SEE: Context.


nitpicking and hairsplitting aside , Yeah I get it. The statement about who is pushing on what has now been taken out of context. I understand the point you all are trying to make. And I agree, I was trying to make a point earlier.
Now, do we play another round of semantics. or can we move forward with our lives?

Please go another round. I'm no done with my popcorn yet.

:thumb:

I have a whole box of microwave popcorn, but no microwave. Keep going, this could take a while.

This pushing and pulling thing is driving me crazy. I guess it means that the tire above the wheel is holding up the wheel and not the bottom. Now that is something to talk about.

Talk about timing.

Alright eff it, some of the stuff on the link, the explainations, make some sence (and make my head hurt) but this notion of the bottom spokes losing tension being responsible for a bike wheel working is driving me nutz. Come on rydaddy throw us out some knowledge, 'splain it for us.

The claim that : 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.

As I see this, when the rim flattens at the bottom this moves one of the anchor points of the bottom spokes closer together there by reducing the tension in those spokes in question. This is the end of the reaction to a load placed on the hub which is connected to a rim by spokes , the spokes on top of the hub pulls the rim above it down in the same direction as the load.
OR
this detensioning of the spokes on the bottom is just the first of a set of actions, and reactions, that takes place in a wheel under load. So it gets all the blame for the reactions that follow, and everything that follows "causes" the rim tobe able to support a load?
Acourding to dannoxyz, its this deformation at the botom of the rim that creates an minor expansion of the rim in the rest of it circumfrence that increases the tension on the rest of the spokes that the hub is attached to.
And one more thing, since the spokes at the bottom are slack and all the other forces in the rim were equal when we started, why doesnt the spokes on the top, pull the hub out of the center of the wheel since the tension is now greater on top than the bottom? Edit: the answer to my own question is the load placed on the hub hold it in place. lazerx said this yestreday

I tried to stop. but now I cant. When I have my stroke I will hold all of you accountable, wait, is that a nose bleed.......

Post 8, 9, and 10 in that thread are really all you need (summed up brilliantly in post 9 of this thread, by the way ;-)

In particular, the graph DannoXYZ posted and this from FBinNY:

"... it isn't the bottom spokes that hold the hub up, nor is it hanging from the top spokes. It's the failure of the lowest spokes to pull the hub down as hard that's the key,"

which properly deals with the semantics argument.

Ok riddle me this, lets suppose that the rim is infinitly strong and will not deform. What happens now? does the wheel collapse because the rim wont deform?

This is what I think:
For a wheel spoked radially, if the rim were not to deform at all, the increase in tension of the spokes at the top would equal the decrease in tension in the spokes at the bottom. The hub would be slightly closer to the bottom of the rim and the tension in every spoke would increase or decrease in proportion to its length change as a result of the new position of the hub slightly below the center of the rim circle. In this case it would be proper to say that the increase in tension in the upper spokes and the decrease in tension in the lower spokes support the load equally. Every spoke would have its vertical tension component change slightly and could be said to support the load as a result of the amount of change in the vertical component of its tension. If you were to add up all the vertical components of tension change (and these components have direction, either up or down--positive and negative) you would find that the absolute value of this sum would equal the load added to the hub.

Thinking about this a bit, it might help shed some light on the problem of an actual bicycle wheel. When you load an actual bicycle wheel, if you add up all the changes in the vertical components of tension in all the spokes, allowing for the fact that some changes are positive (upper spokes) and some are negative (the bottom spokes), this should equal the load you added, should it not? If you compare the amount of tension change in the bottom spokes (which is a bunch) to the change in tension of the other spokes (which is very snall) you see that the change in tension of the bottom few spokes is the dominant tension change.

Something else to consider: spokes in a normal bicycle wheel stretch and contract elastically -- analgous to the way springs expand and contract in response to changes in tension. Springs expand and contract in accordance with a very simple linear mathematical expression called Hooke's law. From Hooke's law and not very much knowledge of elementary physics you can calculate the change in potential energy that results from changing the tension in a spoke. Do this for the spokes in a bicycle wheel and I suggest that you will find that the bottom spokes have a significant change in potential energy and no other spokes do. Energy can be described as the capacity to do work -- this suggests how a decrease in tension can do the work of holding up the load applied to the bicycle wheel hub.

