Re: I fixed a broken spoke! - Page 3 - Cycling Mob: Road Cycling, Mountain Biking and More

12-26-2006, 12:19 PM

Peter Cole wrote:
> jim beam wrote:
>> Peter Cole wrote:
>>> jim beam wrote:
>>>
>>>> wow dude, you're deeply afflicted with nonsensicitis, with a large
>>>> dose of dontgethebasics stubbornius thrown in. really, that has to
>>>> be one of the most willfully obtuse responses i've ever seen on
>>>> r.b.t., and that's saying something.
>>>>
>>>> this stuff is real basic.
>>>
>>>> this stuff is real basic peter and it concerns me that you either
>>>> don't get this stuff or are prepared to deny it.
>>>
>>> OK, you've had a couple of days to think it over, ready to retract?
>>
>> retract that the equation assumes constant L? no!
>
> You do realize that, according to Pythagoras anyway, that if you know
> the length of 2 sides of a right triangle you can compute the 3rd? I
> know you and CF have this abhorrence of "calculation", but you can take
> it too far.

er, i has a passing acquaintance with pythagoras, thanks. in case you
missed my posting to ron, imagine this:

[fixed width font]

v
concrete|---------|concrete

and
v
|--------------------|
| |
| |
| |<thin flexy pole
| |
//////////////////////////

now, which one do /you/ think is doing to show the greatest tension
increase when deflecting the wire at point "v"? by all means, feel free
to use pythaoras to explain.

12-26-2006, 12:31 PM

Ron Ruff wrote:
> jim beam wrote:
> > you can't tell what initial tension is at all! but in our case, we're
> > looking for /increase/ in tension, so that's ok. now, if only we can
> > address the variable L...
>
> L is around 290mm long in our case, and if it changed by 1mm it would
> be a lot. So... we risk an error of 1/290 = about 0.3% in the tension
> calculation by ignoring it. Since there is much less precision than
> this in measuring d, I think we are safe to ignore it.
>
> Now, I wish you guys would make up and play nice and get on to more
> interesting topics...

Dear Ron,

You may be missing where things change.

Push a brake pad so that it touches the rim next to a spoke

Squeeze the spoke pair.

I get a sideways movement of about 6 mm on a front wheel, using my
weaker hand.

Now look at the spoke crossing, where a typical cross-3 spoke goes
around another spoke near the hub at about 4 degrees.

How much does the crossing change when you squeeze the spoke pair?

The ~ 4 degree angle should flatten a bit, but I won't try to guess how
much. My crossing point moved about 10 mm.

In short, the spoke on a typical cross-3 wheel is already going around
a bend. When I squeeze mine, the spoke moves up to 10 mm along the
spoke that it crosses to a new position (and a new counter-tension from
the other spoke).

Meanwhile, the rim at the other end of the spoke moves up to 6 mm
sideways.

The rim also pulls inward some distance (much smaller, I think, but
still exaggerating the "calculated" tension).

At least the hub flange is pretty solid. But this mish-mash doesn't
really resemble the theoretical two fixed points and a nicely centered
sideways deflection.

Here's a page that addresses just the problem of whether the load is
applied at the center of the span:

[Only registered and activated users can see links. ]

Is the squeeze force on the bicycle spoke pair centered relative to the
hub and rim, or to the crossing point and rim? Where does the rim end
up? How far does the crossing flatten? How far does the crossing point
move?

With a tension gauge, the force is automatically centered on a short,
straight spoke span with no crossing bend. Both posts are fixed,
instead of one moving 6 mm sideways and the other sliding along a
counter-tensioned wire.

Oddly enough, the spoke tension gauge keeps giving a different answer
than over-simplified calculations. It's as if the practical details
that the calculations ignore lead to exaggerated results.

I predict that a spoke tension gauge will show that a typical 32 or 36
spoke wheel will see a tension rise of about 55~65 pounds for four
spokes at normal tension that are squeezed as two pairs with 60 pounds
of force.

Anyone with a tension gauge, some rope, and some weights can test my
prediction. I've tested it on a variety of wheels with a tension gauge
that appears to be more than accurate enough.

