Re: Rider's weight vs. spoke tension

On Sun, 22 Feb 2009 21:30:23 -0800 (PST), pm <zzyzx.xyzzy@xxxxxxxxx>
wrote:

On Feb 22, 7:04 pm, carlfo...@xxxxxxxxxxx wrote:
On Sun, 22 Feb 2009 18:36:18 -0800 (PST), pm <zzyzx.xy...@xxxxxxxxx>
wrote:



On Feb 22, 1:06 pm, carlfo...@xxxxxxxxxxx wrote:
On Sun, 22 Feb 2009 12:17:05 -0800 (PST), Nick L Plate

<tj-j...@xxxxxxxxxxxxxxxx> wrote:
On 22 Feb, 20:00, carlfo...@xxxxxxxxxxx wrote:
On Sat, 21 Feb 2009 18:29:45 -0800 (PST), Nick L Plate

<tj-j...@xxxxxxxxxxxxxxxx> wrote:
On 22 Feb, 01:42, carlfo...@xxxxxxxxxxx wrote:
On Sat, 21 Feb 2009 15:58:48 -0800 (PST), Nick L Plate

<tj-j...@xxxxxxxxxxxxxxxx> wrote:
On 21 Feb, 23:44, carlfo...@xxxxxxxxxxx wrote:

So one spoke crossing another doesn't affect pitch when plucked?

Fascinating!

Let us know what pitch you get if you pluck the spoke below the
crossing.

Out of curiosity, what do you think the frets are for on guitars?

You are a clown.  Both spokes in a crossed pair are free to vibrate
between their ends when both are the same mass and length tensioned
the same.  They vibrate the same pitch together as when apart. Why are
you so ignorant on so many things?
TJ

Dear Trevor,

Either your spokes don't touch each other where they cross . . .

(Unlikely, since you talk about kinking them at that point.)

Or else you really have no idea what the frets do on a guitar.

Cheers,

Carl Fogel

Another bloody fretter.

Dear Trevor,

Really, you should find someone who plays a guitar.

Ask them if the pitch changes when they press the string against the
fret with a finger.

They'll explain that the answer is yes, of course the pitch changes.

They may give you an odd look, since it's patently obvious that this
is how string instruments work. When you press a string against the
fingerboard, the string tension remains the same, but the length
shortens and the pitch rises.

I wonder if your wheelbuilding theories are on the same level as your
grasp of how string instruments work?

Why not just pedal past the local bike shop and ask them to show you
how a spoke tension gauge works? They can tell you what your spoke
tension is in a few moments, and then you can tell us what spoke
tension you think is a good idea, rather than just not "anywhere near
100 kgf."

Who knows? You might be rolling around on kinked spokes, tied and
possibly soldered, with only 20 kgf tension. Or you might be making a
huge fuss over what turns out to be 95 kgf.

What have you got to lose by learning what you're doing?

Cheers,

Carl Fogel

When the two strings of a double stringed instrument, guitar or
mandolin, are crossed, there is no magic to change their pitch.  I've
done it.

Stop reading and start doing, and maybe you will learn something for
yourself for once.  I do not require the figures for tension in my
Saturne sprint wheels because the materials have been used together
for long enough in similar assemblies put to worse use than I will.
The spoke tensions are I guess in the order of 60lbf but cant be
bothered at the moment finding out.  I've given you all the
information you need.  Work it out from the mass and tension of a
stringed instrument.  300mm steel 14swg C#  It's you making the
unecessary fuss.  Those wheels can carry greater weight than I by a
considerable magnitude.  I am unconcerned about there ability to carry
loads at any spoke tension.
TJ

Dear Trevor,

Sorry . . .

But I suspect that you've "done it" in the same sense that you've
"reached" 200+ RPM at 70+ mph in Wales on routine rides, "performed"
any calculations in this thread, "sprinted so hard that both your
wheels routinely came off the ground simultaneously, or "seen" a rider
sprinting uphill and swerving 10 feet from side to side with each
pedal stroke.

You keep dodging and weaving and refusing to answer obvious questions
after you make eyebrow-raising claims.

You claim that spokes should be tensioned not "anywhere near 100 kgf."

Fine, measure your spoke tension and let us know what it is.

You dodge off into C# on 12-inch crossed spokes, but then make the
amazing claim that one, two, or three metal spokes crossing and
touching the spoke--which you kink at the crossing!--make no
difference in the pitch.

At this point, it's hard to believe that the spoke's pitch, whatever
it is, is even C#.

Pedal over to the bike shop, ask them to check your spoke tension, and
let us know what it is. Anything else is just you making fun of your
own claims.

