LV2TNDM

[QUOTE=davidad;21393964]
This is irrelevant! The shift cable argument has a little merit because of low derailleur tensions, but brake cables? You really want to make this argument about brake cables? Why would you think a brake cable doesn't reach as high a tension? And even if it only reached 25% of a spoke at 120kgf, it's still a completely relevant analogy. In fact, it's perfect because it exemplifies the fact that brake (and shift) cables ONLY FUNCTION UNDER TENSION! Push on a brake cable just .001 pound and it buckles. Put 1,000 lbs. on a brake cable and it's fine.


Jesus, this is tiresome!


And just for fun, I decided to MEASURE BRAKE CABLE TENSION! I had a pretty strong suspicion that a brake cable would see tensions similar to a spoke...


... and I was right!


Here goes:


Front brake max tension achieved with Deore brake levers and 1994 Deore XT cantilevers: Approximately 440 lbs!


I rigged up a jig to measure spoke deflections for my IceToolz tension meter a few years ago since their rudimentary deflection chart covered only 3 spoke gauges, much less Mavic aluminum bladed spokes and the myriad others on the market. Plus, I had no idea how well calibrated my meter was after using for several years. This eliminates any and all uncertainty.


So I drew up a quick chart ranging in different tension values for this cable: 20, 50, 90, 100, 110, 120, and 150 kgf tension values. (really low values, typical spoke range, and a high value) I then put my meter on the front brake cable and pulled as hard as I could (simulating a MAJOR emergency braking scenario) and went above my anticipated tensions. 150 kgf was measured at 3.45 units of deflection. My gauge hit 4.20 when pulling hard on the brake lever. Given my chart and values, I'm pretty sure this is over 200kgf, or 440lbs. of force. (Sorry, I don't feel like re-rigging my jig with the brake cable again. I really didn't think I'd exceed 150kgf [330 lbs.]. Well, I obviously did.)


So in rebuttal, brake cables REGULARLY* see tensions equal to and above spoke tensions. Matter settled.

*Specifically, FRONT CANTILEVER brake cables. V-brake cables probably see less, given their greater leverage at the brake arm.

LV2TNDM

In no tension wheel does a load "push on the spokes." As Brandt clearly outlines in his book, the load PULLS on all of the spokes above the four to six spokes instantaneously at the bottom position on the wheel. The hub is "hanging" on these spokes, and these spokes rotate through a cycle of very small tension increases (among 26-28 spokes in a 32 spoke wheel) while the 4-6 spokes on the bottom see a larger tension decrease. But if the wheel is properly tensioned, this loss is nowhere near bringing them to zero tension. And they cannot be "pushed" on as they would simply push through the rim (as pointed out by an earlier poster).

Stop using "compression," "push" or any other synonym in a tension wheel when referring to spokes! It is inaccurate and incorrect!

davidad

I've had the book for 20 years. Which page is that exact quote on?

davidad

Look on pages 11 and 12 of Brandt's book.

I have violated rule number again.

LV2TNDM

I reread pp. 11-12 and sure enough he says, "Similarly, because it is prestressed, the wheel can stand on its bottom spokes. Stress changes occur only in the bottom spokes, not in the top ones. Structurally, bottom spokes are acting as compression members in the wheel, and no measurement of their elastic movement reveals that they are anything but rigid columns."


So you are correct, he does write this. However, I think he's trying to explain the wheel's tension and dynamics within the context of prestressed concrete on p. 11. And ultimately, I think this confuses the argument for the layman. Sure, a civil engineer may appreciate the analogy, but it doesn't do me any good. (Hmmm, perhaps I should let the civil engineer who specializes in concrete who lives upstairs take a look at it!)


He prefaces one sentence with "Structurally, ..." so I interpret this to mean "non-literally." He also says, "...acting as," not "actually ARE." I think this clearly shows he's making an analogy here to explain how loads are distributed throughout the wheel.


Why? Because there is NO WAY the bottom spokes can literally be under compression. How could they? If any compressional load acted on the spoke from the hub, pushing downward to the rim, first, the spoke would reach zero tension, and second, the spoke and nipple would push away from the rim hole (or eyelet). It is simply IMPOSSIBLE for a spoke to be compressed between a hub and traditional rim. Plus, elsewhere in the book, he discusses tension/zero tension spoke cycles in wheels built with insufficient tension, especially the NDS spokes in rear wheels. And he further explains sufficient tension is needed in rear wheels to prevent NDS spokes from reaching zero tension values during rotation. So given this, he's admitting that no spoke in a properly-built wheel ever goes below positive tension values. Thus, they CANNOT be under ACTUAL compression.


And on p. 10 he finishes with, "they are being shortened and respond structurally as rigid columns. They are rigid as long as they remain tensioned." I really do not understand the purpose of this statement. It's clearly self-contradictory because a spoke acting as a "column" does not actually support anything under actual compressional forces. It could really only be called "slightly-reduced tension" forces as a column. Again, I don't really know why he went to such great lengths to confuse the issue.


Or he's explaining things in accurate engineering theory speak and it's over my head.

