From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Using Presta Valves
Date: 18 Aug 1998 16:49:10 GMT

Barry Kade writes:

> Better yet, throw the whole damn tube in the trash can and have your
> rim hole reamed to accept Schraeder valves.  Then when you find
> yourself with a repaired flat but no pump, you can use any gas
> station pump.

I don't know where you ride your bicycle but my flat tires generally
don't occur in proximity to a filling station.  In fact much of the
time I am 30 or more miles from such a place.  Fortunately riders
other than you set the standard for what sort of valves are used on
bicycles.  Therefore, your desire to convert to Schrader valves will
not occur soon.

> If anyone can tell me the advantage of Presta, I'd like to know.

Two main advantages are the slender profile that doesn't cut as large
a hole in the rim, and the lack of a spring that allows easy pumping
with a simple pump head (one with no features except a gasket).

The slender form is probably the more important feature because the
valve hole in the rim describes the minimum cross section and
therefore the ultimate strength of the wheel.  When the rim yields at
the stem hole the wheel fails.  If this cross section is small it
limits spoke tension that is the essence of wheel strength.  A rim may
feel heavy and look strong, but if you consider the cross section at
the stem hole it loses much of that robust appearance.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Using Presta Valves
Date: 18 Aug 1998 20:45:09 GMT

Robin Hubert writes:

> I recently received Vredestein's '98 'Top Sport' catalog (e-mail
> request mailed from Holland!).  Inside, there are valve designations
> for their tubes; amongst them are valves I'm not familiar with,
> including Dunlop and Sclaverand valves, in addition to an "Auto"
> valve, which looks like a Schraeder type to me (but their is a
> "Schrader" valve right next to it).  The Sclaverand valves look
> remarkably like Presta's.

You have discovered that 1. Schrader is spelled without an umlaut or
an "ae" and 2. that the German term for Presta is Sclaverand.  That's
it, no more no less.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Schrader -vs- Presta: why?
Date: 16 Dec 1999 18:49:53 GMT

Roger (who?) writes:

> Presta is allegedly easier to inflate because the force required to push
> the air in is lower due to the smaller diameter.  I would have thought
> the diameter of your pump barrel was more important though.

Roger, you keep on posting a compendium of myth and lore on technical
subjects that you don't understand.  This one is typical where you mix
and waffle.  The presta valve is preferred for two reasons.  It's stem
has a smaller diameter and therefore cuts a smaller hole in a rim, and
it does not have a spring loaded valve so that simple pumps, like the
Silca Impero frame fit pump (that has no valve opening device) can be
used.  The choice was made back in the days when racers had to change
their own tires and inflate them.

Neither the diameter of the stem nor the size of the opening in the
valve affect pumping force for any given pump.  That is a function of
back pressure only, in the case of the Schrader valve, principally the
force to open the spring loaded valve.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Schrader -vs- Presta: why?
Date: 16 Dec 1999 21:26:29 GMT

Ed Chait writes:

>> Roger, you keep on posting a compendium of myth and lore on technical
>> subjects that you don't understand.  This one is typical where you mix
>> and waffle.  The presta valve is preferred for two reasons.  It's stem
>> has a smaller diameter and therefore cuts a smaller hole in a rim,

> Ahhh, but you also propagate myth and lore in a subject that you do
> understand.  The smaller valve hole is of absolutely no advantage in
> terms of rim strength.

Oh?  So what is the limit of spoke tension, the measure of load
carrying capacity of a wheel, if it isn't compressive strength of the
rim?  A Schrader stem hole in a rim is the largest gap in rim cross
section and therefore, the weak point and load limit.  Presta stem
holes are generally as small or smaller than spoke socket holes and
therefore, do not constitute the weakest point.

> I guess from a purely engineering approach, there may be an
> insignificant difference, but from a practical approach, rims don't
> fail at valve holes, whether presta or Schrader.

I don't know what you mean by the cryptic qualification that it makes
no difference.  I think your disclaimer makes no difference, it not
making your statement any more correct.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Schrader -vs- Presta: why?
Date: 17 Dec 1999 17:30:00 GMT

Ed Chait writes:

> I, and many others have drilled out holes on narrow rims to
> accommodate Schrader valves.  From a purely anecdotal standpoint,
> I've drilled out some very narrow road rims and have had no trouble.
> I rode them when I weighed 240lbs.

Your comment is like saying it's nice whether when someone asks
whether the tide is high or low.  Drilling does not cause rim failure
unless you tighten the spokes.  As I stated, the strength of a wheel
depends on spoke tension and spoke tension is limited (for
conventional wheels in contrast to 16 spoke wheels) by the rim
compressive strength that is a direct function of cross sectional
material.  A larger hole limits that load.  I don't care how
overweight a rider is, the wheel cannot be as strong with a large stem
hole as with a smaller one.

> When this subject came up about a year ago, I asked on the NG if
> anyone had ever seen a rim fail at the valve hole, whether Presta or
> Schrader.

The larger hole flatly limits spoke tension.

> The only response was from some more learned folks than I, who said
> that rims would never fail at the valve hole due to their design.

> No one responded that they had ever seen a rim failure at the valve
> hole, or from enlarging a hole to Schrader diameter.  Doing this is
> a fairly common practice.

I have seen rims buckle at the stem hole of a drilled out rim from
spoke tension.  Most people have seen it on anodized rims when the
anodizing wears off irregularly at the bulges around drillings.  You
say that no one responded, but that doesn't make your hypothesis
correct.  That's like the BS presented at truck stop lunch counters.
First I tell a C&B story and you don't complain, then you tell your
C&B story and I don't complain.  We can both go away with assurance
that these tall tales are true.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Schrader -vs- Presta: why?
Date: 17 Dec 1999 17:46:34 GMT

Ed Chait writes:

>> Oh?  So what is the limit of spoke tension, the measure of load
>> carrying capacity of a wheel, if it isn't compressive strength of the
>> rim?  A Schrader stem hole in a rim is the largest gap in rim cross
>> section and therefore, the weak point and load limit.  Presta stem
>> holes are generally as small or smaller than spoke socket holes and
>> therefore, do not constitute the weakest point.

