Why the notches in a Presta nut?
Glenn Ammons
to close the Presta valve on one of my inner tubes. Why are they
there?
Thanks.
--glenn
Benjamin Lewis
> I noticed tonight that there are two notches in the nut that is used
> to close the Presta valve on one of my inner tubes. Why are they
> there?
Perhaps it lets the conical end compress slightly, so it stays closed
better? (This is just a guess.)
--
Benjamin Lewis
Don't take life so serious, son, it ain't nohow permanent.
-- Walt Kelly
Appkiller
Muzi stocks them.
Appkiller
Quality is a characteristic of software design; testing merely reveals
the level of that characteristic.
Glenn Ammons <amm...@salsa.cs.wisc.edu> wrote in message news:<fxd7ker...@salsa.cs.wisc.edu>...
jobst....@stanfordalumni.org
> I noticed tonight that there are two notches in the nut that is used
> to close the Presta valve on one of my inner tubes. Why are they
> there?
Are you serious? When you want to let air out, the nut is loosened
and depressed until the conical face seats in the valve body. If
there were no slots, air would not escape past this conical seal.
Pressing the valve partially is impractical because the air seal is
also conical and has a high and sudden break loose force... as you
may have noticed when pumping.
Jobst Brandt <jobst....@stanfordalumni.org> Palo Alto CA
A Muzi
> to close the Presta valve on one of my inner tubes. Why are they
> there?
Look more closely.
The standard way to make that part is by punching a butterfly-shaped bit of
sheet metal, threading the center, and wrapping the ends down across the
valve pin. This makes the little thing easy to grab in your fingers while
leaving only a couple of threads on the valve pin. The whole thing is
simple, lightweight and cheap to make (compared to a solid chunk of
material, that is)
--
Andrew Muzi
http://www.yellowjersey.org
Open every day since 1 April 1971
Benjamin Lewis
> Glenn Ammons writes:
>
>> I noticed tonight that there are two notches in the nut that is used
>> to close the Presta valve on one of my inner tubes. Why are they
>> there?
>
> Are you serious? When you want to let air out, the nut is loosened
> and depressed until the conical face seats in the valve body. If
> there were no slots, air would not escape past this conical seal.
Geez, now I feel stupid.
--
Benjamin Lewis
Clothes make the man. Naked people have little or no influence on society.
- Mark Twain
jobst....@stanfordalumni.org
>> I noticed tonight that there are two notches in the nut that is
>> used to close the Presta valve on one of my inner tubes. Why are
>> they there?
> Look more closely. The standard way to make that part is by
> punching a butterfly-shaped bit of sheet metal, threading the
> center, and wrapping the ends down across the valve pin. This makes
> the little thing easy to grab in your fingers while leaving only a
> couple of threads on the valve pin. The whole thing is simple,
> lightweight and cheap to make (compared to a solid chunk of
> material, that is)
That may sound good but it ain't so. These little knurled brass valve
nuts were machined on screw machines from solid brass rod for a long
time before sheet metal fold-ups with threads were practical. The next
question (that came up previously) is why have a nut at all. The
answer is that Presta valves have no closure spring and would readily
leak if not secured. A rider in my circle of friends thought didn't
believe this so he left it unscrewed. Subsequently, on a fast
descent, air leaked out of his tire fairly quickly as the mass of nut
and valve plunger was depressed by centrifugal force.
jobst....@stanfordalumni.org
>> I noticed tonight that there are two notches in the nut that is
>> used to close the Presta valve on one of my inner tubes. Why are
>> they there?
> Look more closely. The standard way to make that part is by
> punching a butterfly-shaped bit of sheet metal, threading the
> center, and wrapping the ends down across the valve pin. This makes
> the little thing easy to grab in your fingers while leaving only a
> couple of threads on the valve pin. The whole thing is simple,
> lightweight and cheap to make (compared to a solid chunk of
> material, that is)
That may sound good but it ain't so. These little knurled brass valve
nuts were machined on screw machines from solid brass rod for a long
time before sheet metal fold-ups with threads were practical. The
next question (that came up previously) is why have a nut at all. The
answer is that Presta valves have no closure spring and would readily
leak if not secured. A rider in my circle of friends didn't believe
Frank Krygowski
>
> They're for a special presta wrench for the severely arthritic. Andy
> Muzi stocks them.
And Shimano is presently working on a new design that will require a
different wrench. ;-)
--
Frank Krygowski frkr...@cc.ysu.edu
Doug Milliken
On Wed, 4 Dec 2002 jobst....@stanfordalumni.org wrote:
....
> time before sheet metal fold-ups with threads were practical. The
> next question (that came up previously) is why have a nut at all. The
> answer is that Presta valves have no closure spring and would readily
> leak if not secured. A rider in my circle of friends didn't believe
> this so he left it unscrewed. Subsequently, on a fast descent, air
> leaked out of his tire fairly quickly as the mass of nut and valve
> plunger was depressed by centrifugal force.
Long ago I wondered about this (it come up here several years ago) --
because many disk wheels (for time trials, etc) use valve extenders that do
not allow the Presta nut to be tightened. I haven't heard of any air-outs
on these, or seen any movement by the disk wheel mfrs to improve on the
simple thread-on tube extender.
Is the key that tt riders don't decend at a "high enough" speed, avoiding
the problem? Or maybe the pressure used on these tires is high enough
to hold the Presta sealed against centrifugal force?
-- Doug Milliken
A Muzi
> >> used to close the Presta valve on one of my inner tubes. Why are
> >> they there?
>
I(a...@yellowjersey.org) offered:
> > Look more closely. The standard way to make that part is by
> > punching a butterfly-shaped bit of sheet metal, threading the
> > center, and wrapping the ends down across the valve pin. This makes
> > the little thing easy to grab in your fingers while leaving only a
> > couple of threads on the valve pin. The whole thing is simple,
> > lightweight and cheap to make (compared to a solid chunk of
> > material, that is)
>
<jobst....@stanfordalumni.org> wrote in message
news:G%dH9.52557$Ik.14...@typhoon.sonic.net...
> That may sound good but it ain't so. These little knurled brass valve
> nuts were machined on screw machines from solid brass rod for a long
> time before sheet metal fold-ups with threads were practical. The
> next question (that came up previously) is why have a nut at all. The
> answer is that Presta valves have no closure spring and would readily
> leak if not secured. A rider in my circle of friends didn't believe
> this so he left it unscrewed. Subsequently, on a fast descent, air
> leaked out of his tire fairly quickly as the mass of nut and valve
> plunger was depressed by centrifugal force.
Oh, for chrissake it _is_ exactly so.
He asked why the particular shape of a modern one.
I answered to that.
The fact that they were once different is truly interesting but not in
opposition to the current design.
A slot or unevenness can be made in many ways to allow air to escape but
this thing is this way because it's cheaper than making a solid piece.
Mike Elliott
jobst....@stanfordalumni.org says...
> The
> answer is that Presta valves have no closure spring and would readily
> leak if not secured. A rider in my circle of friends didn't believe
> this so he left it unscrewed. Subsequently, on a fast descent, air
> leaked out of his tire fairly quickly as the mass of nut and valve
> plunger was depressed by centrifugal force.
>
I'll be darned. It never occurred to me that them little bits would
be subjected to sufficient centrifugal force to overcome the air
pressure in the tire. I don't have a taken-apart Presta-valved tube
to look at: How much surface is exposed to the tube's internal
pressure? How much centrifugal force is applied to the valve bits at
your higher speeds? By god, with a little math and a battery in my
trusty HP, we should be able to work out a handy nutless Presta valve
maximum safe speed vs tire pressure chart (assuming no road bumps to
bounce the valve bits off the valve seat).
MikeE
jobst....@stanfordalumni.org
>>>> I noticed tonight that there are two notches in the nut that is
>>>> used to close the Presta valve on one of my inner tubes. Why are
>>>> they there?
>>> Look more closely. The standard way to make that part is by
>>> punching a butterfly-shaped bit of sheet metal, threading the
>>> center, and wrapping the ends down across the valve pin. This
>>> makes the little thing easy to grab in your fingers while leaving
>>> only a couple of threads on the valve pin. The whole thing is
>>> simple, lightweight and cheap to make (compared to a solid chunk
>>> of material, that is)
>> That may sound good but it ain't so. These little knurled brass
>> valve nuts were machined on screw machines from solid brass rod for
>> a long time before sheet metal fold-ups with threads were
>> practical. The next question (that came up previously) is why have
>> a nut at all. The answer is that Presta valves have no closure
>> spring and would readily leak if not secured. A rider in my circle
>> of friends didn't believe this so he left it unscrewed.