I think it's great that you actually took the time to make sense of it. Most naysayers won't take the time to try and understand it. I'm not the best at explaining this stuff but what DannoXYZ wrote is about the best short description I have ever read. Now you might understand why double butted spokes make for a more durable wheel. They stretch more under a given tension, thus allowing a greater rim deflection before going completely slack. Spokes do not like going slack. They will fail by fatigue much faster, which is why I think it's important (as a wheelbuilder) to understand how spokes are affected for various types of loading.


You'll have a very durable wheel. As in, the spokes won't fatigue due to radial loading. They will, however, fatigue from drive torque and braking.

EDIT: I like the description above, actually. The all spokes will absorb the load equally, which would increase the fatigue life of the spoke but not make it infinite.

I've been thinking about this for a while and I'm not sure that I understand it. When you add a normal load to a normal bicycle wheel, it doesn't matter what the gauge of the spokes is nor whether they are butted or not as far as how the load is supported. You still have to have a net change in tension to support the load. If it is essentially the few spokes at the bottom that change tension significantly and if we neglect the changes in the tensions of the other spokes, which is reasonable, I think, for this purpose, we would see that no matter the gauge or butt status of the spokes, the tension change in the bottom few spokes will be roughly the same. The bottom of the rim will deform more with butted spokes or lower gauge spokes but unless there is a significant change in how the load is distributed among the spokes, the change in tension in the lower spokes will be roughly the same. Now for either 14/15/14 gauge spokes or for straight 14 gauge spokes, the deformation in the areas of the spokes most subject to fatigue will be the same for the same change in tension. Length change for a given length of spoke is determined only by the length of the section, the change in tension, and the diameter (area).

According to Brandt, with thinner spokes or butted spokes, the load is distributed over more spokes than with straight gauge (example 14 gauge) spokes. But, as best I can tell, he does not demonstrate this with any sort of modeling such as the very nice graph of deformation vs spoke orientation that was posted in the other recent thread on this topic. I will readily agree that with lighter gauge or butted spokes that the load distribution among spokes will be different, but I am not willing to agree that this change in load distribution among the spokes will be significant enough to effect the fatigue life of the spokes. In order to reach that conclusion we need to have demonstrated that the load will be redistributed significantly and that the lesser change in tension of the bottom spokes is significant enough to effect the fatigue life of the spokes.

Another claim that Brandt makes that I have trouble dealing with is his claim that straight gauge spokes can go completely slack and that when slack the nipples can unscrew because there is no tension keeping them from unscrewing. Yes, a slack spoke has no tension, but there is also no torque in a slack spoke (net torque being a result of the tension and the angle of the thread and friction between the threads). A slack spoke won't unscrew. Anyway, a well-built wheel will not have spokes go slack unless it is subjected to excessive load or impact and then, whether it's built with straight gauge or with buitted spokes, the result will be the same, I think.

That's my rant for the day.

Yeah, but its so counter intuitive I feel like a dog chasing its own tail.

It's not that bad.

There are many ways to look at this problem but one thing is certain: if you reach a different conclusion by looking at it in multiple ways, you're doing something wrong.

Here's another way:

We all know (I assume) Newton's second law which states that force is equal to mass times acceleration. Not everybody cares that this is Newton's second law, but everybody knows that if you push or pull on something it moves unless there is something pushing or pulling in the opposite direction. So, F=ma, and if there is no acceleration (that is, movement) all the forces must balance.

