bjtesch

Cheapest on Amazon right now is $21 + shipping. I might buy a new one anyway just for grins.

I did a quick glance at Ian's analysis and it looks pretty good from an engineering standpoint. I'll have to study it later.

bjtesch

I think I could explain this even without having read the book. Of course there is the matter of weight but I don't know if this is significant or not. The other is the matter of elasticity. I've read that there is a limit to how much tension you can get into a spoke due to the limits of the strength of the nipples, friction, etc. So (I'll pull some numbers out of the air) if you have a straight gauge spoke, lets say 2.0mm x 300mm long, and you manage to get 200 pounds tension in that spoke, it will stretch about 0.017". Now if you tighten a double butted spoke, 2.0/1.8/2.0 to the same tension it will stretch more because you actually have higher stress in the thinner center section, spproximately 15% or 20% more. You can see that this amount of stretch isn't very much in either case. If you put load on the wheel, hit a bump, whatever that tends to add load to the spokes on the bottom, if they shorten close to 0.017" then their tension goes to zero. If they are double butted then they maintain their tension better through the same amount of shortening. If your wheels aren't laced tightly enough then there is less lengthening in the spokes but again the double butted spokes lengthen more than the straight gauge spokes and still give you some advantage. Does Brandt say anything similar to this?

rydaddy

It's apparent that you never bothered to read any of this.

dscheidt

More or less (I don't own a copy of the book, though I've read it.). If I remember right, he claims hat swaged[1] spokes (he's pedantical that way) don't increase the absolute strength of the wheel (what load it can support before the bottom spokes come out of tension and collapse), but greatly improve the fatigue resistance of a spoke by putting the strain in the swaged section, and not at the elbow.

[1] what everyone calls butted spokes are really swaged. Butted would mean that the thick sections are made thicker, when they're actually the nominal diameter of the spoke, and the thin section is formed by reducing its diamter. This makes little difference to anyone who isn't an engineer.

Road Fan

Again, we're not really talking about the static compression of one or several spokes. Due to the spoke tension when assembled, we're talking about the bike and rider load reducing the tension of the bottom few spokes, while the tension of the remainder of the spokes increases a small amount. The bottom spokes do not ever go into actual compression in a well-built wheel. You are correct that in that case they would buckle.

Road Fan

Yes.

Road Fan

Yes.

DannoXYZ 

It's important to note that with two pre-stretched rubber-bands, when you turn the board into the vertical orientation, the amount of sag that occurs is less than what would occur if you had a single rubber-band carrying all the weight. Note the title in article wroomwroomoops post#2 - Bicycle Wheel as Prestressed Structure. The mechanical effects are VERY different than un-stretched compression-only wagon-wheels. A spoke and nipple is in effect a bolt and nut. Tension is what makes it all work. Check out this site for some basics on bolted-joints: https://www.boltscience.com.

DannoXYZ 

I believe it has to do with beam-bending chords. The rim has bending forces propagating in two opposite directions. All the loads actually go through the rim and depending upon how the rim is mis-shaped away from perfectly round, the spokes then respond. There are also compression-forces along the circumference as well and the models I've seen doesn't take that into account.

They calculate spoke-stress as if the spokes are hanging from the rim and pulling it inwards and they start the loads at the hub. But it's actually more accurate to model it from the rim's contact patch first. If you take the space between two nipples and flatten it from a curve, you'll see that the distance between the two nipples actually increase. This pushes outwards and tries to increase the rim's circumference and the rim will expand outwards above to maintain the same circumference. The models actually need to do their calculations based upon the rim expanding in response to load, rather than the hub moving donw.

desconhecido 

See this link for $13:https://www.amazon.com/gp/offer-listi...condition=used

mike_s

It's apparent you don't know squat about physics.

bjtesch

Thanks. I don't know why this didn't come up in my search.

dscheidt

This may be the funniest thing I've read on BF.

bjtesch

I don't have "the book" but I've looked at what Ian has on his page. He doesn't go into much detail about his FE program or what elements he used but since he talks about the rim properties he used I'm sure he used beam and axial elements for the rim, which would consider the bending and the axial loads along the circumference. These are pretty basic elements for this type of analysis, I wrote software back in 1977 that would perform this same type of analysis.

sdean911

There are engineers that graduate at the top of their class and there are engineers that graduate at the bottom of their class and guess what? They all think they know what they are talking about. There is no possible way for the bottom spokes to carry any compressive load because the nipples are simply not attached to the rim. If you remove the rim tape and cut a bottom spoke at the hub the spoke would fall on the floor. It cannot support any compressive load at all because if it did it would be pushed right through the tube. You can use all the superposition finite element mumbo jumbo you want but if the constraints are not put into the computer properly the results are nonsence. Lets assume for a moment that the nipples are attached at the rim. Even then the spoke is too long and slender to support any real weight and would simply buckle. The rider is supported by the top group of spokes. The weight distribution on the top spokes is a function of the angle difference from vertical. The more virtical the spoke the higher percentage of load it takes. The spokes from 3 to 9 o-clock do practically nothing except help keep the rim round and are under tension the whole time they are doing that. Yes the tension on the bottom spokes decreases when the wheel is loaded but in no way are they ever put into compression. Half of you get it. The rest of you?