>> Subsequently, on a fast descent, air leaked out of his tire fairly
>> quickly as the mass of nut and valve plunger was depressed by
>> centrifugal force.
> Oh, for chrissake it _is_ exactly so. He asked why the particular
> shape of a modern one.
The question was "why are they there?" not "why are they that shape?"
or "how do they achieve that shape?"
> I answered to that.
Your answer implied that the slots were an artifact of the method of
manufacture, to which I replied as above.
> The fact that they were once different is truly interesting but not in
> opposition to the current design.
That was not the point if you review what was said.
> A slot or unevenness can be made in many ways to allow air to escape
> but this thing is this way because it's cheaper than making a solid
> piece.
No doubt, but that still does not explain why the slots are there.
That was the question and it is where our descriptions differ.
jobst....@stanfordalumni.org
I can't reliably say that deflation will occur, but in this case it
was more than 45mph descent with some rough pavement (the descent into
Portola State Park), a section of road on which we have reached 50mph.
Mike Elliott
jobst....@stanfordalumni.org says...
Well, c'mon - let's do the math! Was your friend within an order of
magnitude of the centrifugal force needed to open the valve? If I
even had the foggiest grasp of physics I could do this in a flash.
(Sound of better-educated people reaching for calculators.)
Smarter guys could even come up with numbers for the road-induced
shocks required to jar the valve open. And remember kids, every time
the valve lets a bit of air out, it becomes easier and easier to open
it the next time!
MikeE
Richard Kaiser
> In article <MciH9.52583$Ik.14...@typhoon.sonic.net>,
> jobst....@stanfordalumni.org says...
>> Mike Elliott writes:
>>
>> >> The answer is that Presta valves have no closure spring and would
>> >> readily leak if not secured. A rider in my circle of friends didn't
>> >> believe this so he left it unscrewed. Subsequently, on a fast
>> >> descent, air leaked out of his tire fairly quickly as the mass of
>> >> nut and valve plunger was depressed by centrifugal force.
>>
>> I can't reliably say that deflation will occur, but in this case it was
>> more than 45mph descent with some rough pavement (the descent into
>> Portola State Park), a section of road on which we have reached 50mph.
>
> Well, c'mon - let's do the math! Was your friend within an order of
> magnitude of the centrifugal force needed to open the valve? If I even
> had the foggiest grasp of physics I could do this in a flash.
>
> (Sound of better-educated people reaching for calculators.)
>
> Smarter guys could even come up with numbers for the road-induced shocks
> required to jar the valve open. And remember kids, every time the valve
> lets a bit of air out, it becomes easier and easier to open it the next
> time!
>
> MikeE
I'm working on it. Force required only requires cutting open a tube
F = tire_pressure * stem_area
Force applied is a little trickier as a wheel has both rotational and
forward motion. If the max is at the top then a good approximation is
F = stem_weight + stem_weight * 2 * bike_speed / wheel_diameter
Intuition tells me this is the max, but more work is needed to prove it.
At the bottom the force probable drops to
F = stem_weight
I know I have seen some math that describes the motion of a wheel, but
I should be able to rederive it as the sum of rotary motion and linear
motion. Add a little complication for the stem not being at the
outer diameter.
Richard "I'm thinking about it" Kaiser
Michael Dart
"Frank Krygowski" <frkr...@cc.ysu.edu> wrote in message
news:3DED74DE...@cc.ysu.edu...
And a different rim.
Mike
Michael Dart
"Richard Kaiser" <rka...@dimensional.com> wrote in message
news:pan.2002.12.04.15...@dimensional.com...
This is why I love reading this NG. Only you guys could take something like
the lowly presta nut and raise it to the level of rocket science!
Mike
Benjamin Lewis
> Force applied is a little trickier as a wheel has both rotational and
> forward motion. If the max is at the top then a good approximation is
> F = stem_weight + stem_weight * 2 * bike_speed / wheel_diameter
> Intuition tells me this is the max, but more work is needed to prove it.
Uh, why do you think forward motion will have any effect? Force is related
to mass and acceleration. I'm guessing forces due to acceleration of the
bicycle are insignificant compared to the force due to the centripetal
acceleration of the wheel. The forward speed of the bicycle is irrelevant.
--
Benjamin Lewis
"Plastic gun. Ingenious. More coffee, please."
-- The Phantom comics
Eric Topp
>I noticed tonight that there are two notches in the nut that is used
>to close the Presta valve on one of my inner tubes. Why are they
>there?
Others have pointed out that the slot in the presta nut is
important for letting air out when the valve is depressed,
but it's even more important for letting air *into* the
valve during inflation. If the slot were not there the
pump pressure would seal the nut against the valve stem.
-=E
jobst....@stanfordalumni.org
>> Force applied is a little trickier as a wheel has both rotational
>> and forward motion. If the max is at the top then a good
>> approximation is F = stem_weight + stem_weight * 2 * bike_speed /
>> wheel_diameter Intuition tells me this is the max, but more work is
>> needed to prove it.
> Uh, why do you think forward motion will have any effect? Force is
> related to mass and acceleration. I'm guessing forces due to
> acceleration of the bicycle are insignificant compared to the force
> due to the centripetal acceleration of the wheel.
Make that centrifugal 'force' and road shock, the second quantity
being more difficult to determine without an accelerometer on the bar
stem. The effective piston diameter of the a Presta valve is 3.4mm
and, when not screwed down, most are easily dislodged from their seat
as is apparent from hand pump inflation.
> The forward speed of the bicycle is irrelevant.
Yes, but indirectly it relates to road shock, the rate at which
pavement irregularities are encountered.
Mike Elliott
Tim McNamara
"Michael Dart" <mrd...@erols.com> wrote:
Not to mention a new way of attaching the wheel to a bicycle: Shimano
Dropout Retention Dynamics. You'll need the SDRD-7846 dropouts to use
those wheels, son.
A Muzi
> In article <G%dH9.52557$Ik.14...@typhoon.sonic.net>,
> jobst....@stanfordalumni.org says...
> > The
> > answer is that Presta valves have no closure spring and would readily
> > leak if not secured. A rider in my circle of friends didn't believe
> > this so he left it unscrewed. Subsequently, on a fast descent, air
> > leaked out of his tire fairly quickly as the mass of nut and valve
> > plunger was depressed by centrifugal force.
"Mike Elliott" <j.michae...@NOSPAMMYcox.net> wrote in message > I'll
be darned. It never occurred to me that them little bits would
> be subjected to sufficient centrifugal force to overcome the air
> pressure in the tire. I don't have a taken-apart Presta-valved tube
> to look at: How much surface is exposed to the tube's internal
> pressure? How much centrifugal force is applied to the valve bits at
> your higher speeds? By god, with a little math and a battery in my
> trusty HP, we should be able to work out a handy nutless Presta valve
> maximum safe speed vs tire pressure chart (assuming no road bumps to
> bounce the valve bits off the valve seat).
This is known at high speeds in schrader vales as well. Notably John Howard
discovered it when the sprung schrader pins in his valves opened during a
motorpaced speed record attempt. The classic steel caps with rubber liners
went on for the next run.
roofi
stems, therefore I have to use valve extenders and must leave the tube
valves undone so I can pump them up via the valve extender. I've never had a
tyre deflate as you described, but that might be due to using more than 120
psi in the tyres.
<jobst....@stanfordalumni.org> wrote in message
news:G%dH9.52557$Ik.14...@typhoon.sonic.net...
J. Kendziera
by MikeE and I'm curious...
Disregarding insignificant forces such as gravitational force acting
on the nut and force due to forward acceleration of the bike we are
left with centrifugal acceleration (from the rotating tire) and
impulse force (from irregularities in the road surface). For the
inital calculation lets disregard the impulse force and assume the
road is totally smooth: How fast would you have to be going?
