Originally Posted by
MassiveD
What are you talking about? If parts break, and they are replaced, how does that prove they were not necessary to the strength of the wheel? If they weren't they wouldn't break. Any of the parts mentioned can be replaced and you will have a fine wheel. You can and should replace spokes and drive on. There is something seriously wrong if too many spokes break because on a properly built and structured touring wheel, the spokes shouldn't break. But if you got the one Peter White wheel where there were 6 bad spokes from the factory, or whatever, just replace them, and all that, and drive on.
You completely missed the point. If a rim wears out or a rim cracks, I doubt that you would find a competent wheelbuilder anywhere who would have a problem with just replacing the rim. Break another rim and a competent wheelbuilder will just replace it. Break a dozen rims and the same thing will happen.
But, break one spoke and a competent wheelbuilder will tell you to watch the wheel. Break two and they will tell you to watch the wheel closely. Break three and any competent wheelbuilder will tell you that the wheel should be replaced. If you break 6 spokes, a competent wheelbuilder will tell you that you can replace the spokes yourself and you can live with the consequences.
Even properly built and tensioned wheels can break spokes. Stuff happens. We ask a lot more of our wheels when we tour than just about bicyclist except downhill mountain bikers.
Originally Posted by
MassiveD
AS far as which part is more important to be carefully sourced, obviously it isn't the spokes, despite your endless (if here restrained through the four pages of the thread) pumping of Alpine III spokes. Commercially most cyclist have no idea what spokes they are riding, compared to what rims or hubs. There are many good makes of spokes, and you can be pretty safe getting no name spokes on a touring bike from a reputable source. No way on earth I would get no name rims or hubs, though partly as a mater of value.
Therein lies the problem. People are very, very careful about choosing the rim and the hub and then pick just any old spoke because they don't understand the dynamics of the bicycle wheel. Spoke breakage is something that comes up over and over again on the Bike Forums while rim failure and hub failure don't. Choosing a spoke that is up to the job instead of concentrating on just the rim and hub would go a long way towards solving that problem.
Originally Posted by
MassiveD
But more seriously, you need to choose a proper rim because it is more likely to break or let down your wheel than the spokes. When this happens it sometimes shows as a broken spoke, and at other times shows as a broken rim. When the rim needs to be replaced, to conserve the spokes one has to replace the rim with a similar size and geometry rim, so you need to choose your rim carefully, but your spokes, meh.
When was the last time you broke a rim? When was the last time you talked to someone who
had broken a rim? I see hundreds of wheels per year at the co-op where I volunteer. The ages range from brand new to some that are dragging 40 years. They are old generic wheels, new name brand and everything in between. With the exception of rims ruined by impact, the number of broken spokes vs broken rims runs about 1000 to 1. I can't recall actually seeing any broken rims.
I, on the other hand,
have broken some rims. I've had several that cracked, a few that have worn out the brake track and a few that have pull a spoke though the rim. It's no big deal to replace the rim and I don't give it a second thought. Broken spokes, on the other hand, are something I give second thoughts to...I even give them 3rd or 4th thoughts.
Originally Posted by
MassiveD
As you say, that is all very simplistic, or sophomoric stuff. You aren't arguing with Jobst, if you make those points. You are bouncing between your understanding of his work, and your simplistic models. Better you than me since I don't get his work either. .
I have not said that this is very simplistic nor sophomoric stuff. You are just saying that as an ad hominem in an attempt to discredit me without actually addressing what I have said. It's extremely complicated and difficult to understand. You don't even seem to understand what I said. Go do the experiment that I suggested or even just look at a wheel. You can't "compress" a wire spoke because there is nothing to compress it against nor is would the spoke stand up to any compression without buckling. See BobG's picture.
Originally Posted by
MassiveD
Jobst's model is as follows (I hope). He, who designed the spoke tensionmeter that is the benchmark of the industry (if little used), measured spoke tension of loaded and unloaded wheels. His observation, which is presumably repeatable, or disprovable, is that the only change in tension occurs in a few spokes under the hub where tension decreases. That is not consistent with the as often as you like to repeat it "fact" that the hub hangs from the rim. If so where are the loads.
