: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.

So try it with a 12 spoke wheel.

Just because the load's shared so much you can't see an increase in tension doesn't mean you should throw out the only explanation that makes sense, IMO...

Imagine a three-spoked wheel for a minute; one of the spokes pointing down. To say the lower spoke bears any weight applied to the hub sounds pretty wrong, doesn't it? Surely you'd notice an increase in tension in the hanging spokes then.

What would happen to this loaded wheel if you backed off the tension in the lower spoke until it was out of the picture? The rim would become slightly egg-shaped and the hub would move up. So how that spoke could possibly be holding the hub up completely eludes me.

spokes in bike wheels (at least standard wheels and not carbon) act in tension. They offer no support when they are loaded in compression.

think of a rope and rock climber. The rope only works under tension, that is, being pulled apart at the ends. If you compress the rope, it offers no support until it has folded in on itself.

check this out: emergency repair kit for a broken spoke.
http://www.peterwhitecycles.com/fiberfix.htm

Nobody has suggested that spokes in a normal bicycle wheel act in compression.

yes, you did.
the spoke would have to act in compression in this situation.

No, that is wrong.

no, you're wrong. :rolleyes:

desconhecido is 100% correct.

I have no idea why this is so difficult to understand. Someone looks at a bicycle wheel and thinks, "when a load is applied to the hub, it must be the spokes above which bear this load." As a first thought, this seems reasonable. Perhaps at first it seems to be the only description which makes sense. But, when the forces acting on the spokes and rim are actually analyzed and tested, when changes in spoke length, rim shape, and the tensions in the spokes are actually analyzed and evaluated empirically, it becomes obvious that the upper spokes do not change significantly in length and do not see a significant increase in tension. What can actually be observed is in contradiction to what seemed to be the only explanation that made sense. How can what is observed be reconciled with the only explanation that makes sense when there is a contradiction? I suppose the answer is "turtles all the way down."

Rofl.

Since we're having so much fun discussing whether losing pre-tension is comparable to acting in compression - why don't we continue with an equally invigorating and rewarding discussion about the difference between deceleration and negative acceleration while we're at it?

well, all I know for sure is that the rim acts in compression and the spokes act in tension. Neither functions properly in reverse, unless they are carbon.

to say that the spokes next to the contact patch on the ground support the wheel through compression is silly.

So what's your beef? is it with the spokes between hub and ground seeing the biggest change in load, or with the admittedly clumsy turn of language stating that the hub is standing on the spokes?

If it's the former there's nothing I can do, and if it's the latter why don't you try to come up with a more fitting phrase? I suppose you could say "the hub is hanging from all spokes simultaneously , but it hangs the least from the bottom spoke(s)". Doesn't exactly roll of the tongue either.

Just got back from work in time to propose the correct language:

"The hub is suspended by the sum of the vectored forces in the spokes. This sum is always zero in the non-collapsed wheel."

------

As to the "what's your beef" question posted by Dabac above:

1. My beef is not with the fact that the bottom spokes experience the biggest change in load. This is true. I have no idea why anybody would have a vendetta against this perfectly normal result of physics.

2. I have a very big problem with the incorrect use of language in the context of structural analysis, in particular with the use of the phrase "standing on the bottom spokes". Not only is "standing" not a regularly used word in this field, it's connotation is the opposite of what it is intended to mean. This word cannot be used to describe a tensile member. When I stand I am supported by my rigid legs. If I'm hanging off a tree branch from my hands, and someone is trying to pull me down to the ground with a rope tied to my ankles, I am not "standing" on that rope. Even if a monkey suddenly jumps onto my head, causing a decrease in tension in the rope, and a smaller (since I have two arms and the force is divided between them) increase in tension in each of my arms, I am still not standing on the rope. That's absurd. Substitution of the correct language with "whatever language makes sense to me" is not trivial, in the same way that you cannot invent your own random mathematical symbols or chemical syntax. Judging from everyone's posts, the misuse of the word "standing" has clearly created a great deal of confusion.

Yep.

But this example is a bit misleading: if you look at the various measurements and FEAs you find that the increase in tension on top does not, even when summed across all spokes, come close to equaling the decrease in tension at the bottom.

There are two equal and opposite forces on the wheel: the force on the hub downwards, and the force from the ground on the rim upwards.

A wheel stays on the ground; it does not accelerate vertically. If you only think about the force applied to the hub, you've only analyzed a wheel that's under constant acceleration, as if you attached a rocket engine to the hub of wheel floating in space. You have to consider both the load on the hub and the load on the rim.

In your example you would have not just a monkey jumping on your head (load on the hub) but another monkey on the ground pushing upwards on the bottom dude's hands (the one who's pulling down on your legs) with force equal to the weight of monkey 1 (load on the rim). Draw that one out on paper, and you will see that the bottom rope loses a lot of tension and the top two ropes don't gain any tension!