Equating forces -
(Force due to tire pressure)=(force due to centrifugal acceleration)
or
P*A=(W*v^2)/(R*g)
where: P=tire pressure; A=valve stem area; W=weight of presta nut;
v=forward velocity of the bike; r=tire radius; g=gravitational
constant
Solving for velocity you get:
v=(P*A*R*g/W)^(1/2)
For some numbers to use, lets assume a standard race bike (pardon my
non metric numbers):
P=100 psi; A=0.0102 in^2; R=13.3 in (700 x 23 tire); g=386.4
in/s^2; W=9.502e-4 lb (0.43 gm)
Which leads to v=2349 in/s or 119 mph (192 km/hr)
119 mph is certainley faster than most, but I do think that if you add
in the forces a rough road would cause it is certainley possible to
cause the nut to release. However, I don't have a good source for
data on impulse forces typically encountered on a bike. Anyone else
out there have any info/data or improvments on my equations that I
missed?
The more accurate equation including these forces would be:
(Force due to tire pressure)=(force due to centrifugal
acceleration)+(Force due to road impulse)
or
v=[(P*A-F)*g*R/W]^(1/2) where F=road impulse force
Jon (just overanalyized the presta nut) Kendziera
Peter Cole
news:4677d6b3.02120...@posting.google.com...
> Well I couldn't resist the statment "Well, c'mon - let's do the math!"
> by MikeE and I'm curious...
> For some numbers to use, lets assume a standard race bike (pardon my
> non metric numbers):
> P=100 psi; A=0.0102 in^2; R=13.3 in (700 x 23 tire); g=386.4
> in/s^2; W=9.502e-4 lb (0.43 gm)
>
> Which leads to v=2349 in/s or 119 mph (192 km/hr)
> Anyone else
> out there have any info/data or improvments on my equations that I
> missed?
Fooling around with presta valves, I noticed that it seems to take less force
on the unscrewed nut to leak air if the force is at a right angle to the valve
axis. The above analysis indicates a pressure force of about a pound, but on a
tire pumped to 100 psi, I was able to get the valve to leak with much less
than a pound of force sideways on a completely unscrewed nut (try it). I think
that road shock induced forces when the valve is at 3 or 9 o'clock might be
the dominant factor. Since (at least on the valves I have seen) there is a
rubber seal at the bottom of the pin, there could also be the conditions for
resonance (mass + spring).
Mike Elliott
jken...@hotmail.com says...
Jon -- nonsense. There can be no such thing as overanalyzing Presta
nuts! These things must be scrutinized completely if r.b.t.'s status
as a geek haven is to remain unchallenged.
MikeE "Taking geekdom another step forward"
Mike Elliott
pete...@nospamatallattbi.com says...
Another mechanism that possibly accounts for the tire deflation that
Jobst may have understandably misattributed to centrifugal force
acting on the valve's piston assembly.
We need road shock data!
MikeE
ant
> stuff snipped
and now.. for the sociology majors in the house--
when i have left my presta nut unscrewed (just forgot), my tire slowly
deflated at a rate much higher than the normal tube air leakage rate.
ive done it twice now. no numbers, but after only a day i could feel
the difference in the tire. and im not doing any 50 mph descents,
here. lord no.
so centripetal force or no, they seem like a good idea.
anthony
jobst....@stanfordalumni.org
> When I have left my presta nut unscrewed (just forgot), my tire
> slowly deflated at a rate much higher than the normal tube air
> leakage rate. I've done it twice now. No numbers, but after only a
> day I could feel the difference in the tire. And I'm not doing any
> 50 mph descents, here. Lord no.
> So centripetal force or no, they seem like a good idea.
If you make that centrifugal force it could have an effect. The
'centriprtal' club seems to have intimidated people to avoid the
correct term in this phenomenon.
Matt O'Toole
<jobst....@stanfordalumni.org> wrote in message
news:9w5I9.52993$Ik.14...@typhoon.sonic.net...
> If you make that centrifugal force it could have an effect. The
> 'centriprtal' club seems to have intimidated people to avoid the
> correct term in this phenomenon.
The centripetal club is made up mostly of high school physics teachers. In
the real world, as adults, we can talk about centrifugal force again. None
of my editors has ever had a problem with it.
I invite all engineers, would-be engineers, and other technical people to
drop out of the Nebbish Society, and join the "PEP" club -- Plain English
Please!
Matt O.
Mike Elliott
ma...@deltanet.com says...
I deal with folks that get fussy about using "centripetal" the same
way that I deal with those who tell me that I should use "bison"
instead of "buffalo," or "pelargonium" instead of "geranium": I
ignore them.
MikeE "Who knows the difference between `centripetal' and
`centrifugal,' and also knows the difference between `literate' and
`annoying.'
Benjamin Lewis
> Anthony Anagnostou writes:
>
>> So centripetal force or no, they seem like a good idea.
>
> If you make that centrifugal force it could have an effect. The
> 'centriprtal' club seems to have intimidated people to avoid the
> correct term in this phenomenon.
Maybe, but it seems most people still don't realise that there really is no
outward radial force, and that "centrifugal force" is merely a useful way
of looking at things.
--
Benjamin Lewis
Well, he didn't know what to do, so he decided to look at the government,
to see what they did, and scale it down and run his life that way.
-- Laurie Anderson
jobst....@stanfordalumni.org
>> If you make that centrifugal force it could have an effect. The
>> 'centripetal' club seems to have intimidated people to avoid the
>> correct term in this phenomenon.
> The centripetal club is made up mostly of high school physics
> teachers. In the real world, as adults, we can talk about
> centrifugal force again. None of my editors has ever had a problem
> with it.
What they fail to do is look it up in Webster's Dictionary:
http://www.m-w.com/cgi-bin/dictionary
where both words are explained... how simple.
> I invite all engineers, would-be engineers, and other technical
> people to drop out of the Nebbish Society, and join the "PEP" club
> -- Plain English Please!
Just correct English would be OK as well.
Frank Krygowski
>
> In article <bn6I9.28$V95....@news2.news.adelphia.net>,
> ma...@deltanet.com says...
> >
> > <jobst....@stanfordalumni.org> wrote in message
> > news:9w5I9.52993$Ik.14...@typhoon.sonic.net...
> >
> > > If you make that centrifugal force it could have an effect. The
> > > 'centriprtal' club seems to have intimidated people to avoid the
> > > correct term in this phenomenon.
> >
> > The centripetal club is made up mostly of high school physics teachers. In
> > the real world, as adults, we can talk about centrifugal force again. None
> > of my editors has ever had a problem with it.
> >
> > I invite all engineers, would-be engineers, and other technical people to
> > drop out of the Nebbish Society, and join the "PEP" club -- Plain English
> > Please!
> >
>
> I deal with folks that get fussy about using "centripetal" the same
> way that I deal with those who tell me that I should use "bison"
> instead of "buffalo," or "pelargonium" instead of "geranium": I
> ignore them.
I assume none of you has the responsibility to teach people about forces
on rotating objects.
If you did (and attempted to do so competently) you would have to
explain the difference between forces toward the center of rotation, and
forces away from the center. Trust me, you'd find LOTS of students
confused about those forces.
Once you explained them, if you wanted students to retain the knowledge,
you'd enforce the lesson by using the vocabulary correctly.
After a while, you could be casual about the terms once again. But when
you ran into some guy on the street (or on the internet) interchanging
the terms, you'd be forced to wonder: is he just being casual? Or is he
confused?
In my mind, it makes more sense to be precise about "centrifugal" than
about "chain stretch."
--
Frank Krygowski frkr...@cc.ysu.edu
Mike Elliott
says...
> Mike Elliott wrote:
> >
> > In article <bn6I9.28$V95....@news2.news.adelphia.net>,
> > ma...@deltanet.com says...
> > >
> > > <jobst....@stanfordalumni.org> wrote in message
> > > news:9w5I9.52993$Ik.14...@typhoon.sonic.net...
> > >
> > > > If you make that centrifugal force it could have an effect. The
> > > > 'centriprtal' club seems to have intimidated people to avoid the
> > > > correct term in this phenomenon.
> > >
> > > The centripetal club is made up mostly of high school physics teachers. In
> > > the real world, as adults, we can talk about centrifugal force again. None
> > > of my editors has ever had a problem with it.
> > >
> > > I invite all engineers, would-be engineers, and other technical people to
> > > drop out of the Nebbish Society, and join the "PEP" club -- Plain English
> > > Please!
> > >
> >
> > I deal with folks that get fussy about using "centripetal" the same
> > way that I deal with those who tell me that I should use "bison"
> > instead of "buffalo," or "pelargonium" instead of "geranium": I
> > ignore them.
