Jonathandavid

If you push on both ends of the guitar, bending it inward as it were, you'd expect the tension to be reduced also. The pushing on both ends of the guitar is compression: two forces pushing inward. Wikipedia gives a perfectly valid definition: "compression is the application of balanced inward ("pushing") forces to different points on a material or structure." It consists of forces. Even if something is stretched to its limits, you can still compress it, because you can still apply opposite force. The net result is probably less tension, but the compression - the forces - is still present.

If you turn on the tuning keys, however, you're reducing the tension by lengthening the string (or opposite). This is not analogous to putting a load on a bicycle wheel.

Jonathandavid

The weight acting on the wheel causes compression in the lower spokes, which lead to the tensional stress within the spoke to be reduced, as experiments show. The net result is still tensional stress, which keeps the spoke in place. You can push on both ends of a tensioned rod, and not nullify all net tension.

But here's the neat thing about the bicycle wheel: changes in weight are effectively 'absorbed', by these changes in tensional stress in the lower spokes, not by increased tensional stress in the upper spokes. This is counterintuitive, odd, and against common sense, but it is what you inevitably observe. I think it was Richard Dawkins who said: "Science. It works, b*****s!"

cyccommute 

You are correct that we have gotten way off into the weeds. To answer Shotland's original question, he has about a third of a good wheel. The hub is a good choice. The rim he has choosen is...meh. It's as good a choice as any of about a dozen that I could suggest and is probably better than about a hundred more. As I said above, the rim hardly matters.

But he has made the classic error in wheel building of concentrating in the rim, then the hub and completely ignoring the most important part...the spokes. Strong spokes equal a strong, durable wheel.

cyccommute 

Your guitar example has described, almost exactly, what happens in a wheel. When you load a wheel with weight, you press on the rim but not the spokes just as you press on the guitar neck and sound box but not the strings. This reduces the tension on the spokes as it does the strings but doesn't result in compression of the strings or spokes.

You cannot simultaneously compress a object and put tension on it in the same plane. The forces are exactly opposite. You could compress a spoke, for example, at an angle to the tension vector, i.e. push it sideways, but that is out of the plane of the tension on the spoke.

To test this, take a piece of string and pull on both ends. Then try to push the ends back together while maintaining tension. You just can't do it.

You just aren't getting it. Every example that you have used is an example of tension reduction and not compression. Even in this case. The weight acting on the wheel acts on the rim which compress the rim. The rim end of the spokes isn't constrained by the rim since it is free to slide in and out of the spoke hole. As I've said over and over again, you reduce the tension but that is not compression.

You are correct that changes in force are effectively absorbed by the changes in tension around the wheel. What is counterintuitive is that it is changes in tension but not compression of the spokes. There is nothing "pushing" on the end of the spoke.

You are absolutely wrong that you can push on both ends of a tensioned rod and still keep the rod under tension if the forces are in the same plane. They are forces in the opposite directions. If you are pulling on a rod, i.e. have it in tension, and you turn around and push on the rod, you have to go to zero tension before compression occurs. The opposite is also true.

Jonathandavid

Then I think we've reached an agreement of sorts. You seem to be aware at least what happens in a physical sense, just not of how this is commonly described (again I am reminded of why I am not a logical positivist, but that's neither here nor there). Opposing forces can, actually, operate 'in the same plane'. Just look at a bouncing basketball. Elasticity of the ball, gravity, the push of my hand, the elasticity of the floor, all in the same up/down line. And since tension and compression are forces, they can operate in the same plane. Remember that they do have components, like many other forces. But you don't always need to know these to solve a particular problem, just the resulting force or relevant component will do.

Reduction of tension is a negative change in tension. Negative tension is effectively compression, because tension cannot actually be negative.

This is what I meant when I said that we seem to actually agree on this: the net stress within the spoke is tension, which keeps it in place. If you can only wrap your head around it if I call it 'tension reduction' that is fine by me, because the physical reality is the same. There is no change in tension in the upper spokes and a reduction in tension in the lower spokes. If a bike 'hangs' in the upper spokes, those spokes would be subject to more tensional stress. As this is not the case, the bike does not hang in the upper spokes.