Greg

desconhecido 

Amazon is a little weird in how it "sorts" used books. When you search for the Brandt book, it gives you a list of available choices one of which is to view "other formats." That's where the lesser priced ones appear. In this case, I assume that the original list is for a particular printing of a particular edition and the other formats are other editions. Sometimes when looking for a book on Amazon that is currently in print only in paperback, you can find almost new or remaindered hardcover editions of the book for a lower price by looking for "other formats." Sometimes the other editions will show very low priced hardcovers that are ex-library books. Of course, condition can be an issue with the former library books.

desconhecido 

Jeez, just when you think it's safe, another one of them shows up. This is like whack-a-mole, Groundhog Day, and "deja vu all over again" all rolled into one.

Look at this site: https://www.astounding.org.uk/ian/wheel/

Or read this abstract: https://scitation.aip.org/getabs/serv...ifs=yes&ref=no

Or read this paper:https://www.duke.edu/~hpgavin/papers/...heel-Paper.pdf

or read the Brandt book. There is no debate about this among people who have actually studied the question.

bjtesch

I graduated summa *** laude. Where does that rate?



My daughter and I used to love that "whack a mole" game at Chuck E Cheese.

rydaddy

:facepalm:

FBinNY 

Well, I guess that the only thing we've proved is that you can lead a horse to water, but that won't get him to taste the Kool-Aid.

At this point it might be time to give this horse the last rites ---.

Carbonfiberboy 

Ooooh, what fun! Here we go again. This is a fun game. Interestingly, and as intimated by Danno, the spokes with the highest load are those to either side of the contact patch, which makes total sense.

But back to the OP's question . . . or what I think he was getting at . . . my low spoke count Rolf's have a lower spoke tension than my 28H, 32H, and 36H wheels. Why? Because the rim is so stiff. The rim above the contact patch doesn't deform much, so the spokes don't get much shorter, hence they don't need to be stretched as much to avoid fatigue. Also, the tension on the spokes at the top of any wheel doesn't increase much in the loaded state over the unloaded. Also, butted (swaged) spokes make a longer lasting wheel than straight gauge, because you stretch them more in the initial build.

So it's not the number of spokes that makes a strong wheel. It's the design of the wheel as a whole, hub, flanges, spokes, rim. My Rolfs require fewer adjustments than my conventional wheels, though I still like conventional wheels because I can build them myself.

mike_s

Very true.

Those claiming the spokes on the bottom support the weight are exactly like politicians who claim that because they only raised your taxes $50, instead of the $100 they wanted to, you're getting a $50 "tax cut."

If you redefine your terms enough, you can claim anything, however nonsensical, to be true. They're like Humpty Dumpty:

rephrased:

eddy m

The answer is the wheel needs all the spokes. There is no dispute among people who understand pre-stressed structures that all the spokes are needed to maintain the pre-load tension, and there is no dispute that the greatest change in tension, the only change large enough to lead to eventual failure, and the change in tension that provides the greatest lift at the hub is the decrease that occurs as the spoke rolls through the bottom. Calling that change "compression" makes perfect sense to me and is commonly accepted among engineers. If you prefer to call that change "reduction in tension", that is a dispute about language, not about physics.

The graph in post #8 above is consistent with what's in Brandt's book, and with the conclusions every peer-reviewed study I have ever seen. If you believe it is not an accurate description of the change in tension as a wheel rolls, you need to back up that belief with something better than your own intuition.

em, p.e.

mike_s

Nope (at least not for a static wheel subject to only vertical load). Load your wheel, then use wire cutters to cut out the 2 (or 4) at the bottom. If they're supporting the load, the wheel will collapse. I await claims of how the wheel is now being supported by non-existent virtual spokes.

FBinNY 

This has gotten worse than a chain lube debate, but mike S proposed an interesting experiment, and it warrants some thought. He's absolutely right that the wheel won't collapse because of the loss of support from the bottom spokes, but if done carefully on a conventional properly tensioned wheel with typical axle loads, he's in for a surprise. Not only won't the hub drop a bit as the lower spokes are cut, in fact it'll rise, as the tension below is relieved by the loss of those spokes.

The opposite of this has been experienced be everyone who's ever broken 2 or more adjacent spokes, a hop in the rim, not a low spot but a high spot.

Unfortunately the debate here has been compromised by sloppy language such as the use of the term compression when reduced tension is meant, but its a simple fact that wheels are tension structures and the the balance of tension forces (only) changes within the wheel to offset external forces on the hub and rim.

Simply put for a stationary loaded wheel there will be an area of higher tension above and lower tension below. The exact distribution of the tension changes depends on the rigidity of the wheel. With a very rigid rim the distribution will be more or less symmetrical above and below, but with a more flexible rim the changes in tension will be like the graph shown above with a large reduction in tension for the few lowest spokes, and a slight increase in tension distributed among the others.


BTW- if mike s's experiment is repeated with a few cut spokes cut at the top he wheel won't collapse either. We don't need to do the experiment because it's been done. Folks have ridden wheels missing spokes for years, and they go round and round with the missing spokes at the top, bottom and sides and manage to stay together despite they're lack of training in physics.