>
> I assume none of you has the responsibility to teach people about forces
> on rotating objects.
Heck no. Well, if my kids started discussing forces on rotating
bodies I'd explain it to them.
> If you did (and attempted to do so competently) you would have to
> explain the difference between forces toward the center of rotation, and
> forces away from the center. Trust me, you'd find LOTS of students
> confused about those forces.
No duh. Betcha $20 that less than 10% of the folks you'd find on the
street has ever used either of these terms in their entire lifetime.
Let's hang out in front of a Walmart and interview shoppers.
> Once you explained them, if you wanted students to retain the knowledge,
> you'd enforce the lesson by using the vocabulary correctly.
Yup, if I had the responsibility of teaching others about things like
this. I'm not going to annoy casual friends and acquaintances by
lecturing them or correcting them.
> After a while, you could be casual about the terms once again. But when
> you ran into some guy on the street (or on the internet) interchanging
> the terms, you'd be forced to wonder: is he just being casual? Or is he
> confused?
His imprecision will drive some batty. Others won't notice. Some
might work to educate the guy and others will benefit as a result.
Some might not care.
> In my mind, it makes more sense to be precise about "centrifugal" than
> about "chain stretch."
Yes. I think it's one's temperament that determines whether someone
else's understanding of these terms is important, and whether
correcting him is appropriate for the social situation. I mean,
you're standing with a friend and he says, "I really like those
geraniums." And you say, "As a matter of fact, those are not
geraniums, they are pelargoniums. It's a common mistake." You've
turned into Cliff Clavin. Yikes.
That said, a technical forum like this is an ideal place to discuss
and clarify such matters. Groups like this tend to be self-selecting
for those of a technical mindset, or seeking technical information. A
great place to refine one's understanding of the natural world as it
relates to the physics of bicycling and bicycles.
MikeE
DirtRoadie
>Others have pointed out that the slot in the presta nut is
>important for letting air out when the valve is depressed,
>but it's even more important for letting air *into* the
>valve during inflation. If the slot were not there the
>pump pressure would seal the nut against the valve stem.
I love it when someone is able to point to a CLEARLY correct analysis while all
the irrelevant petty bickering goes on in the background.
Thanks Eric.
DR
Michael Dart
"DirtRoadie" <dirtr...@aol.com> wrote in message
news:20021207174742...@mb-mv.aol.com...
You're right. I'm so disappointed. Here I was looking forward to charts
and graphs of wheel speed, tire pressure and road surface variablility to
open a unscrewed presta valve and they get hung up on semantics. Sigh.
Mike
G.T.
> I mean,
> you're standing with a friend and he says, "I really like those
> geraniums." And you say, "As a matter of fact, those are not
> geraniums, they are pelargoniums. It's a common mistake."
What's wrong with saying geranimus if you're actually looking at geraniums?
Not being a big fan of flowers I had never heard of pelargoniums so I
looked them up: ?"Any of various herbs and shrubs of the genus
Pelargonium, which includes the geraniums."*
Greg
*American Heritage Dictionary
--
"Walking under ladders for a living
You know we should be running round in pubs and bars
We know we'll never grow short
And we'll never grow tall" - the Mekons
Richard Kaiser
> On Wed, 04 Dec 2002, Richard Kaiser wrote:
>
>> Force applied is a little trickier as a wheel has both rotational and
>> forward motion. If the max is at the top then a good approximation is F
>> = stem_weight + stem_weight * 2 * bike_speed / wheel_diameter Intuition
>> tells me this is the max, but more work is needed to prove it.
>
> Uh, why do you think forward motion will have any effect? Force is
> related to mass and acceleration. I'm guessing forces due to
> acceleration of the bicycle are insignificant compared to the force due
> to the centripetal acceleration of the wheel. The forward speed of the
> bicycle is irrelevant.
After much thought, you are correct. I was thinking about velocity.
I remembered the differential of a constant is zero. There still
is the slight complication of the step not being all the way to
the outside.
If nobody else does so soon I will look for an old tire to
take apart to make these measurements.
Richard Kaiser
Frank Krygowski
>
> Mike Elliott wrote:
> > I mean,
> > you're standing with a friend and he says, "I really like those
> > geraniums." And you say, "As a matter of fact, those are not
> > geraniums, they are pelargoniums. It's a common mistake."
>
> What's wrong with saying geranimus if you're actually looking at geraniums?
> Not being a big fan of flowers I had never heard of pelargoniums so I
> looked them up: ?"Any of various herbs and shrubs of the genus
> Pelargonium, which includes the geraniums."*
>
> Greg
>
> *American Heritage Dictionary
What does that dictionary say about "chain stretch"? ;-)
--
Frank Krygowski frkr...@cc.ysu.edu
Zach Kaplan Cycles
section wheels that has a remote valve opening-closing rod. So with
this type of valve extender one can close the valve. In 2000 the night
before the Central Coast Double Century I accidentally forgot to close
the valve with the rod on one of these Spinergy extenders on my rear
wheel. The ride went fine including some descents in the low to mid 40
mph range but then about 130 miles into the ride on the fastest
descent of the ride I reached 56 mph when the rear tyre suddenly and
completely deflated. The rear end fishtailed somewhat but I was able
to come to a stop upright and complete the ride. The tyre had been
inflated to 85 psi (Ritchey Tom Slick 26x1.4). Now I am very careful
to test by pressing the remote extenders to make sure they are fully
closed. The added weight of the remote control rod pressing against
the valve probably made it more likely to release its air than if it
was just a conventional setup without an extender and the valve left
open.
Zach Kaplan
Doug Milliken <bd...@bfn.org> wrote in message news:<H6Ktt...@freenet.buffalo.edu>...
> On Wed, 4 Dec 2002 jobst....@stanfordalumni.org wrote:
>
> ....
> > time before sheet metal fold-ups with threads were practical. The
> > next question (that came up previously) is why have a nut at all. The
> > answer is that Presta valves have no closure spring and would readily
> > leak if not secured. A rider in my circle of friends didn't believe
> > this so he left it unscrewed. Subsequently, on a fast descent, air
> > leaked out of his tire fairly quickly as the mass of nut and valve
> > plunger was depressed by centrifugal force.
>
> Long ago I wondered about this (it come up here several years ago) --
> because many disk wheels (for time trials, etc) use valve extenders that do
> not allow the Presta nut to be tightened. I haven't heard of any air-outs
> on these, or seen any movement by the disk wheel mfrs to improve on the
> simple thread-on tube extender.
>
> Is the key that tt riders don't decend at a "high enough" speed, avoiding
> the problem? Or maybe the pressure used on these tires is high enough
> to hold the Presta sealed against centrifugal force?
>
> -- Doug Milliken
Mike Elliott
eth...@sbcglobal.net says...
> Mike Elliott wrote:
> > I mean,
> > you're standing with a friend and he says, "I really like those
> > geraniums." And you say, "As a matter of fact, those are not
> > geraniums, they are pelargoniums. It's a common mistake."
>
> What's wrong with saying geranimus if you're actually looking at geraniums?
> Not being a big fan of flowers I had never heard of pelargoniums so I
> looked them up: ?"Any of various herbs and shrubs of the genus
> Pelargonium, which includes the geraniums."*
>
"These days there is a lot more awareness of the need for accurate
naming, but commercial interests have persuaded some to still use the
name geranium for a Pelargonium, without much of an attempt at trying
to explain the problem."
See http://www.pelargoniumsandgeraniums.net/pelorger.html
MikeE
The Izzman Returneth
> Oh, for chrissake it _is_ exactly so.
> He asked why the particular shape of a modern one.
> I answered to that.
> The fact that they were once different is truly interesting but not in
> opposition to the current design.
That's odd, because I've never seen the valve nut that you describe.
All of my Presta tubes past and present have solid-machined valve
nuts, apparently brass. Of course, these tubes were all purchased
at the bike shop. Haven't bought a tube from a department store
(yes, K-Mart stocks Presta) since the early 90's.
The Izzman Returneth
>
> on the unscrewed nut to leak air if the force is at a right angle to the valve
> axis.
Another thing to consider is that it only takes n force to open the
valve *initially*. Once the valve is cracked open, the amount of
force needed to open it further / hold it open drops.
Road shock would then appear to be important, at least when the tire
is fully inflated. When the air pressure drops sufficiently, then
the centrifugal inertia alone could hold it open.
