Tire inflation and hoop stress
Jobst Brandt
given on wreck.bike. Here is an example from long ago:
From: jbr...@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: building the wheels
Date: 7 Jan 1998 18:33:53 GMT
Hoyt McKagen writes:
>> Hoyt, this is apples and oranges. In the case of the wheel, the
>> spokes are shortening due to an applied load.
> That's the basic misconception: the load isn't applied to the spokes
> by the rim and tire; it is taken off by them! <=That has been our
> whole problem, I guess.
> When you inflate the tire, it stretches the rim larger, (I did the
> experiment) and it would tighten spokes which weren't already
> preloaded. Since they have a preload, though, we know they stay in
> tension when we put the weight on the tire.
Oops! Not only did you make this up, but you just showed that you
didn't read "the Bicycle Wheel" as you claimed, because a tire
constricts when inflated and the computation of how much it does this
is in the book in equation 8. Besides, this has been measured. Spoke
tension decreases with inflation. Maybe you could explain how your
wheel without tension looked before you inflated your tire and how you
determined the change, the change being less than 1/1000 inch.
> The expansion is essentially because there is more area outside the
> tire's neutral axis than inside it, and it takes the rim with it
> through the bead-notch interlocking. That rim expansion is mediated
> through the tireplies, which pull on it in response to the pressure.
Now you are just blowing smoke, so to speak. The propensity of a tire
to blow off a rim is matched by the inflation pressure inward at the
tire to rim interface. Thus there is no net force from that, but
there is constriction caused by the cord angle of the tire casing.
Typically an un-glued tubular tire constricts onto the rim with a
force that is proportional to inflation pressure by an amount shown in
equation 8. That is why hose braid, visible in clear clear plastic
tubing, is at 35.27 degrees, not 45 degrees as in tires. A tubular
tire inflated without the benefit of a rim will shorten until the cord
angle is 35.27 degrees. A clincher would try to do the same if it
were not open and had a resilient bead.
> When you distort the tire at the contact patch, you relieve some of
> that tension and the spokes pull the rim in at the contact patch
> only (that is essentially why the flattening is so localized).
> That kicks off a global expansion in the rest of the rim, which
> stretches all those other spokes. But the only element in the wheel
> which can actually stand compression is the rim. Follow? So what
> does it actually stand on?
So now that you have brought the discussion around to what supports a
rim from the ground, please explain what prevents the tire from sagging
to the ground. It can't be the air pressure because it is uniform
around the tire. There must be a net force upward on the rim. You
might give that concept some thought while you are at it.
Jobst Brandt <jbr...@hpl.hp.com>
I was at HP Labs at the time.
Jobst Brandt
thirty-six
28,
2004, at New River Valley Medical Center. He was born in South
Carolina on November 29, 1944. He is survived by his wife Tatiana P.
McKagen, of Blacksburg; father and mother, O.H. and Helen Winsor
McKagen, of Fairfax, Va.; daughter, Nika Irina McKagen, of Blacksburg;
sons, Branden O. McKagen, of Blacksburg and Ian Franzoni, of Durham,
N.C. Services will be private. In lieu of flowers, memorial
contributions may be made to the Leukemia and Lymphoma Society, 1311
Mamaroneck Ave., White Plains, N.Y. 10605. The family will receive
friends from 2 to 4 p.m., Saturday, May 1, 2004 at the McCoy Funeral
Home, 150 Country Club Drive, SW, Blacksburg."
datakoll
> As I perused the archives, I was reminded how much incorrect advice is
> given on wreck.bike. Here is an example from long ago:
>
>
> I was at HP Labs at the time.
>
> Jobst Brandt
did HP run tests on you ?
datakoll
was a DIY housebuilder with a trailer ? atop 3 1/2 shaky block walls
layed up with budweiser.
Naybor was evangelistic on mating block walls with bead styrofoam
insulation, a good thing and not widely practised either above or
below hground in the cold cold PA hills.
