Your analogy is all wrong. A correct analogy of a wheel to a suspension bridge: the rim would be the towers, cable anchoring points and main suspension cables, the spokes would be the vertical hanging cables, and the hub would be the bridge deck. Strengthening any part of the bridge would strengthen the entire bridge.

What is there to compress? If you compress the rim, the spokes float like those in BobG's picture. The only reason that the spokes appear to deform in his picture is because the spokes are pressing on the tube. Remove the tube and tire and the spokes would just slide through the rim. You can't compress something that isn't attached to anything.

The hub would be the towers which are the anchor points for the spokes (cables in a bridge). The deck is the part that is suspended which, in the case of a wheel, is the rim.

"Wire used as a compression member is similar to prestressed concrete used as a tension member. With prestressing, concrete can be used in what appears to be tension. Concrete cannot work in tension just as wires cannot work in compression. However, concrete beams are used as tension members in many bridges. Under load a concrete bridge beam sags at midspan. Sagging com-presses the top surface and stretches the bottom. Although the underside elongates, it is not in tension. If it were, cracks would soon open. Such beams are prestressed in compression by internal steel rods to ensure that no part of the beam will experience tension.

Similarly, because it is prestressed, the wheel can stand on its bottom spokes. Stress changes occur only in the bottom spokes, not in the top ones. Structurally, bottom spokes are acting as compression members in the wheel, and no measurement of their elastic movement reveals that they are anything but rigid columns. Because individual spoke tension results from tension in all spokes, the wheel can be analyzed only by considering all its spokes. The concept that the hub hangs from the upper spokes contradicts all measured and computed behavior of bicycle wheels."

So Cycommute is certainly right that a wheel is prestressed and that this causes the wheel to be rigid, like a wooden wheel of a cart. It's just that the wheel doesn't hang on the upper spokes.

The spokes themselves. Because of the forces acting on them, they become a bit shorter, which is possible because they were prestressed.

Agreed that the hub hangs on the spokes, which in turn hang on the rim. However, you flipped the bridge upside down.

The spokes could be compress if they were attached to the rim. However, there is no compressive attachment of the spoke to the rim expect to compress the rim inward. There's no "attachment" of the spoke to the rim except in tension. The spoke isn't a bolt and nut system nor is it like a wooden wheel.

I agree that the wheel works as a unit but even as it works as a unit you can look at each individual spoke. Assuming that your above quote is from Brandt, the bottom spokes can't work as columns because they have no bottom. A column carries load in compression by the earth pushing up on the bottom of the column. There is nothing in a wire spoked bicycle wheel (the tire isn't included in the discussion) that can push upward on the spoke. If you push upward on the rim, you don't push on the spoke. The upper spokes pull the load upward on the bottom spoke but the rim doesn't push on the spoke.

Going back to my example of removing half the spoke on a wheel, if the spokes are at bottom of the wheel, they will slide out of the rim and can't be compressed unless they hit the ground or the tire, at which point the spoke has a base and can work as a column. But that wouldn't be normal operation.

Exactly. The spoke tension from all the spokes makes the lower ones act like columns, which can therefore compress. You don't need a compressive attachment because of the tension on the spoke: building a wheel puts tension on the spokes. Brandt clearly explains this and indicates how it has been empirically confirmed.

Not only does a spoke not compress at all, it detensions on the lower half of the wheel as the weight of the bike and rider is transferred to the spokes on the upper half of the wheel. Think of it not as compression, but as tension, and it makes more sense. Less tension on the spokes on the lower half of the wheel and more tension on the spokes on the upper half of the wheel. Tension/detension rather than compression/decompression. Tension is a pulling force, whereas compression is a pushing force.

When I pluck the upper spokes like strings, the sound is the same regardless of whether I am sitting on the rack or not. When I do the same with the lower spokes, the sound is lower when I sit on the rack. This is because the increased load on the bike, which puts compressive stress on the lower spokes.

If you keep a rubber band between two fingers and pluck it, moving the ends of the rubber band towards another will produce a lower sound. This is basically the same mechanism.

If you draw the forces on a stationary wheel on a piece of paper, you will see that the downward force acts at the hub, while the upward force acts at the base of the wheel. This way you can easily see that compression of the lower spokes is inevitable. Empirical testing shows that the tension in the upper spokes does not change (see above). Therefore, the hub rests on the lower spokes.