The Izzman Returneth
> On Fri, 06 Dec 2002, jobst brandt wrote:
>
> > Anthony Anagnostou writes:
> >
> >> So centripetal force or no, they seem like a good idea.
> >
> > If you make that centrifugal force it could have an effect. The
> > 'centriprtal' club seems to have intimidated people to avoid the
> > correct term in this phenomenon.
>
> Maybe, but it seems most people still don't realise that there really is no
> outward radial force, and that "centrifugal force" is merely a useful way
> of looking at things.
Centrifugal force *is* correct if you make your frame of reference
a point on the wheel itself. No, it's not an outward radial force,
just inertia.
Or put another way, when you ride a bicycle, do you say that you are
pushing yourself (sometimes all too hard) forward, or that you are
pushing the ground backwards ground and the ground pushes you forward?
The Izzman Returneth
> You're right. I'm so disappointed. Here I was looking forward to charts
> and graphs of wheel speed, tire pressure and road surface variablility to
> open a unscrewed presta valve and they get hung up on semantics. Sigh.
How long have you been reading Usenet to be disappointed instead of
being resigned to The Usual Petty Bickering? <g>
--
P. J. Remner
Benjamin Lewis
> Centrifugal force *is* correct if you make your frame of reference
> a point on the wheel itself. No, it's not an outward radial force,
> just inertia.
If you make the wheel your frame of reference, then it appears that there
is an outward radial force, which is what we call centrifugal force.
--
Benjamin Lewis
Dinosaurs aren't extinct. They've just learned to hide in the trees.
jobst....@stanfordalumni.org
>> Maybe, but it seems most people still don't realise that there
>> really is no outward radial force, and that "centrifugal force" is
>> merely a useful way of looking at things.
> Centrifugal force *is* correct if you make your frame of reference
> a point on the wheel itself. No, it's not an outward radial force,
> just inertia.
> Or put another way, when you ride a bicycle, do you say that you are
> pushing yourself (sometimes all too hard) forward, or that you are
> pushing the ground backwards ground and the ground pushes you forward?
The definition is simple enough for readers here to understand it, but
they must forst read it:
http://www.m-w.com/netdict.htm
Is a good start. There are other references.
Mike Elliott
izz...@yahoo.com says...
It's The Usual Petty Bickering that makes it an experience not to be
missed!
MikeE
Matt J
> Well I couldn't resist the statment "Well, c'mon - let's do the math!"
> by MikeE and I'm curious...
>
> on the nut and force due to forward acceleration of the bike we are
> left with centrifugal acceleration (from the rotating tire) and
> impulse force (from irregularities in the road surface). For the
> inital calculation lets disregard the impulse force and assume the
> road is totally smooth: How fast would you have to be going?
>
Yes, but, if we looked at road bumps, wouldn't they not only jar the
valve, but increase the pressure in the tube? perhaps these forces
would wonderfully cancel eachother out? I mean, if the valve stem is
at 9:00, looking at the bike from the drivetrain side, and it hits a
bump, the valve nut doohickey would be pushed back, but the pressure
in the tire would also be greater, right? Just thought I would add
this.
Matt
jobst....@stanfordalumni.org
> Yes, but, if we looked at road bumps, wouldn't they not only jar the
> valve, but increase the pressure in the tube? Perhaps these forces
> would wonderfully cancel each other out? I mean, if the valve stem
> is at 9:00, looking at the bike from the drivetrain side, and it
> hits a bump, the valve nut doohickey would be pushed back, but the
> pressure in the tire would also be greater, right? Just thought I
> would add this.
Oops! Tire pressure does not increase perceptibly under tire load.
That's why you can inflate the spare tire in your car's trunk to 30psi
and note that when installed and the car let off the jack, it is still
30psi for all practical purposes. You'll note that auto tire shops do
all their inflating and balancing of tires not on the car. Impact of
a road bump can easily inflict 10g's on a valve core with no practical
change in pressure.
Mike Elliott
Nast...@hotmail.com says...
What about when the valve is at 12:00? The wheel is jarred upward by
driving over, say, some horrible piece of twisted metal lying on the
road. The presta valve thingamajigs would prefer to NOT go upward due
to Mr. Inertia ("Hey kids, let's all say `Hi' to Mr. Inertia!"),
and the tube pressure would momentarily increase due to the tire
having smashed into that horrible piece of twisted metal. Both forces
will conspire, not unlike members of a Bolshevik cell, to force the
presta valve to open.
It's all too complicated to contemplate. I think I'll have pasta with
pesto with that presta.
MikeE
Mike Elliott
jobst....@stanfordalumni.org says...
>
> Impact of a road bump can easily inflict 10g's on a valve core [snip]
Yeah? Says who?
MikeE
Matt J
> > Yes, but, if we looked at road bumps, wouldn't they not only jar the
> > valve, but increase the pressure in the tube? perhaps these forces
> > would wonderfully cancel eachother out? I mean, if the valve stem is
> > at 9:00, looking at the bike from the drivetrain side, and it hits a
> > bump, the valve nut doohickey would be pushed back, but the pressure
> > in the tire would also be greater, right? Just thought I would add
> > this.
> > Matt
>
> What about when the valve is at 12:00? The wheel is jarred upward by
> driving over, say, some horrible piece of twisted metal lying on the
> road. The presta valve thingamajigs would prefer to NOT go upward due
> to Mr. Inertia ("Hey kids, let's all say `Hi' to Mr. Inertia!"),
> and the tube pressure would momentarily increase due to the tire
> having smashed into that horrible piece of twisted metal. Both forces
> will conspire, not unlike members of a Bolshevik cell, to force the
> presta valve to open.
>
> It's all too complicated to contemplate. I think I'll have pasta with
> pesto with that presta.
>
> MikeE
I must say I loved the "yeah? says who?" in your previous post. It's
always good not to take things at face value, IMHO. Anyway, I can
understand that putting the biker+rider weight on the tire would not
add significant pressure, but if it goes over a bump at 40+ MPH, thats
a couple times the rider weight, no? and if it's at 9:00, the slowing
down of the bike will make it go in, but the added pressure will push
it out? maybe i'm completely off - though the conspiracy theory -
that's got a ring to it. my russian history teacher would be beaming
right now
matt
The Izzman Returneth
> driving over, say, some horrible piece of twisted metal lying on the
> road. The presta valve thingamajigs would prefer to NOT go upward due
> to Mr. Inertia ("Hey kids, let's all say `Hi' to Mr. Inertia!"),
> and the tube pressure would momentarily increase due to the tire
> having smashed into that horrible piece of twisted metal. Both forces
> will conspire, not unlike members of a Bolshevik cell, to force the
> presta valve to open.
>
> It's all too complicated to contemplate. I think I'll have pasta with
> pesto with that presta.
It must be too complicated to comtemplate.
Pressure holds the valve closed, not forcing it open, in absence of
a tight locknut.
Presta valves open by sliding towards the tube. If the wheel were to be
jarred upwards with the valve at 12:00, your friend Mr. Inertia would
be pulling the valve *away* from the tube, making it tighter. Unless
of course all of the clocks you use have 12:00 at the bottom of the
dial (24-hour clock perhaps?).
--
P. J. Remner
Paul Southworth
But if you leave the little nut unscrewed, the centrifugal effect
must certainly cause the little plunger to go down and all the air
to leak out. And if you disagree you must just not be going fast
enough. :-)
ok where'd I put my drink...
jobst....@stanfordalumni.org
>>> Yes, but, if we looked at road bumps, wouldn't they not only jar
>>> the valve, but increase the pressure in the tube? perhaps these
>>> forces would wonderfully cancel each other out? I mean, if the
>>> valve stem is at 9:00, looking at the bike from the drivetrain
>>> side, and it hits a bump, the valve nut doohickey would be pushed
>>> back, but the pressure in the tire would also be greater, right?
>>> Just thought I would add this.
You may not have noticed but your responses are largely leading
questions with implied meanings. If you know about this subject you
might explain it instead of beating around the bush with 'clever'
remarks. Your snide questions are less entertaining than they are
exposing your not knowing how pressure, force, and motion work.
>> Oops! Tire pressure does not increase perceptibly under tire load.