And that is what makes a bicycle wheel a remarkable thing. If I cut all spokes except the lowest ones, then those floating nipples of yours will cause the spokes to be rammed into the tube/tyre - and some other unpleasantness as well. But, as you aptly described at the beginning of this discussion, spokes are locked firmly in place by pretensioning them. This makes the wheel behave like a wheel. To stick to the lingo that you prefer, the weight of the bike is absorbed as a reduction in tension by the lower spokes. And surely the rim also, which I have mostly disregarded because it is not important for the question regarding if the bike hangs in the upper spokes.

So what? Lots of forces are in opposite directions. If that was impossible, everything would be accellerating all the time in every direction. What a strange world that would be.

Aristotle thought that a constant force on an object made it move at a constant speed. Newton buried that. If something moves at a constant speed, forces have to be operating along the same axis in opposite direction. And if some more force is exerted the object accelerates. Just like other forces, tension and compression can operate along the same axis.

(I think a 'plane' is actually a surface? Forces are typically vectors, so they'd be more like a line rather than a plane...)

alan s 

Here's an experiment you can try at home. Remove all the lower spokes on the front wheel and balance the bike vertically. Then ask yourself what is holding up the front of the bike?

cyccommute 

Again you are missing the point. Tension and compression can't work in the same plane simultaneously. Your basketball isn't experiencing compression and rebound at the same time. If it were, the ball would be at rest. The ball is hit by your hand (compression), rebounds off your hand...not tension, by the way, but a release of the compression...hits the ground where it compresses and rebounds back to your hand. None of these actions are happening simultaneously.


This is were you are just plan wrong. Reduction of tension is a decrease in the force resulting in tension. That is not the same, nor even effectively, compression. It is a reduction in the scalar value of the tension vector. I think the main problem here is how you view tension and compression. They are forces that act in opposite directions...push vs pull...but the absence of one does not mean the presence of the other. In other words, you can compress an object or you can put the object under tension but just because compression and tension don't work together. For example, you can compress a gas but if you release the compression on the gas, you aren't putting tension on the gas.

Tension on a spoke works the same way. When you pull on the end of the spoke with the spoke nipple, you stretch the wire and put the spoke under tension. If you reduce the tension on the spoke by loosening the spoke nipple, you are simply releasing tension on the spoke. You aren't compressing the spoke...effectively or any other way.


On the other hand, if the wheel "stands" on the bottom spokes, the spoke would be compressed. It experiences no compression because there is nothing "compressing" it. In other words, there isn't a force pushing upward on the spoke. As I said before, take a loosely laced wheel and set it on the ground. Which spokes are in contact with the rim? Not the bottom ones.


As you cut spokes, you are releasing the tension on the spokes. If you want to be brave (and maybe a little foolish), take the tire off the wheel (to simplify things), put the wheel in a frame, set the wheel on the ground, get on the bike and have someone cut the bottom spokes. If the spokes were under any kind of compression, the spokes will move towards each other because that is the direction of compressive forces. But you'll find that the cut end of the spoke will do the same as if you cut any other spoke...it will fly out of the rim towards the ground.


Try to simultaneously push (compression) and pull (tension) on a piece of string. That's the easiest example to demonstrate.

By the way we do live in a world of constant acceleration. The force of gravity acts on you all the time. That's acceleration.

Vectors act in planes. The plane can be one, two, three or multiple dimensions. If forces are acting in a single dimension in opposite direction the net acceleration is zero. But, just because tension and compression act in opposite directions, the lack of one does not suggest the presence of the other.

alan s 

Think about tension and compression at a molecular level. With compression, the molecules are being forced together from their normal state, and with tension, they are being pulled apart. This accounts for buckling under excessive compression and snapping under excessive tension.