Where's Brandt's model? All I see is an appeal to authority. Yes, the tension only changes in a small number of spokes where the rim is deformed upwards. It changes in some of the spokes before the contact patch and after the contact patch. It's a stressed structure that spreads the load around the wheel. By the time the load has spread around to the spokes on the top of the wheel, the amount of load that is shared is minuscule and probably unmeasurable. Anyone who builds wheels should know that there is a lot of elasticity in a wheel and what you do in one area doesn't necessarily have a reaction on the opposite side of the wheel.
Originally Posted by
MassiveD
The contention is that in a pre-stressed structure, this change in spoke tensions is what one would expect if the wheel was supported by the ground through the spokes that are between the ground and the hub. One can say the spokes are compressed, or that the wheel stands on the spokes or whatever is in fact the correct terminology when dealing with pre-stressed load paths.
The wheel stands on the ground. The rim, as alan s has said, stands on the ground. But the spokes are not in contact with the ground nor is there any mechanism by which you can put pressure from the ground on the spokes. Brandt calls the spoke a "column" that supports the hub but you have to have a base for a column to work. There is no base on a spoke in a wheel. The end is free to move.
Brandt and others use the idea of "compressing the spokes" too loosely and incorrectly. As I've said above many times, compression is a force that works in an opposite direction than tension but it is not necessarily the opposite of tension. You can compress something or you can put it under tension but if you remove the compression, you aren't placing tension on the object and vice versa. Further, no matter what Jonathandavid says, Brandt's "compressed" spoke is still under tension and you can't have something that is simultaneously being compressed and being tensioned. They are in fact opposite forces and mutually exclusive. Tension or compression, take your pick but you can't have both.
Originally Posted by
MassiveD
Disproving Jobst is basically a two step thing, make credible observations that reveal different results; or describe what is going on in the affected spokes by some other explanation than the one he gave. Saying stuff like that you can't push on a spoke just shows you are arguing with yourself.
More ad hominems. I don't care if you like me or not. Address the question and leave the personality out of it. If you have a better explanation or you can show me how a spoke that is floating free of the hub can be pushed on so that it compresses, do so.
I have made observations and have given a different explanation of the one he gave. You can do it too. Go get a wheel or even just a spoke. Try to put it under tension and compression at the same time. Better yet do it with a piece of string. The results are easier to see with the string. Pull it tight between your hands, then try to push your hands back together while keeping tension on the string. The results will be clear very quickly.
Originally Posted by
MassiveD
Except in that case there would be changes in the tension of every spoke in the wheel, not just the ones directly between the hub and the earth. If you have carefully measured data that shows changes in spokes other than those the wheel is standing on, you have an easy win.
alan s has already answered this one.
Originally Posted by
alan s
Easy win. The wheel is standing on the rim, not the spokes. The slight detensioning of the lower spokes is due to rim compression and deformation.
Edit: Looking at the front end of a bike, the load of the rider, gear and bike compresses the fork and axle/hub, tensions the upper spokes, and compresses the rim all the way around to the ground. The lower spokes are detensioned slightly, and mainly serve to keep the wheel as round as possible and resist lateral forces. The rim compression and slight deformation is concentrated at the point where the wheel meets the ground, which accounts for the noticeable spoke detensioning there.
Well said.
Originally Posted by
Jonathandavid
That doesn't really invalidate Brandt's point, just looks at it from a different perspective. He doesn't say that nothing happens to the rim or that it remains rigid.
Yes, it does invalidate Brandt's point. If you look at "Ian's" analysis, you can see that there is a large decrease in the tension when the rim is at the bottom. Nothing surprising about that. Where they both go wrong (and you along with them) is considering that decrease in tension to be an increase in compression. Once you make that mistake, you
can say that the wheel stands on the spokes. The problem is that Brandt and "Ian" didn't check the model against the observation. I see it happen all the time where people trust their model and say that the world is wrong. The opposite is
always true.