He drove his point home - I remeber he was maybe either a trucker or
welder - by waving a slab of styrofoam in the air shouting 'STYROFOAM
CREATES HEAT" at least 6 or 7 times.
and what did he mean by this ?
carl...@comcast.net
[snip]
Dear Jobst,
Actually, spoke tension on clincher rims will increase _or_ decrease
with tire inflation.
What happens depends on the rim.
On a strongly braced narrow rim, increased tire pressure constricts
the tire and squeezes the rim, so the spokes will show a loss of
tension.
But on a wide single-wall MTB rim, increased tire pressure will spread
the rim edges apart, drawing the center of the rim outward and pulling
on the spokes, _raising_ spoke tension.
We went through the measurements in 2006:
Cheers,
Carl Fogel
datakoll
> On 22 Jan 2010 19:36:42 GMT, Jobst Brandt <jbra...@sonic.net> wrote:
>
> [snip]
>
> Dear Jobst,
>
> Actually, spoke tension on clincher rims will increase _or_ decrease
> with tire inflation.
>
> What happens depends on the rim.
>
> On a strongly braced narrow rim, increased tire pressure constricts
> the tire and squeezes the rim, so the spokes will show a loss of
> tension.
>
> But on a wide single-wall MTB rim, increased tire pressure will spread
> the rim edges apart, drawing the center of the rim outward and pulling
> on the spokes, _raising_ spoke tension.
>
> We went through the measurements in 2006:
>
>
> Cheers,
>
> Carl Fogel
WHAT are you observing there ? spokes move straight back toward and
out of the hub or spokes bow from a straight attitude ??? what ?
thirty-six
> On 22 Jan 2010 19:36:42 GMT, Jobst Brandt <jbra...@sonic.net> wrote:
>
> [snip]
>
> Dear Jobst,
>
> Actually, spoke tension on clincher rims will increase _or_ decrease
> with tire inflation.
>
> What happens depends on the rim.
>
> On a strongly braced narrow rim, increased tire pressure constricts
> the tire and squeezes the rim, so the spokes will show a loss of
> tension.
>
> But on a wide single-wall MTB rim, increased tire pressure will spread
> the rim edges apart, drawing the center of the rim outward and pulling
> on the spokes, _raising_ spoke tension.
That is outside of jobstworld thought. (MA2)
>
> We went through the measurements in 2006:
>
> http://groups.google.com/group/rec.bicycles.tech/browse_frm/thread/e3...
>
> Cheers,
>
> Carl Fogel
Jobst Brandt
> Actually, spoke tension on clincher rims will increase _or_ decrease
> with tire inflation.
> What happens depends on the rim.
> On a strongly braced narrow rim, increased tire pressure constricts
> the tire and squeezes the rim, so the spokes will show a loss of
> tension.
> But on a wide single-wall MTB rim, increased tire pressure will
> spread the rim edges apart, drawing the center of the rim outward
> and pulling on the spokes, _raising_ spoke tension.
> We went through the measurements in 2006:
I wasn't convinced of the accuracy of that test. Also, your
explanation of rim spreading doesn't hold up considering that the
force that spreads the rim is not a pure lateral force, but rather
casing tension that pulls at a predominantly radial direction. In any
event, unless the tire is not a 45° bias ply tire, it will constrict.
As I mentioned, that one is clear from inflating tubular tires off the
rim... or the force that holds it on the rim when in use. Rim cement
keeps the constricted tire mainly from lateral motion. Check your
garden hose cord angle.
Jobst Brandt
carl...@comcast.net
>Carl Fogel wrote:
>
>> Actually, spoke tension on clincher rims will increase _or_ decrease
>> with tire inflation.
>
>> What happens depends on the rim.
>
>> On a strongly braced narrow rim, increased tire pressure constricts
>> the tire and squeezes the rim, so the spokes will show a loss of
>> tension.
>
>> But on a wide single-wall MTB rim, increased tire pressure will
>> spread the rim edges apart, drawing the center of the rim outward
>> and pulling on the spokes, _raising_ spoke tension.