Cheers,

Carl Fogel

12-26-2006, 02:28 PM

jim beam wrote:

> er, i has a passing acquaintance with pythagoras, thanks. in case you
> missed my posting to ron, imagine this:
>
> [fixed width font]
>
> v
> concrete|---------|concrete
>
> and
> v
> |--------------------|
> | |
> | |
> | |<thin flexy pole
> | |
> //////////////////////////
>
> now, which one do /you/ think is doing to show the greatest tension
> increase when deflecting the wire at point "v"? by all means, feel free
> to use pythaoras to explain.

I didn't miss your post to Ron (I'm enjoying this too much), but you
continue to miss the point.

The question isn't which one of your scenarios has more tension, but
rather whether you can precisely determine the wire tension just from
the deflection angle and deflection force identically in both cases.

If you're still confused, try a checking an entry-level physics textbook.

12-26-2006, 04:36 PM

[Only registered and activated users can see links. ] wrote:
> You may be missing where things change.
>
> Push a brake pad so that it touches the rim next to a spoke
>
> Squeeze the spoke pair.
>
> I get a sideways movement of about 6 mm on a front wheel, using my
> weaker hand.
>
> Now look at the spoke crossing, where a typical cross-3 spoke goes
> around another spoke near the hub at about 4 degrees.
>
> How much does the crossing change when you squeeze the spoke pair?
>
> The ~ 4 degree angle should flatten a bit, but I won't try to guess how
> much. My crossing point moved about 10 mm.
>
> In short, the spoke on a typical cross-3 wheel is already going around
> a bend. When I squeeze mine, the spoke moves up to 10 mm along the
> spoke that it crosses to a new position (and a new counter-tension from
> the other spoke).
>
> Meanwhile, the rim at the other end of the spoke moves up to 6 mm
> sideways.
>
> The rim also pulls inward some distance (much smaller, I think, but
> still exaggerating the "calculated" tension).

> Is the squeeze force on the bicycle spoke pair centered relative to the
> hub and rim, or to the crossing point and rim? Where does the rim end
> up? How far does the crossing flatten? How far does the crossing point
> move?

Fortunately, with "calculations" and "theory" (scary, I know), we can
determine the size of these possible error sources and decide whether
they have much of an effect.

If you are so clumsy that you mis-measure the center of a spoke span by
10%, (roughly 0.5" in 10" -- really bad ruler!), then your calculated
tension will be off proportionally -- (10%).

The "sideways" deflection of 6mm on 270mm corresponds to an angle of
1.3deg, or a change in spoke path length of 0.02%.

Complete "flattening" of the ~4deg crossover bend likewise would result
in a path length change of ~0.2%.

Movement of the crossover would make a very small change to the tension.
If the movement (inward) is ~10mm, ~75mm from the hub, the change
crossover bend induced tension is an increase of ~15%. At a 4deg angle,
the ratio of tension increase to force (t = f/2sin(theta)) is about 6,
so if the crossover contact force was 10lb (a high estimate, I think),
the associated tension would be 60lb, and the change from crossover
point movement would be about 10lb (increase).

As to the issue of the rim pulling "inward", that principally determines
the ratio of squeeze force to tension increase (deflection angle), but
it does not affect the calculation of tension as I (Jobst and Ron) have
tried to explain.

12-26-2006, 09:13 PM

Peter Cole wrote:
> jim beam wrote:
>
>> er, i has a passing acquaintance with pythagoras, thanks. in case you
>> missed my posting to ron, imagine this:
>>
>> [fixed width font]
>>
>> v
>> concrete|---------|concrete
>>
>> and
>> v
>> |--------------------|
>> | |
>> | |
>> | |<thin flexy pole
>> | |
>> //////////////////////////
>>
>> now, which one do /you/ think is doing to show the greatest tension
>> increase when deflecting the wire at point "v"? by all means, feel
>> free to use pythaoras to explain.
>
> I didn't miss your post to Ron (I'm enjoying this too much), but you
> continue to miss the point.
>
> The question isn't which one of your scenarios has more tension, but
> rather whether you can precisely determine the wire tension just from
> the deflection angle and deflection force identically in both cases.
>
> If you're still confused, try a checking an entry-level physics textbook.

ok peter, now you're being deliberately obtuse. fact: if the two
anchors are not fixed, deflection is not simply a function of tension,
it's a function of how far the anchors move - try the above
demonstration yourself.