Jonathandavid

You're going from one static equilibrium (tension) to another (less tension). This always requires the application of force, directly or indirectly, because you have to expend energy. In this case the forces work inward on both ends of the spoke. That's compression. Ask a physicist.

alan s 

I'll talk to my physicist about this today when I pick up my compression scrips.

edthesped

So in theory one could lace a wheel with string, or wire brake cable, instead of spokes and it will act the same as a spoke laced wheel?

fietsbob

Tioga did that.. in the 80's .. the string was kevlar and laminated into a Disc.

Tioga-Sugino tension disc wheel https://www.google.com/search?q=Tiog...w=1663&bih=915

alan s 

Fiber spoke. Morrison-Barrios FiberFix Spoke Replacement - Tools & Accessories | Adventure Cycling Association

edthesped

If one can lace a wheel with string and use it the spokes must be in tension...

cyccommute 

Tension requires the application of force, yes. But applying less force...or reducing the force...to go from higher tension to lower tension is simply a change in the scalar of the force. It's not a change in the direction of the total force. Just because you reduce the force doesn't mean that you transition from a pulling force (tension) to a pushing force (compression). You've only reduced the amount of force on the spoke. The spoke is still under tension...just a little bit less of it. But it's not compression because you aren't pushing on the end of the spoke. You are just pulling on it a little less hard. To transition to compression, you would have to release all the tension and then push on the both ends of the spoke. That just doesn't happen in a bicycle wheel.

Others have provided really good examples but, yes, you can build a wheel with string.

Exactly my point. The Fiberfix works well but, since it's a string, you can't push on it. It's under tension at all times.

fietsbob

& the fiber-fix is an emergency stop gap spare . temporary to get you to a shop to get a proper replacement..

I went with spare spokes already in the wheel.. when 1 broke in a 48 spoke wheel 37 were left
(vs a 36 spoke... 11 were already in-place)

Miles2go

You'd have to break the remaining spares in the right order though.

Fiber fix. Wonder if they have an expiration date. Been carrying the same two from the beginning of time. Among all the bikes I've owned (nearly embarrassing), I've never suffered a broken spoke. Even had a ravenous lap dog jump into the non-drive side (radially-laced) of a Campagnolo race wheel with no ill effects….well, unlucky for the dog though; those were bladed spokes.

edthesped

I was expecting a Q.E.D. at the end of your post

cyccommute 

The FiberFix is a Kevlar line surrounded by a polyester sheath. It's a line that is used in 2 line kite flying and within the sheath is very long lasting. Outside the sheath, the Kevlar is somewhat unstable in UV light and starts to break down rather rapidly (month's of use rather than years). If you keep the FiberFix in the package and keep it out of the sun, you should be able to carry it around for decades without it losing strength.

nickw

You are wrong, your intuition is steering you in the wrong direction here. Throw rational thinking out the window, it is what it is:

The Bicycle Wheel as Prestressed Structure

Bicycle wheels achieve their structural efficiency by making use of prestressing in three ways. Tests show that the bottom spokes carry virtually all the load by compressive forces, which reduce the tensile prestress set up in the spokes when the wheel was made. The test results are compared with an analysis that considers the spokes as a disk that can carry force in one direction only. This is shown to give good agreement, as does an analysis that considers the rim as a straight beam on an elastic foundation. The behavior of the wheel with an inflated tire is also considered, and it is shown that good comparisons with theory are obtained if the reaction from the road is assumed to be distributed over a specific length of the rim. Prestressing is shown to be important also in the mechanism by which the various forces are transmitted through the tire from the road to the rim.

I don't think the term 'standing' has helped anybody understand the concept and is a bit misleading....but the fact remains, bottom spokes carry load.

Jonathandavid

Yes, I would say you can. Such a wheel, when properly tensioned, would act like a normal wheel with thick, rigid spokes.

Jonathandavid

I tried not to use the expression too often because it is a generalization of what is really happening, and also semantically questionable. What is meant by "the wheel stands on the lower spokes" is that these spokes undergo compression, while other spokes do not undergo more tension. So there's something pushing on some spokes, but no extra pull on others.

Cycommute doesn't want me to use the term 'compression' for spokes that still suffer overall (net) tensional stress. They're still tight between the rim and the hub. I think you can say that there is some compression added to the tension, reducing the overall tension, but this seems semantic more than about reality.