>
>> We went through the measurements in 2006:
>
>> http://groups.google.com/group/rec.bicycles.tech/browse_frm/thread/e3864af31619d8d4/511343ee76d906fd#511343ee76d906fd
>
>I wasn't convinced of the accuracy of that test.
[snip]
Dear Jobst,
Who are we going to believe, you or our lying eyes?
"Here's a 24-second 3mb video showing my Park tension gauge needle
dropping from over 24 to under 24 (less tension) as 60 psi of air
hisses out of a 26 x 1.75 mtb clincher tire on a no-box-section steel
rim:"
http://www.youtube.com/v/NeG5o3WX7Mw
Watch the Park tension gauge's needle drop as the air hisses out of
the tire.
Cheers,
Carl Fogel
datakoll
> On 23 Jan 2010 16:41:34 GMT, Jobst Brandt <jbra...@sonic.net> wrote:
>
>
>
>
>
> >Carl Fogel wrote:
>
> >> Actually, spoke tension on clincher rims will increase _or_ decrease
> >> with tire inflation.
>
> >> What happens depends on the rim.
>
> >> On a strongly braced narrow rim, increased tire pressure constricts
> >> the tire and squeezes the rim, so the spokes will show a loss of
> >> tension.
>
> >> But on a wide single-wall MTB rim, increased tire pressure will
> >> spread the rim edges apart, drawing the center of the rim outward
> >> and pulling on the spokes, _raising_ spoke tension.
>
> >> We went through the measurements in 2006:
>
>
> >I wasn't convinced of the accuracy of that test.
>
> [snip]
>
> Dear Jobst,
>
> Who are we going to believe, you or our lying eyes?
>
> "Here's a 24-second 3mb video showing my Park tension gauge needle
> dropping from over 24 to under 24 (less tension) as 60 psi of air
> hisses out of a 26 x 1.75 mtb clincher tire on a no-box-section steel
> rim:"
> http://www.youtube.com/v/NeG5o3WX7Mw
>
> Watch the Park tension gauge's needle drop as the air hisses out of
> the tire.
>
> Cheers,
>
>
> - Show quoted text -
THE GAUGE USED MEASURES 360 DEGREES ?
AMuzi
>>>> Actually, spoke tension on clincher rims will increase _or_ decrease
>>>> with tire inflation.
>>>> What happens depends on the rim.
>>>> On a strongly braced narrow rim, increased tire pressure constricts
>>>> the tire and squeezes the rim, so the spokes will show a loss of
>>>> tension.
>>>> But on a wide single-wall MTB rim, increased tire pressure will
>>>> spread the rim edges apart, drawing the center of the rim outward
>>>> and pulling on the spokes, _raising_ spoke tension.
>>>> We went through the measurements in 2006:
>>>> http://groups.google.com/group/rec.bicycles.tech/browse_frm/thread/e3...
>> Jobst Brandt <jbra...@sonic.net> wrote:
>>> I wasn't convinced of the accuracy of that test.
>> [snip]
> carlfo...@comcast.net wrote:
>> Who are we going to believe, you or our lying eyes?
>> "Here's a 24-second 3mb video showing my Park tension gauge needle
>> dropping from over 24 to under 24 (less tension) as 60 psi of air
>> hisses out of a 26 x 1.75 mtb clincher tire on a no-box-section steel
>> rim:"
>> http://www.youtube.com/v/NeG5o3WX7Mw
>> Watch the Park tension gauge's needle drop as the air hisses out of
>> the tire.
datakoll wrote:
> THE GAUGE USED MEASURES 360 DEGREES ?
Can't be. Too hot for most tires.
--
Andrew Muzi
<www.yellowjersey.org/>
Open every day since 1 April, 1971
carl...@comcast.net
Or our lying dial calipers?
***
Mike Schwab suggested the rim-spreading explanation, which was
confirmed by simple and repeatable measurements:
mike.a.sch...@gmail.com wrote:
"I would say adding air pressure pushes the sidewall out, levering
through the right angle, and lifting at the spoke holes, lifting at
the spoke holes. Can you measure the width inflated?"
carlfo...@comcast.net wrote:
"The wider 1.75" tire at lower pressure on a wider 26" simple no-box
steel rim lost spoke tension as 60 psi was released.