SethAZ 

I've listened to a lot of Hambini's videos. He's very opinionated, and definitely irreverent, and most certainly will unleash abuse and scorn on various bike and bike part companies that goes beyond what society normally would find acceptable. I listen anyway because he does say some things I think I learn from, and also, I just dig that accent. Hehe.

scott967

At the risk of jumping into this thread mid-stream, I think you are considering the net forces on the spoke, while what you referenced is only the additional sinusoidal force applied as the loaded wheel rotates. Clearly when the wheel is built there is a tension force on the spoke. What I get from what you quoted is that the tensioned spoke acts as a column WRT to the ADDED compressive force so when considering the effect of the compressive force, treat the spoke as a column. Again, it says nothing about the net force, which has to be tension.

scott s.
.

LV2TNDM

Yes, I think he's considering it this way. As a whole, the wheel undergoes compression and its resistance to that compression is what holds the bicycle by the dropouts. I think it's an unnecessary simplification that serves no purpose. Of COURSE the wheel is opposing the compressive force of the dropouts on the hub. Otherwise the bike wouldn't function. The downward force of gravity is being opposed by an equal and opposite force from the wheels. But the way the wheel achieves this is not by compression. He explains earlier in the book about how a wagon wheel IS a compression wheel and acts completely differently than a tension bicycle wheel. (Unless, of course the wagon wheel's spokes are so well attached that the top spokes undergo tension forces as the bottom spokes undergo compressive forces, thereby sharing the overall load or force. Then it gets more complicated.)

But that doesn't happen in a bicycle wheel. Tension wheel spokes, as almost always configured, cannot tolerate individual compression loads. They can only go to zero tension and stay there. They cannot be compressed because the are not supported in that direction - they simply push out of the spoke holes or eyelets.

This, again, is why his discussion makes no sense to me. Again, I may just be incredibly stupid. My wife tends to think this

davidad

ndexHomeAboutBlogFrom: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Weight of a bike
Date: 22 Aug 2000 00:38:48 GMT

Alan Hosker writes:

> Is the weight of the bike suspended on the spokes above the hub or
> is the weight pushing down on the spokes below the hub? Intuition
> tells me that it is hanging down from the upper spokes but I read
> once that wheelbuilding experts believe that it is pushing down on
> the spokes below. I find this hard to understand. What do you
> think?

First you must define your terms. Obviously you can't stand any load
on a wire as long and thin as a spoke, that is unless you pretension
it. That is why a bicycle wheel is tensioned and tensioned fairly
highly, almost to the yield strength of the rim. So in fact the hub
is hanging from all the spoke in one sense, since they all pull on it
in all directions. How a wheel responds to loads is not a belief but
rather a straight forward structural analysis and, as you point out,
not obvious. That is why the bicycle wheel remained misunderstood
until the mid 1960's... and then the analysis was rejected by people
who should have known better. It is still rejected by many engineers,
people who refuse to accept all results that conflict with their own
perception.

However, structurally the pretension can be analytically ignored
because it is only there to prevent the wires from buckling in
compression. Other than its tension the bicycle wheel is the same as
any other wheel such as a wooden wagon wheel. The spoke under the hub
is shortened in compression by a load on the axle, while the upper
spokes remain unaffected by the load. If you watch a dynamic display
of a structural analysis program on your screen, there is no way of
distinguishing between a wire spoked wheel and one with thick
compression members. That leaves it up to the observer to come to
terms with the description that "the wheel stands on the bottom
spokes".

You can test this by plucking a front wheel spoke (near the nipple)
before and after you put weight on the handlebars. You'll find that
the only spokes to change are those about the tire contact patch on
the floor. These spokes are compressed and lose tension. If the load
is great enough, they will become slack and the wheel can collapse
sideways. In any case, a wheel can only bear loads that do not
consistently slacken the preload.

> I apologize if this has been asked before.

It gets asked regularly but most of those who ask also give a
dissertation on how it REALLY works, except that they are always
wrong... otherwise they wouldn't have asked in a contentious manner.
That is what usually makes this such a testy question. As you see,
you got straight forward answers, in contrast to the last times this
came up, where the name calling began in the question.

Jobst Brandt <jbrandt@hpl.hp.com>

Your wife is a wise woman.

guy153

There is actually a company (in Canada I think) who make string spokes.

Nice video about them here:

LV2TNDM

Brandt says, among other things, "structurally the pretension can be analytically ignored..." and "These spokes are compressed and lose tension."

OK, not that I'm pretending I know more than Brandt here (insert your witty quip here), but if you "ignore" the pretensioning of the wheel, then you're simply ignoring the most important part of the wheel, its tension. It's the single most important aspect of a bicycle's spoked wheel.

And It's simply impossible to literally COMPRESS a spoke as Brandt describes. Yes, it loses tension when the axle sees loads compressing the wheel. But to consider the resultant forces on the bottom spokes as compression is, in my opinion, a misnomer. They remain under tension, and very high tension.

I think what Brandt is doing is, by ignoring the pretension, effectively re-setting the tension values in the wheel to 0.00. Then, when a load is applied to the wheel, the bottom spoke tension drop slightly, yet into a negative tension value. Negative tension is compression, ergo compression.

Given this, there is no further need for argument or citing further explanation by Brandt. I understand what he is saying.

I will take the stance that this is a silly explanation of wheel dynamics (again, me being stupid again). But since there is no ACTUAL compression on the spokes (otherwise, they would protrude through the bottom of the rim), I refuse to call it compression. I guess I'm just difficult (wife quip please).

But thanks for bringing this up. I was obviously wrong as it relates to Brandt's discussion in the book and on the topic. I understand the theoretical approach Brandt is taking.

davidad

It is not theory, it is fact as he has stated.