I've a modest background in geology and did have to study on static situations involving elasticity, stress and strain. So I've tried to express myself correctly, but may not have succeeded all the time. But by now it has been abundantly described what the 'go of it' is, your quote sums it up very well.

nickw

Another good article, independent analysis of Jobst, same conclusion:

Ian's Bicycle Wheel Analysis

He is fine with the 'standing' term, as explained here:

In asking whether the hub hangs or stands on the spokes, I really mean to question where the structurally active spokes are. That is, if the spokes which resist load are below the hub, I'd describe the hub as standing on the lower spokes. If the spokes that resist load are above the hub, then I'd say the hub was hanging. That is:

Something hangs from a support if it is below the support.
Something stands on a support if it is above the support.
Some people don't like these definitions. They feel that it can only be described as standing if compressive stress is involved. However, the most reliable dictionary definition I can find ( The New Shorter Oxford English Dictionary, half a million definitions in two volumes each 1900 or so pages) makes no such restriction - the verb is the third entry for stand, and meaning 12 (the first within the second major grammatical division) is the relevant one - "Of a thing: be in an upright position with the lower part resting on or fixed in the ground or some other support". This is compatible with the definitions above - if the hub has its lower part supported by the spokes below it, from which the supporting action is predominately received, the definition fits. None of the meanings in the dictuionary specify that compressive stress be involved. (Incidently, for the Americans, Merriam-Webster is the same - stand means "to rest or remain upright on a base or lower end", with no requirement for compressive stress.)

cyccommute 

First, throwing rationing thinking out the window leads to magical thinking which isn't a good thing. Second, and yet again, if you have compressional forces on the spokes, that compression has to come from somewhere. The spoke is under tension. Compressing the spoke releases the tension and puts the spoke into another mode entirely. Look at BobG's picture from post 22

That's what compression of a spoke looks like. He is compressing a detensioned spoke against the tire and the spoke buckles. We don't see that kind of buckling in normal usage of a tensioned wheel. There is compression there but it is of the rim, not the spoke.

If you look more closely at the picture, you can see that the spokes on top of the wheel are longer than the spokes on the bottom. The spokes on the top of the wheel are under a slight amount of tension while the bottom ones are being compressed. The fact that they are buckling makes them effectively shorter. You can see much the same thing happening in this partially laced wheel



The spokes on the top of the wheel, from which the hub hangs, are resting in the spoke holes while the spokes on the bottom have dropped out of the bottom of the rim. The hub is clearly "hanging" from the top of the wheel. When rotated the hub and pulled the spoke against the rim as well as added the rest of the spokes, the bottom spoke will still stand proud of the rim on the bottom. If I were to put the rim on the ground and push downward on them, they would buckle just like BobG's do.

As I layered in tension, the spoke nipple pulls up against the rim but there is no compression of the spoke. The spoke is still under tension and the hub is hanging from the top spokes. Adding tension doesn't change that. Can you show at what point spokes on the bottom of the wheel transition from tension to compression during the tensioning process? How they transition from tension to compression when you put weight on the wheel without buckling like BobG's have?

What you have forgotten to mention is that the string would act like a normal wheel with rigid spokes under tension. If you compressed any of the strings, the wheel would collapse.


Since you have studied stress and strain...essentially tension and compression...then you should know what they are and what they aren't. It's not that I don't "want" you to use the term compression but that you can't use that term to describe a reduction of tension. The spokes aren't being compressed because they aren't being pushed on by anything. The rim is being compressed...yes, it really is being compressed...but reduces the tension on the spokes which isn't the same as compressing the spokes. You can clearly see what compression of a spoke looks like in BobG's picture but that compression isn't present in a properly tensioned wheel wheel.

nickw

Your turning your back to science and structural analysis, which is certainly not magical but obviously not intuitive as many people have made the same mistake as yourself.

Go read the links and educate yourself, it lays it out for you, step by step as Jonathandavid has attempted several times.

If we all believed only what made 'sense' imagine where we'd be as a society.....