>> That's why you can inflate the spare tire in your car's trunk to
>> 30psi and note that when installed and the car let off the jack, it
>> is still 30psi for all practical purposes. You'll note that auto
>> tire shops do all their inflating and balancing of tires not on the
>> car. Impact of a road bump can easily inflict 10g's on a valve
>> core with no practical change in pressure.
> What about when the valve is at 12:00? The wheel is jarred upward
> by driving over, say, some horrible piece of twisted metal lying on
> the road. The Presta valve thingamajigs would prefer to NOT go
> upward due to Mr. Inertia ("Hey kids, let's all say `Hi' to
> Mr. Inertia!"), and the tube pressure would momentarily increase due
> to the tire having smashed into that horrible piece of twisted
> metal. Both forces will conspire, not unlike members of a Bolshevik
> cell, to force the Presta valve to open.
I think you have this upside down. When the valve is at the bottom,
aka 6:00, its piston is forced open by centrifugal force and road
shock acceleration. That is when air escapes as the valve piston is
loosened from its seat and the inflation pressure of the tire is
compromised. One way to alleviate this is to cut off the nut and end
of the valve piston with diagonal cutters, thereby materially reducing
the mass of the valve piston.
> It's all too complicated to contemplate. I think I'll have pasta
> with pesto with that Presta.
> Impact of a road bump can easily inflict 10g's on a valve core with
> no practical change in pressure.
Yeah? Says who?
It is much like a basketball rebounding from the court or, if a
bicycle is ridden across a typical cattle guard at a speed resonant
with wheel bounce, a pinch flat will occur easily. This indicates a
depression more than ten times that of the static load. Similarly,
riding over a Botts Dot (*) lane marker will have a similar
acceleration.
(*) Botts Dots:
http://www.ttnews.com/members/printEdition/0000575.html
Trent Piepho
Matt J <Nast...@hotmail.com> wrote:
>Mike Elliott <j.michae...@NOSPAMMYcox.net> wrote in message news:<MPG.185eccb0f...@news.west.cox.net>...
>> will conspire, not unlike members of a Bolshevik cell, to force the
>> presta valve to open.
If the valve it at 12:00, it needs to be pushed upward relative to the stem to
open. A bump would push the valve downward, or rather push the wheel up.
>understand that putting the biker+rider weight on the tire would not
>add significant pressure, but if it goes over a bump at 40+ MPH, thats
>a couple times the rider weight, no? and if it's at 9:00, the slowing
Think about it in terms of volume reduction. The amount of air in the tire
stays the same, but the volume of the tube decreases when it gets flattened
against the rim by the bump. Even if you pinch flat, how much do you think
the total volume of the tube decreases? 5%? 10%? Probably more like 1%.
And so that's the corresponding increase in tire pressure.
jobst....@stanfordalumni.org
>>> Yes, but, if we looked at road bumps, wouldn't they not only jar
>>> the valve, but increase the pressure in the tube? perhaps these
>>> forces would wonderfully cancel each other out? I mean, if the
>>> valve stem is at 9:00, looking at the bike from the drivetrain
>>> side, and it hits a bump, the valve nut doohickey would be pushed
>>> back, but the pressure in the tire would also be greater, right?
>>> Just thought I would add this.
You may not have noticed but your responses are largely leading
questions with implied meanings. If you know about this subject you
might explain it instead of beating around the bush with 'clever'
remarks. Your snide questions are less entertaining than they are
exposing your not knowing how pressure, force, and motion work.
>> Oops! Tire pressure does not increase perceptibly under tire load.
>> That's why you can inflate the spare tire in your car's trunk to
>> 30psi and note that when installed and the car let off the jack, it
>> is still 30psi for all practical purposes. You'll note that auto
>> tire shops do all their inflating and balancing of tires not on the
>> car. Impact of a road bump can easily inflict 10g's on a valve
>> core with no practical change in pressure.
> What about when the valve is at 12:00? The wheel is jarred upward
> by driving over, say, some horrible piece of twisted metal lying on
> the road. The Presta valve thingamajigs would prefer to NOT go
> upward due to Mr. Inertia ("Hey kids, let's all say `Hi' to
> Mr. Inertia!"), and the tube pressure would momentarily increase due
> to the tire having smashed into that horrible piece of twisted
> metal. Both forces will conspire, not unlike members of a Bolshevik
> cell, to force the Presta valve to open.
I think you have this upside down. When the valve is at the bottom,
aka 6:00, its piston is forced open by centrifugal force and road
shock acceleration. That is when air escapes as the valve piston is
loosened from its seat and the inflation pressure of the tire is
compromised. One way to alleviate this is to cut off the nut and end
of the valve piston with diagonal cutters, thereby materially reducing
the mass of the valve piston.
> It's all too complicated to contemplate. I think I'll have pasta
David Damerell
>depression more than ten times that of the static load. Similarly,
>riding over a Botts Dot (*) lane marker will have a similar
>acceleration.
>(*) Botts Dots:
>http://www.ttnews.com/members/printEdition/0000575.html
Botts Dots, nothing. These devices "invented" by Botts in the 1950s are no
different from cateyes, invented by Mr. Percy Shaw in the 1930s. But I
suppose he is insufficiently American?
--
David Damerell <dame...@chiark.greenend.org.uk> flcl?
Mike Elliott
No, my watch is right. Here, let me hold it up so you can see it . .
. see? 9 o'clock at the top and 71 o' clock at the bottom.
Oh, wait.
Right then, forget everything I said.
MikeE
Mike Elliott
jobst....@stanfordalumni.org says...
> Mike Elliott writes:
>
> >>> Yes, but, if we looked at road bumps, wouldn't they not only jar
> >>> the valve, but increase the pressure in the tube? perhaps these
> >>> forces would wonderfully cancel each other out? I mean, if the
> >>> valve stem is at 9:00, looking at the bike from the drivetrain
> >>> side, and it hits a bump, the valve nut doohickey would be pushed
> >>> back, but the pressure in the tire would also be greater, right?
> >>> Just thought I would add this.
>
> You may not have noticed but your responses are largely leading
> questions with implied meanings. If you know about this subject you
> might explain it instead of beating around the bush with 'clever'
> remarks. Your snide questions are less entertaining than they are
> exposing your not knowing how pressure, force, and motion work.
That wasn't me (MikeE) that wrote that. It was someone else who had
picked up this thread.
>
> >> Oops! Tire pressure does not increase perceptibly under tire load.
> >> That's why you can inflate the spare tire in your car's trunk to
> >> 30psi and note that when installed and the car let off the jack, it
> >> is still 30psi for all practical purposes. You'll note that auto
> >> tire shops do all their inflating and balancing of tires not on the
> >> car. Impact of a road bump can easily inflict 10g's on a valve
> >> core with no practical change in pressure.
Still not me. That was you, I think.
>
> > What about when the valve is at 12:00? The wheel is jarred upward
> > by driving over, say, some horrible piece of twisted metal lying on
> > the road. The Presta valve thingamajigs would prefer to NOT go
> > upward due to Mr. Inertia ("Hey kids, let's all say `Hi' to
> > Mr. Inertia!"), and the tube pressure would momentarily increase due
> > to the tire having smashed into that horrible piece of twisted
> > metal. Both forces will conspire, not unlike members of a Bolshevik
> > cell, to force the Presta valve to open.
Okay, that part was me.
> I think you have this upside down. When the valve is at the bottom,
> aka 6:00, its piston is forced open by centrifugal force and road
> shock acceleration. That is when air escapes as the valve piston is
> loosened from its seat and the inflation pressure of the tire is
> compromised. One way to alleviate this is to cut off the nut and end
> of the valve piston with diagonal cutters, thereby materially reducing
> the mass of the valve piston.
That would be you again. And you are absolutely correct! Had the
whole darn thing upside down.
> > It's all too complicated to contemplate. I think I'll have pasta
> > with pesto with that Presta.
Uh, me again.
> >> Impact of a road bump can easily inflict 10g's on a valve core with
> >> no practical change in pressure.
That was you again.
> > Yeah? Says who?
Me again. Wanting to know where the 10g's number came from. Earlier
in the thread it was calculated that a bicycle would need to be going
somewhere around 119 mph in order for there to be sufficient force on
the presta bits to overcome the tire pressure. That was on a flat
surface. It was speculated that road shocks might easily jar the
valve open at far lower speeds, but no one was able to come up with
typical peak values for these shocks. In a fashion that I hoped would
come across as cheeky or playful I was hoping to get you to post a
clue about where the 10g number came from. That my tone came across
as "snide" points to my poor writing skills, not to my intention.