12-26-2006, 10:03 PM

Peter Cole wrote:
> [Only registered and activated users can see links. ] wrote:
>> You may be missing where things change.
>>
>> Push a brake pad so that it touches the rim next to a spoke
>>
>> Squeeze the spoke pair.
>>
>> I get a sideways movement of about 6 mm on a front wheel, using my
>> weaker hand.
>>
>> Now look at the spoke crossing, where a typical cross-3 spoke goes
>> around another spoke near the hub at about 4 degrees.
>>
>> How much does the crossing change when you squeeze the spoke pair?
>>
>> The ~ 4 degree angle should flatten a bit, but I won't try to guess how
>> much. My crossing point moved about 10 mm.
>>
>> In short, the spoke on a typical cross-3 wheel is already going around
>> a bend. When I squeeze mine, the spoke moves up to 10 mm along the
>> spoke that it crosses to a new position (and a new counter-tension from
>> the other spoke).
>>
>> Meanwhile, the rim at the other end of the spoke moves up to 6 mm
>> sideways.
>>
>> The rim also pulls inward some distance (much smaller, I think, but
>> still exaggerating the "calculated" tension).
>
>> Is the squeeze force on the bicycle spoke pair centered relative to the
>> hub and rim, or to the crossing point and rim? Where does the rim end
>> up? How far does the crossing flatten? How far does the crossing point
>> move?
>
> Fortunately, with "calculations" and "theory" (scary, I know), we can
> determine the size of these possible error sources and decide whether
> they have much of an effect.
>
> If you are so clumsy that you mis-measure the center of a spoke span by
> 10%, (roughly 0.5" in 10" -- really bad ruler!), then your calculated
> tension will be off proportionally -- (10%).

mockery doesn't make you right here peter.

>
> The "sideways" deflection of 6mm on 270mm corresponds to an angle of
> 1.3deg, or a change in spoke path length of 0.02%.

and rim elasticity is how many times greater than that?

>
> Complete "flattening" of the ~4deg crossover bend likewise would result
> in a path length change of ~0.2%.

calculated, not measured.

>
> Movement of the crossover would make a very small change to the tension.
> If the movement (inward) is ~10mm, ~75mm from the hub, the change
> crossover bend induced tension is an increase of ~15%. At a 4deg angle,
> the ratio of tension increase to force (t = f/2sin(theta)) is about 6,
> so if the crossover contact force was 10lb (a high estimate, I think),
> the associated tension would be 60lb, and the change from crossover
> point movement would be about 10lb (increase).
>
> As to the issue of the rim pulling "inward", that principally determines
> the ratio of squeeze force to tension increase (deflection angle), but
> it does not affect the calculation of tension as I (Jobst and Ron) have
> tried to explain.
>
no, you're dismissing the very crux of the argument against you as
though it doesn't count - you're not explaining the difference between
calculated and actual tension difference. /you/ need to measure. /you/
need to explain discrepancy, and /not/ with some hand waving about
"clumsy mis-measurement". you've already been told what it is - how
much more simple does it need to be made until you get it?

12-27-2006, 05:01 AM

jim beam wrote:
> Peter Cole wrote:

>> The question isn't which one of your scenarios has more tension, but
>> rather whether you can precisely determine the wire tension just from
>> the deflection angle and deflection force identically in both cases.
>>
>> If you're still confused, try a checking an entry-level physics textbook.
>
> ok peter, now you're being deliberately obtuse. fact: if the two
> anchors are not fixed, deflection is not simply a function of tension,
> it's a function of how far the anchors move - try the above
> demonstration yourself.

I didn't say "deflection is a function of tension", I said deflection
angle gives the ratio of forces -- a very different statement.

Apparently you haven't ever studied vectors. How could you grasp
material science without them? Even so, I thought the rough idea would
be intuitive, I guess not.

Entry level textbooks are full of this kind of "rope and weight" problem
simply because the vectors are so easily revealed by the rope paths.
There can be no confusion about the direction of force, it's only left
to work out the magnitude.

