I'd put it down as being simplified rather than wrong.
Indeed they are, but the increase that they see in load is much less than the decrease in load seen by the spokes immediately below the hub. The difference is so big in comparison that one can argue that the load increase for the upper spokes aren't significant. (for a given wheel configuration of course.)

If starting from an otherwise sensibly configured and decently built wheel.

I agree with this, and would like to add that the upper spokes are trying to pull the top of the rim down, creating a flattened oval. This means that the spokes on the sides of the wheel (even those somewhat below the horizontal line, i.e. the 4 and 8 o'clock spokes) are also experiencing increased tension. The only spokes experiencing decreased tension are those between the 5 and 7 o'clock positions.

Correct.

Explain how that is possible. For a given force applied to the hub at the axle, the hub will deflect some amount relative to the centerline of the rim. This will result in an increase in tension for some spokes with a decrease in tension in other spokes. All spokes are starting out with tension applied and assuming a reasonably built wheel, will still have some amount of residual tension even with the decrease as a result of the load. How do you conclude that these changes will be so largely disproportionate that the increases can be basically ignored?

The rim displaces toward the hub. Spokes near the bottom lose tension. The tiny amount of tension gain in the upper spokes is due to the rim losing its round shape. Jobst Brandt's book explains this quite well.

Yes, Brandt's book explains this. Tension decreases in just a couple spoke at the bottom. All the other spokes experience either a slight increase in tension or no increase in tension at all. Any deformation in the shape of the rim -- that is, deviation from the round shape -- requires a change in spoke tension where the deviation from roundness occurs. Because most spokes see little or no change in tension, it follows that little or no deviation from roundness occurs except at and very near the bottom. My recollection from his book is that Brandt determined this both through very careful measurements of actual wheels and also through finite element anlaysis modeling.

Assuming which rim? I can see how a change in roundness would affect tension but I'm skeptical that any decent rim would see that big of a displacement along the bottom (especially given that there is a tire/tube distributing the load).

The spokes at the top must also see an increase in tension from the applied load. The rim losing it's round shape near the bottom should mostly affect those spokes near the deformed area.

The primary displacement of the rim is at the bottom, where it contacts the tire which contacts the ground. No matter the rim, this happens, though it's very small (rim strength obviously controls the amount of displacement). The rest of the rim will sort of 'warp' but much less so than at the bottom. The lower spokes take the most of the load. The other spokes change in tension due to the 'warping'. I really can't explain it that well but Brandt has good drawings of the displaced shape of a rim due to all sorts of loading.

Quite obviously, if the tension in some spokes decreases, the tension in other spokes must increase. But, only a few spokes at the bottom decrease in tension so you've got 7 or 8 times as many spokes that do not decrease in tension. Then there is the fact that the strength of the rim resists change of shape. That is, some of the decrease in tension in the spokes at the bottom is countered by changes in compressive load in the rim. The net result of al this is that the increase in tension in the spokes at the top is slight and is dwarfed by the decrease in tension at the bottom. As Brandt put it, the hub does not hang from the top spokes as traditionally described, it stands on the bottom spokes.

Why the hell should I. The idea like you is bull****.

:popcorn

You guys need to go back to physics class: yes, Brandt too.

Spokes are tensile members and are unable to take compressive load. The bottom spokes do nothing to resist vertical load on the hub.

Not at all, but maybe a lesson on how to communicate clearly w/o using excess terminology would have been useful.

True, no one is arguing that point - when a spoke is seen as a separate element.

You're missing a key feature: Wheels are pre-stressed structures. (in vertical) their dominant reaction to load is by losing some of their pre-tension. The only place on a wheel where you can readily measure a change between with axle loading and w/o axle loading is on the spokes directly underneath the hub.
So although it's questionable in terms of literature and language the engineering perspective is clear, the hub stands on the lost pre-tension in the bottom spokes.

That's a big assumption (unless you have some proof to back it up). I can believe that a rim goes out of round when a load is applied to the hub. However, the only way to apply a load to the bottom of a rim by using a hub attached to the rim with spokes is to pull down on the rim (you can't push with a spoke). The only spokes that can provide that pulling force are the spokes at the top.

And in order for the rim to go greatly out of round at the bottom, the spokes next to those bottom spokes would need to stretch (increase in tension) to allow for the out of roundness (the material of the rim has to go somewhere). All of this is complicated by how many spokes are used, the cross section of those spokes, and the cross section of the rim. I need to go and read Brandt analysis and see exactly what he modeled and measured though.

:roflmao:

Sorry, but you're wrong. This has been argued a million times. Pretensioned structures behave differently.

This is completely wrong. It doesn't matter whether the components of the wheel are seen as seperate elements or an assembly: a structural analysis is still done using free body diagrams which analyze each component individually. Each spoke is a two force member. Each section of rim between adjacent spokes can be simplified into a chord running between the adjacent nipples, the chord also being a two force member. The rim takes compressive force. All the spokes take tensile force.

In the free body diagram of a bottom spoke: let the spoke end at the hub be point A; let the spoke end at the rim be point B. Let points C and D be the nipples of the adjacent spokes. Let segments CB and BD be the adjacent sections of rim. The reaction at point A is an upward force. The reaction at B is a downward force. The rider adds a load L, in the downward direction at point A.

http://i89.photobucket.com/albums/k2...eelPhysics.jpg

*Text in bottom left corner of image should read "|Fch|=|Fbh|". I made a mistake and don't want to spend another 10 minutes modifying and re uploading the image.