> It is much like a basketball rebounding from the court or, if a
> bicycle is ridden across a typical cattle guard at a speed resonant
> with wheel bounce, a pinch flat will occur easily. This indicates a
> depression more than ten times that of the static load. Similarly,
> riding over a Botts Dot (*) lane marker will have a similar
> acceleration.
MikeE
jobst....@stanfordalumni.org
>> (*) Botts Dots:
>> http://www.ttnews.com/members/printEdition/0000575.html
> Botts Dots, nothing. These devices "invented" by Botts in the 1950s
> are no different from cateyes, invented by Mr. Percy Shaw in the
> 1930s. But I suppose he is insufficiently American?
Had you read the item you would have noticed that these dots, although
previously used were different in that they were attached to the road
with a special tough but pliable epoxy and that they were white
porcelain, Mr Bott's contribution to their usefulness and wide spread
use. Subsequently reflective colored plastic lane markers became
practical. Metal (aluminum) ones have been used in Europe for a long
time and frequently come loose exposing the spike that holds them.
I assume Mr. Percy's contribution was to put reflectors in the nailed
metal ones.
There is progress.
Kronk
<j.michae...@NOSPAMMYcox.net> wrote:
>In article <qKdJ9.53794$Ik.15...@typhoon.sonic.net>,
>jobst....@stanfordalumni.org says...
>> Mike Elliott writes:
>>
<...>
>> >> Impact of a road bump can easily inflict 10g's on a valve core with
>> >> no practical change in pressure.
>
>That was you again.
>
>> > Yeah? Says who?
>
>Me again. Wanting to know where the 10g's number came from.
Suppose a cyclist is descending at 50 mph and encounters a road
irregularity that bounces a rim up from its momentum track one quarter
inch over a linear distance of roughly nine inches.
50 mph = 880 inches per second
At this speed it would take .01 second to travel 8.8 inches.
Elevation zero to elevation .25" in 1/100th of a second requires an
upwards acceleration of .5" per .01 second per .01 second.
The speed .5 inch per .01 second is the same as 50 inches per second,
so the above translates into 50 inches per second per .01 second--or
5000 inches per second per second--or 416 ft. per second per
second--or about 13 g's.
Can the tire transmit that much acceleration force to the rim?
Suppose the tire is one inch wide at 100 psi with a four inch contact
zone. The casing tension in that zone would have an aggregate
vertical component of about 400 lbs. so that casing region could
accelerate 40 lbs. at 10 g's (though under a hard impact the
load-affected zone of the tire probably increases to more than 4
inches). Now, while the average wheel load is usually greater than 40
lbs., large scale road undulations and other factors could well reduce
the load on a wheel to much less than 40 lbs. for the .01 second
duration of such a bump, so I'd say it's quite plausible that the tire
could transmit that much acceleration to the rim, briefly, at certain
times. The harder the tire, and the lighter the rider, and the more
the rider rests weight on the pedals rather than on the bars and seat,
and the more large undulations there are in the road, the easier it
would be for high-speed bump accelerations at the rim to exceed 10
g's.
Kronk
Mike Elliott
I love it. Hard numbers. Now -- how can we stir these numbers into
the centrifugal "force" vs. tire pressure discussion earlier on in
this thread to see how likely it is that the presta valve could be
opened while descending a mountain road at high speeds? On a smooth
surface it was calculated that you'd need to whiz along at 119mph in
order to overcome the tire pressure. Maybe assume that the presta is
being subjected to 10g bumps, how rapidly would the wheel have to
rotate in order for the piston to come unseated?
MikeE
Trent Piepho
Mike Elliott <j.michae...@NOSPAMMYcox.net> wrote:
>I love it. Hard numbers. Now -- how can we stir these numbers into
>the centrifugal "force" vs. tire pressure discussion earlier on in
>this thread to see how likely it is that the presta valve could be
>opened while descending a mountain road at high speeds? On a smooth
>surface it was calculated that you'd need to whiz along at 119mph in
>order to overcome the tire pressure. Maybe assume that the presta is
>being subjected to 10g bumps, how rapidly would the wheel have to
>rotate in order for the piston to come unseated?
You need find the force needed to unseat the valve when it's at the 3 and 9
o'clock positions where the force on the nut is acting perpendicular to the
valve stem. In this case the threaded stud acts as a lever for the mass of
the nut, reducing the force required to overcome the tire pressure.
It would look something like this:
*|* <--- force on nut
\|/
|
: | : <- stop of presta stem
: | :
/ | \
:=====: <- right edge of rubber seal is the fulcrum
: :
: ^ : <- presta stem
: | :
: | :
|force of air
Find the mechanical advantage by dividing the distance from the center of the
nut to the edge of the seal inside the valve stem, by the distance from the
edge of the seal to it's center. Though to be exact, you'd need to integrate
the mass of the seal and plunger over the distance from the fulcrum.
Mike Elliott
t...@venturi.cfr.washington.edu says...
This thread, which started as a simple inquiry about the slots on
presta nuts, took a side trip when Jobst mentioned in an aside that a
friend had experienced tire deflation due to centrifugal force
opening the valve during a rapid descent. Through mighty number-
crunching on the part of educated folks, it was determined that on a
flat surface, one would need to go about 120mph in order to force the
valve open. As it that wasn't enough, acceleration due to road shocks
applied parallel with the axis of the valve was thrown into the mix
as a further valve-opening mechanism.
But this -- this mechanism, where road-shock induced acceleration is
applied at right angles to the valve is, to me, very interesting. Who
feels like analyzing *this* system?
Didn't someone mention earlier that only on r.b.t would guys take the
lowly presta valve and make it into rocket science?
MikeE
Kronk
<j.michae...@NOSPAMMYcox.net> wrote:
The centrifugal force part of that is easy.
Suppose that our 50 mph wheel of the previous example has a radius of
13.3 inches, where the radius to the valve stem is 12 inches. The ten
percent reduction in radius nets a ten percent reduction in
circumference and thus a ten percent reduction in circumferential
speed for a given rpm. So 50 mph at the wheel circumference would be
45 mph at the valve.
45 mph = 66 ft. / sec.
Radial acceleration = ( 66 ft. / sec.) * (66 ft. / sec.) / 1 ft. =
4356 ft. / sec. / sec. = 136 g's acceleration.
So all you need to get the centrifugal force from that is to multiply
the weight of the valve core by 136.
Will that be enough to depress the valve radially? Let's say the
valve aperture is almost 2.9 mm. in diameter, for a surface area of
about 6.5 mm. That's roughly .01 square inches, so at 100 psi, the
plunger needs one pound radial force in order to be depressed
radially. I'm not sure how much a valve core weighs, but at 136 g's,
it would have to weigh about 3.3 grams in order to generate a pound of
force. I think it more likely the core weighs less than a gram--I'll
guess about a half gram--so by that guess it would only supply about
one sixth of the force needed for a radial displacement.
Radially, the extra 10 g's from a bump would be insignificant.
However, bumps don't always operate in radial alignment with the
valve. Do we have enough information to figure how much force at
right angles to the valve core would be enough to unseat it? Well, I
know I don't. For that, I would need to know what shape seat the
valve has (probably something vaguely conical), the effective aperture
diameter of the seat, and the lever length of the valve core wire.
For knocking a plunger off axis in a conical seat, air pressure on one
side of the plunger cancels pressure on the other side, so that moving
it off center is almost entirely a function of overcoming the friction
of the valve seat. The smaller the valve seat aperture, and the less
the air pressure, and the greater the centrifugal force, the less the
friction that has to be overcome to move the plunger off-axis.
My suspicion is that centrifugal force is a minor effect when it comes
to valve leakage at fast downhill speeds. I'd bet valve leakage is
almost entirely due to bump accelerations knocking the core back and
forth in the valve stem.
Kronk
Trent Piepho
Mike Elliott <j.michae...@NOSPAMMYcox.net> wrote:
>But this -- this mechanism, where road-shock induced acceleration is
>applied at right angles to the valve is, to me, very interesting. Who
>feels like analyzing *this* system?
It's not just road shocks, think of the amazing torque cyclsits generate when
they accelerate their wheels! As soon as the spokes finish winding up, that
presta nut will be whiplashed against the edge of valve stem.