Dear Mike
D'oh!
I only thought about measuring the wheel diameter, which is far too
big for my calipers. Your suggestion is far more sensible.
And yes, that looks like the explanation.
raw
rim
widths
26" 700x26 700x26
no-box no-box box ma-3
psi steel aluminum aluminum
---- ------ -------- --------
0 1.082" 0.901" 0.085"
60 1.118" 0.907" 0.086"
120 n/a 0/912" 0.088"
total
rim
spread
26" 700x26 700x26
no-box no-box box ma-3
psi steel aluminum aluminum
---- ------ -------- --------
0 x x x
60 0.036" 0.006" 0.001"
120 n/a 0.011" 0.003"
At the same 60 psi, the wide 26" no-box steel rim spread 6 times as
much as the narrow 700c no-box aluminum rim, and 36 times as much as
the 700c boxed aluminum MA-3 rim.
http://groups.google.com/group/rec.bicycles.tech/msg/a5eba5e94f0c6f5d
Of course, no one just pumping up the mtb tire would be likely to
notice the roughly 1/32nd of an inch widening of the steel rim.
http://groups.google.com/group/rec.bicycles.tech/msg/a5eba5e94f0c6f5d
***
Cheers,
Carl Fogel
Jobst Brandt
>>> Actually, spoke tension on clincher rims will increase _or_ decrease
>>> with tire inflation.
>>> What happens depends on the rim.
>>> On a strongly braced narrow rim, increased tire pressure constricts
>>> the tire and squeezes the rim, so the spokes will show a loss of
>>> tension.
>>> But on a wide single-wall MTB rim, increased tire pressure will
>>> spread the rim edges apart, drawing the center of the rim outward
>>> and pulling on the spokes, _raising_ spoke tension.
>>> We went through the measurements in 2006:
>> I wasn't convinced of the accuracy of that test.
> Who are we going to believe, you or our lying eyes?
> "Here's a 24-second 3mb video showing my Park tension gauge needle
> dropping from over 24 to under 24 (less tension) as 60 psi of air
> hisses out of a 26 x 1.75 mtb clincher tire on a no-box-section steel
> rim:"
http://www.youtube.com/v/NeG5o3WX7Mw
> Watch the Park tension gauger's needle drop as the air hisses out of
> the tire.
The change seen on that clip is so small that it might be overlooked
without a reverse test of re-inflating the tire. Tension changes I
have seen are easily and rapidly seen without careful scrutiny. Do
you have a similar video of a common aluminum box section rim about
20mm wide? I'm sorry, I don't have videos. My information came from
the days of tubulars and was retested on clinchers to make sure the
effect hadn't gone away, but sampled only by change in pitch of
plucked spokes.
Jobst Brandt
thirty-six
> Carl Fogel wrote:
> >>> Actually, spoke tension on clincher rims will increase _or_ decrease
> >>> with tire inflation.
> >>> What happens depends on the rim.
> >>> On a strongly braced narrow rim, increased tire pressure constricts
> >>> the tire and squeezes the rim, so the spokes will show a loss of
> >>> tension.
> >>> But on a wide single-wall MTB rim, increased tire pressure will
> >>> spread the rim edges apart, drawing the center of the rim outward
> >>> and pulling on the spokes, _raising_ spoke tension.
> >>> We went through the measurements in 2006:
>
>
> >> I wasn't convinced of the accuracy of that test.
> > Who are we going to believe, you or our lying eyes?
> > "Here's a 24-second 3mb video showing my Park tension gauge needle
> > dropping from over 24 to under 24 (less tension) as 60 psi of air
> > hisses out of a 26 x 1.75 mtb clincher tire on a no-box-section steel
> > rim:"
>
> http://www.youtube.com/v/NeG5o3WX7Mw
>
> > Watch the Park tension gauger's needle drop as the air hisses out of
> > the tire.
>
> The change seen on that clip is so small that it might be overlooked
> without a reverse test of re-inflating the tire.