Now, look at a normal radially spoked bicycle wheel without a load. All the forces on the hub balance -- there's no motion. When you put a load on the hub and the wheel again becomes static, there is a slight increase in the upward force (tension in the upper spokes) but not nearly enough to counteract the load you added and keep the hub from accelerating. So, how come the hub doesn't accelerate? Because the tension in the lower spokes decreases in response to the load and keeps it from accelerating. Compare the increase in tension in the upper spokes to the decrease in tension in the lower spokes. The increase in tension in the upper spokes is not enough to keep the hub from acceleration, but the slight increase in tension in the upper spokes added to the great decrease in the magnitude of the tension in the lower spokes is just enough to do it. Does that help in understanding how a decrease in tension can act to support a load?

quote: desconhecido
Another claim that Brandt makes that I have trouble dealing with is his claim that straight gauge spokes can go completely slack and that when slack the nipples can unscrew because there is no tension keeping them from unscrewing. Yes, a slack spoke has no tension, but there is also no torque in a slack spoke (net torque being a result of the tension and the angle of the thread and friction between the threads). A slack spoke won't unscrew. Anyway, a well-built wheel will not have spokes go slack unless it is subjected to excessive load or impact and then, whether it's built with straight gauge or with buitted spokes, the result will be the same, I think. End quote:

I can throw even more wood on this fire. When I started building wheels I used butted spokes. I had a horrible time keeping them tensioned. A couple sets would have to be re tensioned every couple hundred miles. It is what led me to bf in the first place. I used spoke freeze, I marked the position of the nipples and spokes with a black sharpie and rode them, guess what? 225 miles the nipples had moved, not all but enough to lower my tension readings. The one thing I did notice is once it starts it progresses at an ever increasing rate. So last year I was riding my bike in a different state, I travel for work and was able to bring my bike with me, the rear wheel didnt feel right. Sure enough, the spokes were all slack again, read well below 100kgf on the drive side. So I went to the lbs and bought all straight gage spokes and rebuilt the wheel. This wheel has over 1000 miles on it now and is still just as tight as the day I built it. This led me to rebuild all of my rear wheels with straight gage spokes, and I havent had one wheel loosen, or go out of true in about 4000 miles total between about 3 sets. My latest set is 105 hubs, straight gage spokes, mavik reflx rims. They are without a doubt the stiffest wheels I own.
The above countradicts everything I learned about spoke selection except for spoke fatigue wich remains to be seen.

Re: butted spokes - good point... and I think my interpretation was a bit off. From Roger Musson's book...

"The thinner central portion of the butted spoke will stretch slightly when building and the cyclic changes in tension as you ride the bike will be in this region rather than generating high cyclic loads on the spoke elbow and threads which could result in fatigue issues - not just within the spoke but on the hub and rim spoke interface."

So in other words, the real benefit of butted spokes versus plain gauge is that the spoke elbow takes less abuse from cyclic loads.

As for the nipples unwinding when slackened... I have seen it happen. My friend recently had a nipple completely de-thread itself from a spoke on a rough descent during a road race. :eek:

There are a lot of variables involved in how a wheel will perform. Without knowing a lot of stuff, I don't think it's possible to say why you were having trouble with the butted spokes but not with the straight gauge spokes. I know from my experience that a wheel built with butted spokes can perform well over many thousands of miles. I've got some wheels that I bought from Colorado Cyclist back in 1997 and they are still fine wheels -- DT 14/15/14 spokes. Everybody should also know that it's possible to get well performing, strong, stiff, and durable wheels using 14gauge spokes. There are millions of wheels out there with straight gauge spokes that never give a bit of trouble. Now, there are many people much more experienced than I am who have built countless wheels that believe that butted spokes result in a better wheel and I do not have the expertise to contradict them, they are probably right.

Trust me I have been skirting the edges of this one in an effort to make sence of it. Doesnt the load play a part in balancing the forces?
I think where I have a problem is with the terminology you have used, for instance ; refering to somthing that is being acted upon, the bottom spokes, in a manner that would leave one to believe they had an active roll. But I think I have a glimmer of understanding at least the load path, and why.

