bjtesch

I don't know what kind of engineering degree that Jobst has or doesn't have, but I know what kind I have and I'll put it up against any engineering school anywhere in the world. My specialty is structural engineering and as such I've studied very much more in the way of structural behavior than most electrical engineers, mechanical engineers, civil engineers, petroleum engineers, maybe even most aerospace engineers, which could explain why Probst couldn't discuss superposition of stresses. I know how the wheel behaves and I have tried to explain it. I've also said it is counterintuitive to anyone with a lesser comprehension of structures. Even the other people in my office have a bit of a problem grasping it although I think they would grasp it if they thought about it long enough. I find nothing to fault with Ian's analysis that has been linked above, I would expect that Jobst has performed similar analysis. Superposition of stresses is an important principle in understanding how the behavior works, and yes you can add compression load to a spoke that is in tension. I will perform my own FE analysis, not so I can backup what Jobst or Ian has said, but because some of the other posters have become interested in how butted spokes affect the results.

Tunnelrat81

Looking forward to it. Those of us without engineering education really do appreciate your efforts here. I hope you get that message though our questions. This is the most in depth explanation/debate that I've heard on BF or elsewhere and it's been enjoyable.

-Jeremy

DannoXYZ 

We are getting two things mixed up:

1) engineering concepts
2) the words used to describe those concepts

I think we're all on the same page with #1 but are disputing on how to describe it. Perhaps if we break up the sequence into individual steps, it may make it easier to handle:

1. unloaded static wheel has all spokes tensioned to a certain pre-load value.

2. a downward load is applied to the hub, this generates what Jobst calls a "compression force" because it is moving the hub closer to the ground. This force also brings the spoke's threaded and hook end closer together.

3. this "compression force" reduces the pre-loaded tension on the bottom spokes. The material in the spokes doesn't experience actual compression, but rather a lowering of the pre-load tension. Whether this means the bottom spokes are "carrying" the load" or is "in compression" or is "standing on the spokes" is a matter of semantics and how you describe the phenomenon.

4. the rim expands outside of the load-zone on the ground to maintain the same circumference

5. expanding rim pulls harder on spokes not in load-zone and increases their tension. The total of increased tension equals the loss tension on the bottom spokes in the loaded zone.


The debate is really about how to describe those spokes at the bottom and what they experience and whether that can be considered "load"? I don't like to use either the "stands on bottom" spokes or "hang from top spokes" terms. I prefer to say a wheel carries load by "reducing tension on bottom spokes and increasing tension on top spokes".

Mr IGH

This is really all he's saying, it's not really very novel. To Josbt, it's a breakthrough. To a good engineer, it's a reference point issue. The fact that Josbt thinks it's a breakthrough is telling. The fact that he claims this is proof the wheel is standing on the bottom spokes makes me wonder what he really understands about this. My conversation with him was all the confirmation I needed.

Agreed, any freshman engineering student understands that. You have to met Josbt and hear him preach about his breakthrough, he reminded me of Elmer Gantry...all hat.

Mr IGH

agreed, it isn't exactly novel. It's a fairly simple linear system and the limit is when the tensioned spoke's overall force changes from tension to compression. To "stand" on a spoke would require withstanding compression, it can't so it isn't

bjtesch

This is the superposition of stresses.

A. You perform an analysis on the wheel to determine the forces due to tensioning the spokes. The result is that the spokes are in calculated tension, the rim is in calculated compression.
B. You perform an analysis on the wheel to determine the forces do the rider's weight. The result is that the spokes on the bottom are in calculated compression, the spokes on the top are in calculated tension. The forces in the rim have become more complicated, probably some tension in some places, compression in others, and also bending here and there. Input the wheel geometry into any finite element program you choose to use, it will calculate compression in the bottom spokes, as well as the forces in all of the other elements.
"A" and "B" are independent of each other, and neither tells the whole story. You can do "A" or you can do "B" first, doesn't matter, but you have to do them both and not get too involved in drawing conclusions based on the results of either analysis alone by itself.
C. When finished with "A" and "B", you add them together. Now the result is that the calculated stress in the bottom spokes is tension, so they don't buckle.

If the system is linear, then you can do this "adding together" thing. If the system isn't linear, either due to geometry changes as loads are applied, or because of the response of any of the materials to stress, then you cannot analyze in this way, you have to do it some other way. Nonlinear analysis can get to be really complicated.

interested

It is Jobst, not Josbt.
Jobst Brandt is a mechanical engineer from Stanford. He has worked for Porsche (designing brake systems etc), Stanford Linear Accelerator Center, Hewlett-Packard (The old HP, when they where engineer driven and made awesome products). With that CV he must have above average skills.

Here is another engineer agreeing with Jobst and even providing some data and a FE analysis
https://www.astounding.org.uk/ian/wheel/

It is easy to prove Jobst Brandt wrong if it is just simple freshman stuff; just make a good experiment and mathematical model, write a paper and get it accepted in a peer reviewed academic journal.