You put me in fits.
> Tension changes I
> have seen are easily and rapidly seen without careful scrutiny. Do
> you have a similar video of a common aluminum box section rim about
> 20mm wide? I'm sorry, I don't have videos. My information came from
> the days of tubulars and was retested on clinchers to make sure the
> effect hadn't gone away, but sampled only by change in pitch of
> plucked spokes.
With stupidly bowed spokes. ROTF
Jobst Brandt
>>>> Actually, spoke tension on clincher rims will increase _or_
>>>> decrease with tire inflation.
>>>> What happens depends on the rim.
>>>> On a strongly braced narrow rim, increased tire pressure
>>>> constricts the tire and squeezes the rim, so the spokes will show
>>>> a loss of tension.
>>>> But on a wide single-wall MTB rim, increased tire pressure will
>>>> spread the rim edges apart, drawing the center of the rim outward
>>>> and pulling on the spokes, _raising_ spoke tension.
>>>> We went through the measurements in 2006:
>>> I wasn't convinced of the accuracy of that test.
>> Who [whom] are we going to believe, you or our lying eyes?
>> "Here's a 24-second 3mb video showing my Park tension gauge needle
>> dropping from over 24 to under 24 (less tension) as 60 psi of air
>> hisses out of a 26 x 1.75 mtb clincher tire on a no-box-section
>> steel rim:"
http://www.youtube.com/v/NeG5o3WX7Mw
>> Watch the Park tension gauge's needle drop as the air hisses out of
>> the tire.
> Or our lying dial calipers?
I don't contest that inflation tends to spread rims. The more
significant elastic response is to the radial stress and that has been
measured by assessing spoke tension. The reason for that change is
direct and repeatable. We've had a discussion on that when
non-believers suggested it was caused by air pressure on the bed of
the rim, something that doesn't hold for tubulars or garden and
industrial hoses that have a special bias ply to prevent constriction.
I recall long ago how a garden cheap hose, coiled at the water faucet,
shrunk when the water was turned on with the hose nozzle closed. The
distortion of tubular tires inflated off a rim brought that image back
and subsequent switch to clinchers made me wonder why they should be
any different... as pressure hoses, and for instance, clear Tygon
hoses with nylon weave, make obvious.
I think if you give this a logical review, you'll see that bicycle
tires, both clinchers and tubulars, constrict when inflated and the
force can be evaluated with a bit of trigonometry. It would be a good
general science exercise for high school.
Jobst Brandt
carl...@comcast.net
>Carl Fogel wrote:
>
>>>> Actually, spoke tension on clincher rims will increase _or_ decrease
>>>> with tire inflation.
>
>>>> What happens depends on the rim.
>
>>>> On a strongly braced narrow rim, increased tire pressure constricts
>>>> the tire and squeezes the rim, so the spokes will show a loss of
>>>> tension.
>
>>>> But on a wide single-wall MTB rim, increased tire pressure will
>>>> spread the rim edges apart, drawing the center of the rim outward
>>>> and pulling on the spokes, _raising_ spoke tension.
>
>>>> We went through the measurements in 2006:
>
> http://groups.google.com/group/rec.bicycles.tech/browse_frm/thread/e3864af31619d8d4/511343ee76d906fd#511343ee76d906fd
>
>>> I wasn't convinced of the accuracy of that test.
>
>> Who are we going to believe, you or our lying eyes?
>
>> "Here's a 24-second 3mb video showing my Park tension gauge needle
>> dropping from over 24 to under 24 (less tension) as 60 psi of air
>> hisses out of a 26 x 1.75 mtb clincher tire on a no-box-section steel
>> rim:"
>
> http://www.youtube.com/v/NeG5o3WX7Mw
>
>> Watch the Park tension gauger's needle drop as the air hisses out of
>> the tire.
>
>The change seen on that clip is so small that it might be overlooked
>without a reverse test of re-inflating the tire.
[snip]
Dear Jobst,
It isn't overlooked--anyone can look at it.
Why do you think the Park tension gauge needle went the other way when
Dianne tested box-section 700c rims?