I edited my earlier post. Just to clarify, the rim is compressed, not the spokes.

Tension and compression are scientifically well defined. A spoke in a wheel undergoes tension, as the nipple and the flange hub pull on it. When the spoke is on the lower part of the wheel, the weight of the bike acts as a force pointing down. Since the bike is not moving vertically, there is also an equal force going up. This is acting from the ground on the base of the wheel. These two forces push on the ends of the spoke. This is compression. I do not see how anyone can deny that there is compression acting on the spoke when it is so blatantly obvious that there is.

The net result of this tension and compression is thankfully still tension, or the spoke would buckle, and the wheel presumably also. But the tension is less than on other spokes. You can measure this, and people have. A lower spoke on a bike undergoes less tensile strength than an upper spoke. This has been explained ad nauseam. The remarkable thing, however, is that the upper spokes do not increase in the amount of tension! This can be measured also, and again people have. As I have also explained, you can check this on your own bike by playing the wheel like a harp, and noticing that those higher spokes do not give a different tone when someone sits on the bike. Increasing the weight does not change the sound the upper spokes make.

The explanation for this can, as it turns out, be found in the pretensioning of the bike with the spokes. This gives the wheel its properties. It is counterintuitive, because this implies that pieces of cable can be made to act like stone columns. But that's what the spokes do.

can a spoke be compressed from one end?

There is another explanation that fits what you have observed better. The upper spokes are under higher tension so the pitch is higher. The lower spokes are under lower tension because the load is deforming the rim and thus have a lower pitch. The spoke isn't being compressed because there is nothing to push upward on the spoke to provide compression.

Yes, it is the same mechanism as I said. You are lowering the tension on the rubber band and the vibrational energy is changing. The rubber band isn't being compressed, however.

Sure you can draw the forces on a wheel but you have to draw them properly. The floating rim is the key. The upward force on the rim is independent of the force on the spoke because the rim floats on the spoke nipples. The upward force on the spoke is provided by the upper spokes and the tension they have that is pulling the lower spokes up to keep them in contact with the rim as the rim deforms.

I assume that since you've read the book on wheel building that you have some experience with building a wheel. Consider the simplest example of a wheel...a radially spoked front wheel. When you lace the wheel but before you add tension, the spokes can move freely through the rim. If you set the untensioned wheel on the ground, the spoke on the top of the wheel will fall down into the holes on the rim while the spokes on the bottom of the wheel will fall through the rim and hang in free space. The hub is being held up by the spokes at the top of the wheel and the spokes at the bottom of the wheel aren't experiencing any force other than that due to gravity. They aren't being compressed at all. In fact, no spoke is experiencing compression. Adding tension doesn't change that. No spoke anywhere on the wheel is experiencing compression. If you put weight on the wheel, no spoke is going to experience compression because there is nothing to compress the spoke.

Herein lies the problem. There is a force acting from the ground on the wheel but it is acting on the rim of the wheel. The spoke is only being acted by the hub and the other spokes in the wheel. You have forces pulling up on the spoke from the hub and forces pulling down on the spoke from the rim but there is nothing "pushing" on both ends of the spoke as you would need for compression.

Go back to the radially laced but untensioned wheel. You can load the rim and compress it but the spokes that aren't in contact with the rim won't be under compression. You could tension the wheel a little and you still wouldn't see any compressive forces on the lower spokes. You could continue this until you tore the spokes out of the rim and you still wouldn't have anything that could compress the ends of any spoke.

Yes, the tension is less. But that doesn't mean that the spoke is under compression.

No. And that is my point. As Jonathandavid said, compression is well defined. You can't compress something by pushing on one end and letting the other end hang in free space which is what every single spoke in a wheel does. You can place the spoke under tension but you can't compress it.

Here's another way to look at it - something compressed beyond the load it can withstand will buckle (rim), whereas something tensioned beyond the load it can withstand will snap (spoke). The rim is compressed and the spoke is tensioned.

No, there isn't. You can only achieve a lower tone on a tensed spoke by a) adding bits of spoke, b) changing the temperature or c) subjecting the spoke to compression. Since a) and b) are not the case, it's c).