Someone needs to just get some weights and see how much force in different
directions it takes to unseat the valve.
Richard Brockie
> Had you read the item you would have noticed that these dots, although
> previously used were different in that they were attached to the road
> with a special tough but pliable epoxy and that they were white
> porcelain, Mr Bott's contribution to their usefulness and wide spread
> use. Subsequently reflective colored plastic lane markers became
> practical. Metal (aluminum) ones have been used in Europe for a long
> time and frequently come loose exposing the spike that holds them.
>
> I assume Mr. Percy's contribution was to put reflectors in the nailed
> metal ones.
I am working from memory here - it is a while since I have paid
attention to the details of a cat's eye road reflector, and I am now in
the US and cannot find a webpage which has a useful description.
I don't think that the cat's eyes which are/were widely used in the UK
are simply nailed. The invention that Percy came up with was to house a
pair of marbles (glass balls) in a mechanism which sat proud of the road
surface, yet retracted when run over by a car. The balls are mounted in
heavy rubber in such a way that the act of retraction cleans debris from
the exposed surface of the balls. As such, I think that these sit in a
recess in the tarmac to allow the glass balls to retract fully. It is
certainly the case that driving over cat eyes produces a much less
pronounced noise through the tyres of a car compared with Bott's dots.
I encourage a UK reader to correct the above (cross-posted to
uk.rec.cycling).
--
R.
<> Richard Brockie "Categorical statements
<> The tall blond one. always cause trouble."
<> richard...@yahoo.com
<> http://members.telocity.com/~richard_brockie/
Benjamin Lewis
> I am working from memory here - it is a while since I have paid attention
> to the details of a cat's eye road reflector, and I am now in the US and
> cannot find a webpage which has a useful description.
Here's one:
http://www.jenkins-ip.com/mym/spring2000/t_news01.htm
--
Benjamin Lewis
"Love is a snowmobile racing across the tundra and then suddenly it flips
over, pinning you underneath. At night, the ice weasels come."
--Matt Groening
Mr R@t (2.3 zulu-alpha) [comms room new build]
|| balls to retract fully. It is certainly the case that driving over
|| cat eyes produces a much less pronounced noise through the tyres of
|| a car compared with Bott's dots.
||
|| I encourage a UK reader to correct the above (cross-posted to
|| uk.rec.cycling).
||
Richard - actually *you* are quite correct.
Percy's invention is still pretty much the very same design deployed on the
UKs roads to this day.
The only disadvantage was that naughty schoolboys would sometimes try and
extract the reflective marbles. In true schoolboy fashion, obtaining those
colours that involved the most personal risk (i.e dodging traffic on a busy
slip road) were thought of as most "valuable" to the lads - although I can
put my hand on my heart and say that I *never* nicked any ;)
Alex
christian mock
> all their inflating and balancing of tires not on the car. Impact of
> a road bump can easily inflict 10g's on a valve core with no practical
> change in pressure.
OK, but if I've done the math correctly, at 45 mph =~ 20 m/s the
centrifugal acceleration on a 700C rim's diameter is already at 125g.
Given a tire pressure of 7 bar, the force needed to open the valve is
at 6.3N, so the valve core would have to weigh in at 5 g(rams) to
actually open from the centrifugal force. To me, this means there's a
large margin of safety, and some 10g's of shock don't matter.
ciao,
cm.
--
>> [1] MSFP nennt das dann "Hooverbutton",
> ^^^^^^ Weils so gut saugt?
Das. Und weil's aussieht wie J. Edgar in Fummel.
Martin Herrmann, Peter J. Holzer, Felix Deutsch in dasr
Just zis Guy, you know?
wrote:
>I assume Mr. Percy's contribution was to put reflectors in the nailed
>metal ones.
No, Mr Percy's contribution was to design a completely different
system, and several years beforehand at that. The Catseye is made
primarily of rubber, sits in a cast iron receptacle, and retracts when
a car wheel does over it; as it retracts the rubber mounting wipes the
reflectors keeping them clean. The system is very effective, and no
spikes are involved.
http://www.jenkins-ip.com/mym/spring2000/t_news01.htm
Guy
===
** WARNING ** This posting may contain traces of irony.
http://www.chapmancentral.com (BT ADSL and dynamic DNS permitting)
NOTE: BT Openworld have now blocked port 25 (without notice), so old
mail addresses may no longer work. Apologies.
Michael Dart
"Mike Elliott" <j.michae...@NOSPAMMYcox.net> wrote in message
> In article <at5sj2$7n3$1...@venturi.cfr.washington.edu>,
> t...@venturi.cfr.washington.edu says...
> > In article <MPG.186003809...@news.west.cox.net>,
> > Mike Elliott <j.michae...@NOSPAMMYcox.net> wrote:
> > >I love it. Hard numbers. Now -- how can we stir these numbers into
I'm starting to enjoy this too.
> Didn't someone mention earlier that only on r.b.t would guys take the
> lowly presta valve and make it into rocket science?
>
That would be me.
Also wondering who gets to be the crash test dummy screaming down a mountain
at 119mph, with his presta nuts unscrewed, and heading straight at a row of
Bott's Dots. ;^)
Mike - has Schrader's on his DH bike.
Mike Elliott
What a wonderful idea. We'll need a full film crew.
MikeE
Mike Elliott
c...@tahina.priv.at says...
> <jobst....@stanfordalumni.org> wrote:
>
> > all their inflating and balancing of tires not on the car. Impact of
> > a road bump can easily inflict 10g's on a valve core with no practical
> > change in pressure.
>
> OK, but if I've done the math correctly, at 45 mph =~ 20 m/s the
> centrifugal acceleration on a 700C rim's diameter is already at 125g.
> Given a tire pressure of 7 bar, the force needed to open the valve is
> at 6.3N, so the valve core would have to weigh in at 5 g(rams) to
> actually open from the centrifugal force. To me, this means there's a
> large margin of safety, and some 10g's of shock don't matter.
>
> ciao,
Well, if this is an accurate assessment of the problem, and if folks
actually do experience depressurizing of the tires while screaming
downhill at great speed, it is tempting to suggest that it must be
the road-shock-applied-at-right-angle-to-the-presta-valve
(RSAARATTPV) theory that explains it all.
MikeE
Matt J
> > I'm starting to enjoy this too.
> >
> > > Didn't someone mention earlier that only on r.b.t would guys take the
> > > lowly presta valve and make it into rocket science?
> > >
> >
> > That would be me.
> >
> > Also wondering who gets to be the crash test dummy screaming down a mountain
> > at 119mph, with his presta nuts unscrewed, and heading straight at a row of
> > Bott's Dots. ;^)
>
> What a wonderful idea. We'll need a full film crew.
>
> MikeE
I'll scream down the mountain. Loud. But they'd have to be more than
just my *presta* nuts.
Matt
Alan Collier
room new build]" <ratsn...@sovtel.su> wrote:
>Richard Brockie wrote:
>
>|| balls to retract fully. It is certainly the case that driving over
>|| cat eyes produces a much less pronounced noise through the tyres of
>|| a car compared with Bott's dots.
>||
>|| I encourage a UK reader to correct the above (cross-posted to
>|| uk.rec.cycling).
>||
>
>Richard - actually *you* are quite correct.
>
>Percy's invention is still pretty much the very same design deployed on the
>UKs roads to this day.
not on all roads, though. You'll find that motorways use much smaller
ones that don't have the marbles - they reflect all over. I used a
road the other day that had fancy new ones. They must have solar
panels in the tops, white LEDs inside, and light sensors on the side.
When they 'see' you coming, they light up for quite a long way ahead
of you. You get an interesting effect at the end of corners, when a
whole row of them suddenly light up at once. You can tell they're lit
up most easily by looking in the mirror - normally you would see red
relected from your rear lights. These ones stay white!
Alan
David Damerell
>David Damerell writes:
>> (*) Botts Dots:
>> http://www.ttnews.com/members/printEdition/0000575.html
>>Botts Dots, nothing. These devices "invented" by Botts in the 1950s
>>are no different from cateyes, invented by Mr. Percy Shaw in the
>>1930s. But I suppose he is insufficiently American?
>Had you read the item
I did, although I see from the other responses that I was wrong to state
that Botts Dots are identical to cateyes when they are in fact inferior.
>There is progress.
Yes, and there's also ignorance of a superior existing design - something
you have been known to comment on yourself. :-)