Just so I can think about it, when you talk about counteracting the acceleration of the hub, you are talking about a downward direction in response to the load correct?
and finally, I think I understand your statement that, a decrease in tension can act to support a load. I mean I see the point you are making but to me it is an in complete statement. You are giving an awfull lot of credit to those 4 spokes. Couldnt I also say that the rims ability to deform and reform in response to an aplied load allows the lower spokes to unloadetc.etc. ? In my limited grasp of what is happening in the wheel this deformation seems just as important as the unloading of the spokes at the bottom. Or then is it because all this detensioning happens in such a limited amount of spokes?

No doubt happens every day. But I would still like to find an answer to the why . I am happy what I have done has worked but it is frustating that everytime I build a rear wheel with butted spokes the loosen. And yes everything else is correct, the right size nipples, correct tension etc.

That statement by Musson is the sort of thing that drives me nuts. The tension in a spoke is constant its entire length. Any cyclic change in tension in the thinner center section of the spoke occurs exactly the same throughout the length of the spoke and occurs at the rim and at the hub as well. At least for non-relativistic speeds. The only way his conclusion can be correct is if butted spokes result in significantly smaller cyclic changes in tension and I see no support for that. It very well may be true, I don't have the expertise to say, I haven't done the necessary experiments, and I do not do finite element analysis.

I believe that nipples can unscrew and I can accept that repeated cycles of very low, or zero tension, coupled with cycles of increasing tension can do it. I just dispute the claim that the nipples unscrew because there is not tension to keep them from unscrewing. Nipples don't unscrew in a tensioned spoke because there is no net torque in the unscrewing direction. The only way to get them to unscrew is to have the tension induced torque exceed the counteracting torque which is caused by the friction between the threads. Both the friction force and the force component of the tension which causes the unscrewing torque increase and decrease as tension increases and decreases butnot necessarily in the same manner throughout the possible tension range. So, it may be that very low spoke tension results in insufficient friction to counteract the unscrewing torque -- that explanatiion is possible and even, perhaps likely.

Man, I thought I was done ranting for the day.

So the load + the increase in the tension in the upper spokes is = to the detensioning of the spokes in the bottom of the rim? so the hub stays centered in the wheel ( - the rims deformation) due to this ballance of load/tension/detensioning?

I hear you. I don't claim to fully "get it" either. But try thinking about it from a strain standpoint. If the middle section stretches more, there is more strain there. When the spoke is unloaded, the strain in the center will be reduced while the ends are still under the same initial amount of strain. A double butted spoke is like 3 springs in series. Maybe this is the key difference? Sorry for fueling your rant!

"Just so I can think about it, when you talk about counteracting the acceleration of the hub, you are talking about a downward direction in response to the load correct?"

Yes, that's right. Just remember Newton's second law -- if the forces are not balanced, you're going to have acceleration -- i.e. motion. If the weight applied to the hub is not countered, gravity will pull it down.

"and finally, I think I understand your statement that, a decrease in tension can act to support a load. I mean I see the point you are making but to me it is an in complete statement. You are giving an awfull lot of credit to those 4 spokes. Couldnt I also say that the rims ability to deform and reform in response to an aplied load allows the lower spokes to unloadetc.etc. ? In my limited grasp of what is happening in the wheel this deformation seems just as important as the unloading of the spokes at the bottom. Or then is it because all this detensioning happens in such a limited amount of spokes?"

The behavior of the rim is very, very important. That's why I mentioned the section in the Ian site (it's referred to a couple times in this thread) that discusses indeterminacy. You can't thoroughly analyze what happens in a bicycle wheel just by looking at the spokes and tensions and hub and load without looking at what the rim does. The rim defines the behavior of the different spokes as load changes.

Oh crap! two more problems
the spoke streches in its entire length, not in the butted portion? I understand the heavyier portion is too support the elbow, more material but if I understand you you are saying the thick portion of the spoke streches as much as the narrow portion or is there a rate variation that occures?
The rattling spoke theary, the only part of this that I dont like is that none of my spokes ever tighten when they are unloaded, only loosen.

And the center section , the narrow portion is a slightly weaker " spring"?