What do you think would cause the needle to drop instead of rise?
Cheers,
Carl Fogel
carl...@comcast.net
[snip]
Dear Jobst,
No one is likely to contest that the single-wall MTB rim spread 36
times as far as the 700c box-section rim.
And you're the only person who seems to think that the needle moving
the wrong way on a spoke tension gauge is somehow not believable or
inaccurate.
Dianne did the same thing with 700c box section rims with the same
Park spoke tension gauge--and the needle moved the other way for the
other kind of rim.
The explanation suggested by Mike Schwab and confirmed by dial caliper
measurements of the rims with tires inflated and uninflated makes
perfect sense--the spreading of the wide MTB rim, which is suffers
much greater applied hoop stress, pulls the spokes outward and
increases the tension far more than the hoop stress reduces it.
Cheers,
Carl Fogel
sergio
> I think if you give this a logical review, you'll see that bicycle
> tires, both clinchers and tubulars, constrict when inflated and the
> force can be evaluated with a bit of trigonometry. It would be a good
> general science exercise for high school.
If I understnd what is discussed here, that is no excercise at all.
Common sense, or a continuity argument, is just enough.
When the tube is held, straight, between end points, while being
inflated it expands radially, equally so in every direction if it is
itself rotationally invariant.
Upon starting from a bent configuration it does tend to expand as
well: nowhere can the deformation be reverse.
Sergio
Pisa
Jobst Brandt
>> I think if you give this a logical review, you'll see that bicycle
>> tires, both clinchers and tubulars, constrict when inflated and the
>> force can be evaluated with a bit of trigonometry. It would be a
>> good general science exercise for high school.
> If I understand what is discussed here, that is no exercise at all.
> Common sense, or a continuity argument, is just enough.
> When the tube is held, straight, between end points, while being
> inflated it expands radially, equally so in every direction if it is
> itself rotationally invariant.
> Upon starting from a bent configuration it does tend to expand as
> well: nowhere can the deformation be reverse.
The tube cross section expands, but the length of the tube shrinks at
the same time if the casing has a 45° bias ply as bicycle tires do.
You must have observed the response of a tubular tire when inflated
unsupported by a rim. That occurs because it constricts and the base
tape and seam resist shortening so the tire turns inside-out with the
tread inward and base tape outward.
To avoid this effect, hoses have a special neutral cord bias of 35.27°
instead of 45° to avoid that.
Jobst Brandt
Peter S.
> [snip]
>
> Dear Jobst,
>
> Who are we going to believe, you or our lying eyes?
>
> "Here's a 24-second 3mb video showing my Park tension gauge needle
> dropping from over 24 to under 24 (less tension) as 60 psi of air
> hisses out of a 26 x 1.75 mtb clincher tire on a no-box-section steel
> rim:"
> http://www.youtube.com/v/NeG5o3WX7Mw
>
> Watch the Park tension gauge's needle drop as the air hisses out of
> the tire.
You may be right, but that video alone is rather unconvincing in
itself, eg. the average spoke tension may rise when the rim expand
even though some spoke lose tension like if the rim was slightly oval
to being with. Furthermore the Park Tool tensiometer is used in
contradiction to its instruction manual that emphasizes that the TM-1
should be held horizontally when measuring, and also that it should be
released gently between every measurement before a reliable deflection
reading can be reached, which isn't done neither. Finally it isn't
shown where on the spoke the the measurement takes place nor what the
thickness of the spoke is.
--
Regards
carl...@comcast.net
Dear Peter,
So random spokes on several occasions just happened to lose tension
when I deflated that tire on that single-wall wide MTB rim several
times and finally made a movie of it . . .
And random spokes on several narrow box section 700c wheels just
happened to gain tension when Dianne did the same thing several times
with extensive photographs of her Park tension gauge . . .
What do you think the spoke thickness has to do with the _direction_
in which the needle moves? Or whether the tool is hanging sideways or
upside down? Think about how that simple tool works.
When you deflate the tire, the needle on the spring gauge moves.