--
Regards

zzyzx_xyzzy

Technical vocabulary should serve to clarify particulars. If people are wound up about the technical application of 'load' or 'stand' but are actually in agreement about what the actual forces in the spokes are, it seems like they are having a very stupid argument on both sides.

Mr IGH

Been there, done that, served on peer review committees, served as a chairman for sessions. Where's Jobst's peer reviewed papers? Love to see 'em.

rydaddy

If I recall correctly, you are an EE, right?

desconhecido 

That might explain some of what we're seeing here though EEs study everything necessary to understanding this problem. Some of them just don't realize it.

interested

I am not talking about writing papers in general, but writing a specific one that supports your view against Brandt (and others), which I believe you haven't actually done (or else, a citation would be nice).

The subject seems to generate controversies, and bicycle wheels are both very common and old seen as an invention, so I don't doubt that an academic journal would accept a paper that could settle this old question of whether a hub hangs or stands on the spokes.


--
Regards

dscheidt

When he wrote the book, it was novel. Nobody had bothered to an analysis, and there were lots of people who should have known better that would have told you that wheel behaved in all sorts of ways that it doesn't.

Mr IGH

Where's Jobst's peer reviewed paper? Where's his ME from?

me: BSEE, Iowa, 1985

Road Fan

All I can say is, it happens all the time. Really good engineers take a moment as they do quantitative work and think about how they would explain what they've done. Clarity is always the goal but it can be hard to achieve.

One problem is that a long of terms used in science and engineering have tightly limited definitions, and are completely clear when used in a technical discussion. The same words have much broader usages in casual speech, and hence non-science people can't always understand what the engineers are saying. IMO it's the fault of the engineers for failing to translate effectively, since we are equipped to see the difference, and most non-scientists are not since they don't know the technical usages of the terms.

Road Fan

My contention is, that point does not need to be proven. It is adequate to have a model that predicts that the hub is in equilibrium (sum of the forces acting on the hub equals zero) when it is loaded and when it is not loaded, and that stands up to some physical verification. We have that. The rest is not necessary as science, at least to explain why hubs don't fall down and why wheels are so strong.

bjtesch

I would not think so.

When I was in college only the CE and Aerospace departments offered the kind of structural analysis courses that would begin to analyze this type of problem. I don't think the ME's did either but I can't be so sure about that.

For that matter I took an EE course but I can't begin to visualize anything involving electricity past ohm's law and AC currents.

desconhecido 

I don't know, doesn't look so different from an EE problem to me. The physics is pretty simple and all the mathematical relationships necessary have pretty basic analogs in EE.

edit: When I say "understanding the problem" I mean understanding the solution as presented by Ian, Brandt, or Gavin, and, I'm sure, others. I'm not saying that EE students study structural engineering problems or are typically familiar with the necessary tools. Back about 40 years ago I was taking physics courses, mechanical engineering courses, and electrical engineering courses all at the same time and I was amazed at how much that I learned in one department would solve problems presented by the others. Superposition is superposition, etc.

Road Fan

While I fully accept that structural engineering is a specialty in its own right, with both elementary and advanced topics, most EEs have had similar fundamentals. Statics and dynamics, strength of materials, mechanics of deformable bodies, linear systems theory (lots of that), partial differential equations, field theory, and some materials science. My school at least wanted us to become engineers with a focus in EE, ME, et cetera. The EE class that really taught me that the same math could address different domains was classical control systems. It integrated active electrical systems, statics and dynamic (kinetics, kinematics, and some dynamics), and mechanisms into a single linear mathematical framework. But no shells or whatever else is advanced.

So we understand about writing force equations, free body analysis, things that stretch and compress, distributed forces, the properties of different types of basic joints, multivariate analysis, and linear systems math. We have only gone that far into the structures world, but we do know we can apply our math and the fundamentals we all learned to problems we see. Some of us know where our limits are, as well. Like I said, I want to listen to the smart guys. But I may ask questions as I try to understand.

I think we can follow these arguments just fine, though maybe a bit slower.

dabac

Having spent a couple of years reading/posting on a forum that Brandt also frequented I can say that he's also above average committed to his own ideas and views. Even when people pulled off decently documented and repeatable experiments contradicting his predictions/statements he'd cling to his own theories well past the point of ridicule.

For those interested in ancient forum threads something can probably be found by typing "jobst brandt" and "carl fogel" into the search engine of your choice.

Although in this recurrence of the standing or hanging argument I'd blame flawed language rather than flawed engineering.

Al Criner

I think non-engineers (like me) sometimes become aware of this spoke discussion and concentrate on the spokes, as if that is all there is. The wheel is a stucture and the rim is big part of it. A bike does not stand on a spoke or a few spokes, it is supported by the wheel structure. Brandt's spoke discussion describes what actually occurs with the spokes, which is interesting, but it is only part of the explanation of a wheel.