most likely theory is that in a tension spoke wheel the hub neither stands nor hangs on the spokes but is suspended within the spokes. The hub is always supported by all of the spokes.
The best example I have of this is having 6 spokes in a row ripped out of a wheel during a ride. I was able to ride the bike 3 miles back to the car. When the spokeless area was at the bottom the wheel still supported my weight, as it did through the whole rotation. It even stayed true. The only problem was it was out -of-round and had lost some lateral stability. |
| your model and Brandts are pretty much the same | Seb
Jun 16, 2002 6:25 AM | | I don't see how your "4 spoke model" really deviates much from anything Brandt says. You did ignore the side spokes, which is a mistake- they WOULD expereince increased tension. If the rim is not flexing, then theier angle of incidence would change, which would stretch them / place them in tension. That would mean the decrease in tnesion in the bottom spoke is blanced by an increase in tension for ALL the other 3 spokes. Whats more, the rim actualy does deform slightly, becoming a little oval, so the side spokes actually share a good bit of the tension.
In any case, as you noted the practical limit on the wheels strength is reached when the bottom spoke goes slack. Saying the bottom spoke is supporting the wheel by compresion is an equaivalent (but confusing) statment to saying it supports the wheel by its decrease in tension.
Maybe I am niave enough not to have preconceptions, but the FEA's in Brandt's book show pretty much what I would expect to happen. I don't care what terms he uses to describe them, because the equations used to generate them are the basic physics or elasticity and flex, the same regardless of what words are used. |
| your model and Brandts are pretty much the same | Dougal
Jun 16, 2002 3:13 PM | | Exactly how much deflection are you expecting in the model?
It'd have to be more than a centimetre at the hub for the side spokes to give any meaningful vertical force to the hub. You aren't going to get that much deflection without wrecking the wheel.
I don't believe that wheel flex is appreciable, you state it goes oval. To go oval the side spokes would have to stretch a lot. In reality the rim is highly preloaded inwards. To deflect it out of round noticably you have to use a force higher than the preload (once again when the bottom spokes are slack).
Dougal.8m.com |
| tension vs. vertical force | Seb
Jun 19, 2002 1:22 PM | | I never said the side spokes take any verticle load, just that thier tension increases when the bottom of the rim is forced upwords by a verticle load. I don't recall the toal defoprmation, but even a very small one would result in signifigant tension changes, if it is a radial change.
So no, the side spokes aren't contributing much in the way of vertical support, but they ARE crucial to preventing rim deformation from vertical loads.
Consider an arch- the stones at the base don't have the same vertical load on them as the keystone, but if you let them slide out, the whole arch will colapse. Thats why you see rods running horizontallyt across the bottom of some arches, to prevent them from splaying out and collapsing. How does that rod support a vertical load, if its horizontal? Through the arch.
In a wheel, the top half of the rim could be considered the arch, and the (horizontal) spokes as that rod. Yes, the hub "hangs" from the top of the rim (apex of the arch), but that doesn't mean the load borne by the other spokes isn't contributing to its strength! |
| Bad terminology to induce argument... | GregR
Jun 17, 2002 2:58 PM | | Anyone who reads the rec.bicycles usenet groups finds out soon that Brandt is a retro-grouch that will argue over semantics, and when someone offers a different opinion, he questions their theory and requests data analysis from their conclusion (which of course most people dont have the time or access to devices to obtain the data). The old put-up or shut-up, which forms a retreat in most cases.
I am not saying he is always wrong, I believe he has taken the time to try to understand the principles, but his terminology could be better. I have read the book, and have gotten good insight into things I would have not thought of previously.
Basically what he is saying is a wheel is a pretensioned structure, where tensions are beleased when loads are applied. A wheel is suported by the tension of the all the spokes, but the only ones that have any change in tension are the spokes under the load-affected zone (the bottom 4 or so). The rest of the spokes do not increase tension because the rim is deflected elastically at the load-affected zone (thus causing the decrease in spoke tension), while the rest of the spokes remain in tension supporting the wheel. Where he goes wrong is by claiming that because the only spokes that are affected are the ones in the load-affected zone, that they are the ones supporting the load and therefore why a wheel stands on its spokes, whis is a direct contradiction to the theory that a wheel is supported by the tension of "all" its spokes.
G |
| Thanks for that.. | Matt
Jun 17, 2002 6:53 PM | | Your answer helps. Both to give insight into the book, and more importantly the wheel. I get what you're saying. I still don't like the "standing" terminology. |
| This I agree with | shiggy©®
Jun 17, 2002 10:31 PM | | Thanks Greg. You were able to say it better than I have been trying to. |
| at the risk of revealing my utter ignorance on this topic... | singleminded
Jun 27, 2002 11:57 AM | | I gotta say that I find this debate as puzzling as it is interesting. I'm not sure what the point is -- or why one should think either that a wheel sometimes stands on its spokes or that a hub sometimes hangs from the wheel.
..........
It seems to me that just one thing fundamentally distinguishes a bicycle wheel from a wagon wheel: the spokes and rim are easily bent; they are capable of a lot of flex.
What does this difference mean? It means that securing your rim to your spokes is not enough to make a functional wheel. Under use, the rim and spokes would bend to the point where the wheel was not even approximately round. It would no longer be a wheel at all.
So what do you do? You make the spokes and rim bend less. How do you do that? You make the spokes taught. You reduce the amount the spoke can bend by pulling on it at both ends -- by creating "tension." You can't walk a tightrope if the rope isn't pulled tight; it will bend too much.
You make this tension happen by applying it more or less evenly at each point of connection. When you've done that you've both reduced the amount of bending possible in each area of connection AND balanced the pulling forces across the circumfrence of the rim, creating roundness.
Nothing is sitting on anything. Nothing is hanging from anything. Everything is in "tension" because everything is pulling on everything else. Words like "sitting," "standing," or "hanging" are absurd in this context.
The AMOUNT of pulling, or "tension," in any given area of the wheel changes as forces like the rider's weight act on a particular part of the wheel. Indeed, it's possible for a limited number of spokes to momentarily lose all of the tension that you created during the build. But the rim in that area remains tensioned by the pull of the other spokes. And the hub and other spokes continue to be in tension with one another and the rim. Thus, the system as a WHOLE remains in tension. That's why Shiggy could ride his wheel with 6 spokes missing. |
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