It moves one way when hanging on a spoke on a narrow box-section 700c
rim that expands sideways only 0.001 to 0.006 inches when inflated.
It moves the other way when hanging on a spoke on a wide single-wall
MTB rim that expands sideways 0.36 inches when inflated.
Cheers,
Carl Fogel
The gauge moves the
Peter S.
> So random spokes on several occasions just happened to lose tension
> when I deflated that tire on that single-wall wide MTB rim several
> times and finally made a movie of it . . .
As I said, your interpretation may be correct, and perhaps you have
made some proper measurements to back it up, but the movie shows only
one data point being gathered, and that by an instrument being used
against instructions. Not very convincing in itself.
> And random spokes on several narrow box section 700c wheels just
> happened to gain tension when Dianne did the same thing several times
> with extensive photographs of her Park tension gauge . . .
>
> What do you think the spoke thickness has to do with the _direction_
> in which the needle moves?
Say that the spoke thickness was 1.6 mm, then 24-25 on the TM-1 is way
off the chart. Using a measuring tool beyond its stated capacity may
result in weird data and severely lessens the persuasive power of any
data gathered. So not knowing the spoke thickness reduces the
persuasive power of the video.
> Or whether the tool is hanging sideways or
> upside down? Think about how that simple tool works.
Regarding Park Tool's explicit instruction to operate the TM-1 in the
horizontal plane, then I must assume that Park Tool do so for a
reason. Perhaps gravity influence the lever enough to make a
difference. Again, using a measurement tool against instruction
severely lessens the persuasive power of any data gathered and
therefore he persuasive power of the video.
So given the alternative of believing "my own lying eyes" or Jobst
Brandt based on a single data point gathered, using an instrument used
against instruction without crucial data being known like spoke
thickness or where on the spoke things were measured, I would be wise
to choose Jobst Brandt.
> When you deflate the tire, the needle on the spring gauge moves.
>
> It moves one way when hanging on a spoke on a narrow box-section 700c
> rim that expands sideways only 0.001 to 0.006 inches when inflated.
Sure that is what to be expected.
>
> It moves the other way when hanging on a spoke on a wide single-wall
> MTB rim that expands sideways 0.36 inches when inflated.
"Correlation does not imply causation" as the saying goes. Again an
elliptical wheel may result in one or more spoke becoming looser while
the overall tension actually rises.
It would therefore be much more convincing if you measured average
tension on the entire wheel before and after tire inflation and made
your measurements according to how the TM-1 manual says, eg.
horizontally and gently releasing the lever at each measurement.
--
Regards
carl...@comcast.net
<phs...@gmail.com> wrote:
>On 24 Jan., 06:58, carlfo...@comcast.net wrote:
>
>> So random spokes on several occasions just happened to lose tension
>> when I deflated that tire on that single-wall wide MTB rim several
>> times and finally made a movie of it . . .
>
>As I said, your interpretation may be correct, and perhaps you have
>made some proper measurements to back it up, but the movie shows only
>one data point being gathered, and that by an instrument being used
>against instructions. Not very convincing in itself.
[snip]
Dear Peter,
If you read the original thread, you'll see that I repeated the
experiment several times before starting the thread because I was so
surprised that the needle moved the "wrong" way on that particular MTB
rim. Then I made the movie to show what happened, again and again.
You'll also see that I mentioned Dianne's extensive experiments and
photos of 700c box rims, showing the same kind of motion on the same
Park spoke tension gauge, but with the tension behaving as expected.
The two kinds of rims behave differently.
The instrument is nothing more than a spring-loaded arm pressing
against the middle of a spoke span against two fixed posts. The motion
of arm is magnified by the indicator needle.
If spoke tension drops, the spring pushes the spoke farther sideways
and the indicator moves one way.
If spoke tension rises, the spring is pushed back the other way and
the indicator moves in the opposite direction.
It's the same as a simple spring scale in the bathroom. If you stand
on the scale and then pick up a weight, the needle moves one way. If
you drop the weight, the needle moves the other way.
Cheers,
Carl Fogel