Cheers,

Carl Fogel

A fret is only a fret because it is far more rigid than the string.

Crossed strings, presuming equal tension and equal chord lengths, will
have many vibratory modes, but it should be obvious that all of the
modes present in uncrossed strings will also be modes of the crossed
strings -- that is so say, if two uncrossed spokes are vibrating in
sync at the same amplitude at the points where they would contact,
bringing them to contact changes nothing about the vibration.

It would be an interesting problem to work out the theoretical normal
modes of a crossed, unequally tensioned pair of spokes, and it should
be doable for anyone who you can work through the first few chapters
of Crawford's "Waves" -- but after acquiring an actual tensiometer it
became clear to me that spoke count and rim stiffness have enough
influence that tone is useless to assess overall spoke tension.

-pm

Dear PM,

I think that we agree here that without a spoke of a known tension for
reference, no one is going to figure out spoke tension by musical note
and spoke length, thickness, and crossings.

Yeah, pretty much.

But don't hesitate to correct me if, even though we're on roughly the
same side, my explanation seems wrong to you.

I agree that the spoke count and rim stiffness are going to confuse
what tension a pitch, spoke length, and thickness mean.

(The pitch _does_ work for checking how even the tension is--plenty of
wheelbuilders on RBT routinely tension by tone. But that's just
checking for even tension, not determining tension itself.)

Yup. I read a few spokes with the tensiometer, and go around the wheel
plucking the rest to see if any are way out of line.

I suspect that Trevor doesn't understand that a spoke/string will
vibrate either as a noticeably shortened spoke/string, or else as a
full-length spoke/string divided into unequal parts, in the manner of
a violin string playing a harmonic below the mid-point, with a finger
touching the string lightly in a spot where the string can vibrate on
both sides:
 http://www.thesession.org/discussions/display/13924/comments

If the crossing isn't at a good point, you just get mush, harmonically
speaking. Given that Trevor kinks his spokes and ties and solders
them, I imagine that there might be a few more complications.

I disagree with this explanation. In the case of the finger on the
string, the finger reduces the movement of string to zero at the point
where it touches. What you seem to be missing is that in the case of
spokes the "crossing" is not at a fixed point in space like the finger
or like fret on a fretboard. The crossing moves along with the spokes.
Trevor's example of lacing a mandolin so that the paired strings are
crossed is actually more appropriate here than your example of finger-
on-fixed-fretboard.

Let me try help you visualize this. Imagine first that you you have
two uncrossed string, of identical tension. Imagine drawing a spot on
each, at a "crossing" location. It does not matter where, as long at
is is in the same position on both spokes. Then pluck each string to
excite the same vibratory mode.

Now focusing on the spots you marked, you could find that spot on the
first spoke moves up and down, 440 times per second. Being at the same
tension, and at the same location on the spoke, so does the other
marked spot on the other spoke.

Imagine these spoke vibrating in slow motion, so you can see it moving
up and down. While you do this, bring the spokes together in slow
motion, until there is just the barest gap between them -- the spot on
spoke 1 moves up while the spot on spoke 2 moves up, then the spot on
spoke 2 moves down while the spot on spoke 1 moved down in concert.
[This of course presumes that the vibrate in phase, but after all when
you work through the math, all that crossing the spokes entails for
the symmetric modes is that the spokes are phase locked, which doesn't
change the frequency.]

Now make the slightest movement so that these spokes that are
vibrating in concert while not touching are made to touch. What
happens? Nothing in particular--the first spoke wasn't moving any
differently at the point of contact from the second spoke, so that
contact between the two does not transmit any energy. So the frequency
does not change.

That's what I mean when I say that the normal modes of a crossed pair
of equal tension spokes include all those of an uncrossed spoke. Of
course caveats apply:

* There are probably additional modes arising from the crossing
* I haven't worked out what happens with uneven tension,
* Who knows which of the myriad modes you are hearing when you pluck a
spoke?

-pm

Dear PM,

I think that I follow your explanation, but I don't think that it
applies to actual plucked string/spokes, much less bowed ones, which
can demonstrate the details of the problem.

Plain fingering on a violin involves merely pressing the string down
hard on the fingerboard, shortening the string at the same tension,
and raising the pitch. No matter where you press, you get a note,
whose pitch depends on how tight and heavy the string is and how much
you shorten the string's length.

Harmonics, as they're called are entirely differnent.

When you touch a violin string for a harmonic, you divide it into two
sections, both vibrating.

You just touch the string delicately for a harmonic. You don't push it
to the fingerboard.

The string is under the same tension, but now both parts vibrate,
above and below the finger delicately touching the string.