It comes down to simple trig, which is where Pythagoras comes in.

BTW, "obtuse" has two possible meanings in this context:

a : lacking sharpness or quickness of sensibility or intellect :
INSENSITIVE, STUPID

b : difficult to comprehend : not clear or precise in thought or expression

I wouldn't be so quick to use that word if I were you.

12-27-2006, 06:39 AM

jim beam wrote:
> Peter Cole wrote:

>> Fortunately, with "calculations" and "theory" (scary, I know), we can
>> determine the size of these possible error sources and decide whether
>> they have much of an effect.
>>
>> If you are so clumsy that you mis-measure the center of a spoke span
>> by 10%, (roughly 0.5" in 10" -- really bad ruler!), then your
>> calculated tension will be off proportionally -- (10%).
>
> mockery doesn't make you right here peter.

I think you have things reversed.

>> As to the issue of the rim pulling "inward", that principally
>> determines the ratio of squeeze force to tension increase (deflection
>> angle), but it does not affect the calculation of tension as I (Jobst
>> and Ron) have tried to explain.
>>
> no, you're dismissing the very crux of the argument against you as
> though it doesn't count - you're not explaining the difference between
> calculated and actual tension difference. /you/ need to measure. /you/
> need to explain discrepancy, and /not/ with some hand waving about
> "clumsy mis-measurement". you've already been told what it is - how
> much more simple does it need to be made until you get it?

First, the measurements/calculations Carl & I made weren't all that far
off, about 15%.

I'm more than willing to attempt to quantify all of the potential
sources of error in my approach. I think I did just that. It's common
engineering practice to include an addendum to an analysis describing
potential errors.

Carl's setup isn't without several sources of potential error. His
primary tool, the Park tensiometer, also has its limitations.

Carl challenged me to make actual measurements of squeeze force to
tension rise in one of my own wheels. I did that. Both of you asserted
that my method could not be accurate because it did not account for rim
deflection. I explained how, not only did it account for rim deflection,
it actually measured it as well. Neither of you seem to grasp the
physics/math which supports my claim, although it's fairly basic.
Whether this is from aptitude or attitude, I don't know. I'll give you
the benefit of the doubt and assume the latter, but that's hardly
flattering, either.

12-27-2006, 05:38 PM

In article
<1166948028.987967.312030@42g2000cwt.googlegroups. com>,
"Ron Ruff" <[Only registered and activated users can see links. ]> wrote:

> I don't know if the Park tensiometer is any good but I picked one up
> from Performance when they were having their $20 off $50 with free
> shipping sale, and I'm having fun with it. It seems to be pretty
> consistent at least, if not very precise...

Consistent and precise mean much the same thing.

> ie a single digit on the
> scale can represent a 4 to 20 kg variation in tension. The force it
> exerts on the spoke is not that small either... considering that it
> takes ~3lb to squeeze the handle and a leverage ratio of ~8 to 1 gives
> a force of ~24lb. Supposedly their conversion table compensates for
> this...

You think that it may not be as _accurate_ as the scale
allows.

A clock that reads the correct time within a certain
bound is accurate to that bound. A clock whose
oscillator measures the same interval to within a
certain bound is precise to that bound. The oscillator
may have little twitches & anomalies; diurnal
variations; sensitivity to barometric pressure,
temperature, and humidity that overall add up to little
variation making the clock accurate for time telling,
but poor as a frequency standard. A clock may have a
very precise oscillator that does not oscillate at the
design frequency, making it a poor instrument for
telling time.

--
Michael Press

12-27-2006, 05:50 PM

In article <[Only registered and activated users can see links. ].prodigy.com >,
Michael Press <[Only registered and activated users can see links. ]> wrote:

> In article <1166948028.987967.312030@42g2000cwt.googlegroups. com>,
> "Ron Ruff" <[Only registered and activated users can see links. ]> wrote:
>
> > I don't know if the Park tensiometer is any good but I picked one
> > up from Performance when they were having their $20 off $50 with
> > free shipping sale, and I'm having fun with it. It seems to be
> > pretty consistent at least, if not very precise...
>
> Consistent and precise mean much the same thing.

Hardly, and neither necessarily mean accurate.