When you repeat this tedious exercise for each spoke, and arrange the resulting free body diagrams around point A, the hub, you'll notice that the net reaction at point A, being the sum of the horizontal and vertical components of the forces in each spoke, is zero. Note that at this point, L has been accounted for in the modified forces within the spokes, and is no longer used in the calculation.

If you didn't bother reading the above, the point is that:

1. There's no such reaction as "losing pre-tension". The tension is inherent within the spoke. The reaction is the compressive force in the rim, as well as the tension in the opposing spokes, which resist the pull from the spoke in question. The load is applied to the "joint" at point A (the hub). The applied load requires the rebalancing of the forces in the two force member (the spoke), causing a reduced tensile force and a reduced reaction at both the hub and rim ends. If we don't use the correct engineering terminology, we'll never get our points across.
2. Physics doesn't care which forces are easier to measure. There's no such thing as "standing" on a tensile member. Under gravity and when supported by one member, you hang from a tensile member and bear down on a compressive member.

What looks really cool is a bike that just works all the time. If the primary goal is aesthetics, then make it a museum piece. If you want to ride it, form follows function. YMMV

Again, you are completely wrong. It's obvious you have zero understanding of the concept of prestressed structures. I happen to design them for a living. Just take a wheel (with tire and air), set it on the ground vertically, and put all the weight you can at the 12:00 position. How are the forces being distributed? Does the wheel suddenly collapse? Draw that FBD and explain to me the load path. ;)

I hope you actually read my post instead of just "blahblahblah". I took some time writing it and it's correct.

The wheel doesn't collapse for the same reason corrugated piping doesn't collapse when buried underground. Forces push the sides of the cylinder (or circle) in, preventing ovalization in the horizontal axis. You can replicate this experiment by popping the lid and bottom off of a tin can. Place the now extremely flexible cylinder between two blocks of wood, as shown in the diagram below. You can now stand on top of this assembly without crushing the can. The vertical load is transfered compressively through the rim, all the way around the perimeter, to the bottom. The tin can scenario is an especially good example of this, as there is nothing inside the can at all, and the upper and lower halves of wood are not touching.

Yes, we have actually done this experiment at the office. That's how bored we get at lunch.

http://img.chinaypages.com/0808/phot...ated_pipe_.jpg

http://i89.photobucket.com/albums/k2...Terror/Can.jpg

Anyway, "zero understanding" is a pretty insulting judgment on my professional ability. I design buildings for a living. I'd be seriously concerned if the structural engineer on my team (or I, myself) couldn't draw a free body diagram of a spoked wheel being loaded at 12 o'clock and supported at 6 o'clock. We're liable when things go wrong, so we know what we're talking about and don't bluff about things we don't understand.

Apples and oranges my friend. Read the book - you will be enlightened. I stand by my statement that you don't understand prestressed structures. I design bridges, btw.

Just tell me the load path in my example above. I agree an FBD is way too simple.

Absolutely not. The only difference is that in the spoked wheel, the stabilizing force is being applied via tension from within, where as in the example I provided, the force is exerted by compression from the outside. The direction of said forces are the same and the mechanism of load transfer is identical.

*Sigh*

I knew I shouldn't have looked at this thread :bang:

Again, not a flaw in the reasoning - it's a language shortcoming.

You have two options:
A) Either the hub is hanging from spokes who don't show a significant increase in tension between loaded and unloaded axle.
b) or the hub is "standing" on the reduced pre-tension of a tensile-only construction element.

Your choice. Clumsy as it is I prefer option b.

Measuring spoke tensions with a Park tensiometer on a bike sitting on the ground loaded with nothing but itself I can't determine by measurements alone which is the bottom spoke, that gets lost in the noise. Enlisting my brother to sit on the bike I can measure a decrease in bottom spoke tension, but any increase in the other spokes pretty much gets lost beyond the resolution and repeatibility of the tensiometer.

So there you have it - Of course the wheel isn't standing on the spokes, in the manner we usually think of as "standing".
But given a reasonably decent measuring device, the only change between loaded and unloaded axle is a reduction of the tension previously registered to the spokes that occupy the space in a rather narrow sector between the hub and the ground.

Better yet - ignore the whole issue of "standing" or "hanging". Every spoke tries to pull the hub towards the rim, and the consequences of an axle load is that the spoke that pass between the hub and the ground for a short moment will pull less.

That's right. Brandt has suggested, in one of his numerous on-line discussions of this, that an easy way to see which spokes see significant tension change when a bicycle wheel is loaded is to pluck the spokes both loaded and unloaded and see which spokes have a discernible change in pitch. The bottom spokes do, the top spokes do not.

Here is an interesting analysis the results of which agree with Brandt: http://www.astounding.org.uk/ian/wheel/

Of particular interest is the drawing showing deviation from roundness; greatly exaggerated, of course, because there really isn't much deviation from roundness.

Of all the things that have been written here the worst is, "[t]he bottom spokes do nothing to resist vertical load on the hub." That's beyond wrong. It would be nice to know what buildings this person has designed so I could avoid them.