The resulting pitch is not the pitch of the full vibrating string.

That is, if you press delicately at the midpoint of the A string, you
get an A one whole octave higher, with equal lengths of string
vibrating on either side of the finger.

But if you press _below_ the midpoint, you get various notes (if you
press at the few spots that work), all with a much higher pitch than
if you pressed down firmly at that point and had only one side of the
string vibrating.

Again, the pitch is _not_ the pitch of the open untouched string. It's
_much higher.

Harmonics are a sneaky way to get a high note from the lower
positions. (The tone also changes, becoming an odd, light, whistling
tone.)

Harmonics are beastly tricky for two reasons.

First, only certain spots will even work--press a tiny bit high or
low, and the divided lengths of the string won't vibrate worth a damn.
You just damp the whole string.

Second, the touch must be very delicate to allow both sides of the
string to vibrate. Anyone can press a string down hard on a
fingerboard and get a plain note, no matter how out of tune. Playing a
harmonic requires not only that delicate touch to allow the string to
vibrate on both sides, but also the delicate touch in the right
spot--if the relationship between theupper and lower lengths of the
string isn't darned near perfect, you get nothing except a dull scrape
as if you'd pushed a wad of kleenex onto the string.

(If you press above the midpoint, I hate you because I was never
anywhere near good enough to work up there and have only three
harmonics that I'm willing to try for.)

In other words, touching a string delicately enough to let it vibrate
on either side of the touch point does _not_ produce the same pitch as
letting the whole string vibrate. Most of the time, splitting a string
doesn't let the far side vibrate in any significant way--you just get
the upper half's pitch, and wish that you'd pressed harder for a plain
note or pressed in just the right spot, to allow both sections to
vibrate usefully.

Spoke crossings are not likely to be in the right spot to allow that
kind of note, which would be higher, anyway.

But your explanation at least has me wondering, so thanks. I think
that your chief point is to treat the two parts of one string as
equivalent to a single full length of string because the crossing
point is with a string of equal tension.

I'm not sure if it would be a practical test, but does your approach
predict that the spoke could be plucked just as well below the
crossing as above it? That is, same note, no matter which side?

As you point out, getting that equal tension might be a considerable
practical stumbling block.

I think that we agree that pitch, raw spoke length, and gauge aren't
enough.

If nothing else, where is the equivalent of the bridge and fingerboard
on a wheel?

There's 10~14 mm of nipple threaded onto a spoke--where is that end?

There are several crossings, at least one of which Trevor kinks and
ties and solders. He says the second. On a 3x, the crossing that
touches may be the third--a quick look shows the crossing around 70 mm
from the elbow on a handy wheel.

And where does the elbow end stop, assuming that you don't take the
crossing as the primary length, the equivalent of a bridge? On a low
hub, a spoke may be pressing for 5~10 mm against the hub flange as
hard as any guitar string against a bridge.

So is the 300 mm spoke actually 300 mm minus 10~14 for the nipple end,
minus 5~10 mm for the hub flange, and minus 70 mm (where we may
disagree) for the length up to the crossing?

And that's just for the straight gauge spokes.

Maybe a radial wheel with straight gauge spokes would be a better
candidate for estimating spoke tension by plucking, but we'd still
need a tension gauge reading to test the theory.

Thanks again for going through the two-string idea.

Cheers,

Carl Fogel
.

Relevant Pages

  • Re: Riders weight vs. spoke tension
    ...  They vibrate the same pitch together as when apart.. ... When you press a string against the ... fingerboard, the string tension remains the same, but the length ... They can tell you what your spoke ...
    (rec.bicycles.tech)
  • Re: Riders weight vs. spoke tension
    ... a violin string playing a harmonic below the mid-point, ... touching the string lightly in a spot where the string can vibrate on ... first spoke moves up and down, ... Harmonics, as they're called are entirely differnent. ...
    (rec.bicycles.tech)
  • Re: Riders weight vs. spoke tension
    ... a violin string playing a harmonic below the mid-point, ... touching the string lightly in a spot where the string can vibrate on ... first spoke moves up and down, ... Harmonics, as they're called are entirely differnent. ...
    (rec.bicycles.tech)
  • Re: Riders weight vs. spoke tension
    ... When you press a string against the ... fingerboard, the string tension remains the same, but the length ... shortens and the pitch rises. ... They can tell you what your spoke ...
    (rec.bicycles.tech)
  • Re: Riders weight vs. spoke tension
    ... When you press a string against the ... fingerboard, the string tension remains the same, but the length ... shortens and the pitch rises. ... They can tell you what your spoke ...
    (rec.bicycles.tech)
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