If you take a big harp and pluck a string, the tone will become lower when someone puts a big weight on it. This weight causes compression in the strings. The weight and the counteracting force from the ground are the two opposing forces here. Compression is the opposite of tension: if you pull the ends of the string apart, you're adding tension, if you apply forces on the ends towards eachother, it's 'negative tension', i.e. compression. So forces counteracting the tension are called compression, and a spoke that is tight in a wheel can undergo compression and remain tight in the wheel because the tension is high enough. There is no need for different nipples.

If a wheel would 'hang on the upper spokes', a load on the bike would increase the tension on the upper spokes and produce a higher tone when these spokes are plucked. This is not the case. Therefore, the weight results in the compression, which is the same as decreased tension or a negative change in tension, of the lower spokes, not increased tension in the upper spokes. That is why Brandt, an experienced engineer who really knows the science behind this, is right.

You seem to believe that forces resulting in less tension are not the same as compression. This is akin to believing that I am not subject to gravity because I am currently not falling. Scientific hogwash.

This is a prime example of thread drift. OP asks a simple question. Thread turns into a debate over wheel-building theory and nobody answers the question. Classic!

Sorry, but no, it relieves some of the tension, hence the lower pitch. Putting weight on the instrument to lower the pitch, as in your example, is no more adding compression to the strings as removing some of the tension via the tuning pegs. In both cases the strings are not under compression forces.

The same principle is apropos to the bike wheel.

That doesn't follow at all. Terrible example.

I'd consider a killer combo for "touring" (what does that word mean to you? It's somewhat important.) a solid and efficient hub, a proven rim (not "New!, the rim you've all been waiting for!"), like the Velocity Dyad or Mavic Open Sport, and on top of this and of the most importance, assembled by a skilled wheel builder, be that yourself or whomever.

So in short, no, that's not a killer combo for me. Don't need the beef of the Phil hubs, even though I have some fun off the pavement when I tour. Don't want to prove a rim for touring applications if I don't' have to.

YMMV.

The suggest line say "wheel onions," so how is this off topic?

Compression is two forces pushing on an object in opposite directions. Tension is two forces trying to pull it apart. Both can occur at the same time: if a force is counteracted by another force, it doesn't disappear. For example, if I lift something, gravity still acts on the object. My muscles may be winning out, but the gravity is still present.

'Relieving some of the tension' means that you have to apply two forces opposite to the tension, 'negative tension' as it were. That's compression. The fact that you might not 'relieve' all of the tension is irrelevant; the two forces acting as compression still apply (see above). This is very basic physics. The weight of the bike and the force from the ground compress the lower spokes in a wheel. The fact that the spoke remains subject to tensional stress does not change that: the tension applied by tightening the nipple remains also, and is bigger than the compression from the weight and the ground. You can apply compression to an object undergoing tensional stress, which is what happens here.

You're joking right. Tell me you're joking, because that's funny.

This thread is obviously making some people very tense, and others very compressed.

That may be arguable...

d) by changing the tension of the spoke. Since the spoke can't be compressed, it's d).

If you take a big harp and tighten or loosen the strings you can change the tone. I'm not a musician but I do understand the principles of tuning a stringed instrument. People don't tune stringed instruments by adding weight to them. Weight, i.e. compression, on the sound box tends to dull the sound because it dampens the resonance.

As for a string, if you pull the string at each end...like you would a spoke...you have the string under tension. You do not, and probably cannot, compress a string. The string (and spoke) would buckle and bend. That's not what is happening here nor in the case of the harp. If you change the tone of the string or spoke by releasing tension, that is not compression. It's a reduction of tension.

As you said above, compression and tension are well defined. Compression is, in it's simplest form, pushing on two ends of a rod. Tension is pulling on two ends of a rod. Reducing the compression on a rod isn't the same as putting tension on it nor is releasing tension on a rod the same as compressing the rod. They really aren't as opposite as you suspect. They are two entirely different processes.

Again, how do you compress a wire spoke that has one end floating freely in space? The rim cannot press on the end of the spoke so you have no compression. You have less tension when the bottom of the rim is loaded but you have no compression of the spoke. You can't compress something that you can only push on one end. He may be an engineer but he has made an error in his thinking.

When I reduce the tension on my guitar strings the tone gets lower; not because the strings are compressed, but because there is less tension.

The rubber band is not compressing it just has less tension, acting like an elastic guitar string.