> Really?  Then why is it that when you overtension a rim, as you
> recommend when building up a wheel, it doesn't fail at the valve
> hole?  No doubt many folks have built up Schrader rims using your
> book.

It does.  The misalignment you see when stress relieving is causes by
local yielding.  Because a wow at one place affects alignment of the
whole wheel, you still get a potato(e) chip although it has a more
pronounced jog at the stem hole if you were to look.  Some Fiamme red
label tubular rims had unnecessarily large stem holes when these were
made with sockets.  These easily bulged conspicuously at the stem.

>>> I guess from a purely engineering approach, there may be an
>>> insignificant difference, but from a practical approach, rims
>>> don't fail at valve holes, whether presta or Schrader.

>> I don't know what you mean by the cryptic qualification that it
>> makes no difference.  I think your disclaimer makes no difference,
>> it not making your statement any more correct.

> Have you personally seen a rim fail at the valve hole, or even heard
> of one doing this?

That depends on what you call failure.  Yes I have seen rims fail to
with stand stress relieving at the stem hole.  What you seem to expect
is that the wheel fails in use by breaking apart there.  That won't
happen because the rim is in compression from spoke tension and that
is its primary load.  If the stem hole is too large for this load,
then the rim will bulge there as rolling bending loads cause cyclic
overload.  This will relax spoke tension and peak stress.

I sometimes wonder what your motivation is to cite only that part on
which you can build a specious argument.  You seem to think readers
are so dull that this ploy is not obvious.  The omitted part of my
response addressed why a large stem hole is undesirable.  You seem to
want to ignore that if the stem hole causes a smaller rim cross
section that it limits spoke tension and that lower spoke tension
reduces the strength of a wheel.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Schrader -vs- Presta: why?
Date: 17 Dec 1999 23:39:51 GMT

Len (who?) writes:

>> As I stated, the strength of a wheel depends on spoke tension and
>> spoke tension is limited (for conventional wheels in contrast to 16
>> spoke wheels) by the rim compressive strength that is a direct
>> function of cross sectional material.  A larger hole limits that
>> load.  I don't care how overweight a rider is, the wheel cannot be
>> as strong with a large stem hole as with a smaller one.

> I agree that a larger hole decreases strength of a rim. I suppose
> ideally no stem at all would be better. But it is not possible as of
> yet.

What means "as of yet", rims have spokes and this requires holes about
the size of a presta valve.  As long as the stem hole isn't
substantially larger than the spoke holes, ho haven't lost anything...
but then you knew that had you read the thread up to now.

> I personally prefer a schrader valve, and can argue its merits, but
> as they made right now, the are to big for bicycle wheels.

Maybe you can explain how you mean that.  As is is, people break off
stems so I don't think they are too large.  They are probably just
large enough.

> Now if they were to produce a smaller Schrader, I would buy it. For
> now I'll go on using presta.

Huh?  Then it wouldn't be a Schrader valve but something else.  The
Schrader valve serves far more vehicles and industrial applications
than you seem to recognize.  This includes aircraft, earth movers,
trucks, rubber tired rail vehicles, cars, motorcycles and a slew of
stationary machines.  None of these folks share your disdain for the
reliable and convenient Schrader valve.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Schrader -vs- Presta: why?
Date: 20 Dec 1999 19:32:36 GMT

Drew who writes:

> Presta valves work the way a valve should work: Air pressure inside
> the tube keeps the valve closed, and it is only when the pressure
> developed inside the pump exceeds that in the tube that the valve
> can open to allow air in.

The Presta valve makes compromises that seem to be overlooked.  The
Presta valve has a conical stopper that by its taper exerts the force
to seal without the spring load that the Schrader valve uses.  The
disadvantage of this is that, by friction, it jams and must be made
free before pumping.  Without this, a Presta valve can require more
than 250psi to open.  During manual inflation after each pump stroke,
each closure at higher pressure requires a higher break-loose
pressure, above and beyond the back pressure.  This can often be heard
as a sharp sound at the onset of air passage.  The free air space
through the valve is also smaller than that of a Schrader valve and
restricts flow enough to, for instance, make filling a large truck
tire substantially slower (if one were to use a presta valve).

For these reasons, Presta valves will most likely not find much use
anywhere other than on lightweight bicycle tires.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Schrader -vs- Presta: why?
Date: 4 Jan 2000 22:30:24 GMT

anonymous writes:

> I heard that the reason for Presta valves is that at high speeds,
> the wheels on a road bike are spinning so fast that the centrifugal
> force out at the rim can supposedly cause Shrader valve stems to
> depress, letting out some of the air.  Presta valves are threaded
> internally, and so resist the c-force.  I don't know how valid this
> is, but supposedly that was an original reason (?).

Almost but no cigar.  Presta valves are not threaded internally, their
valve shafts are threaded and carry a lock nut.  The release of air
with a Schrader valve by centrifugal force occurred on John Howard's
land speed record bicycle while drafting behind a motor paced
enclosure on a salt lake at near top speed (160mph).  This will not
occur on any bicycle riding you or I intend to do.  On the other hand,
Presta valves have released air from centrifugal force when the lock
nut was not secured and only air pressure held it closed.  The added
weight of the lock nut and shaft under the absence of spring load can
release on a rough road at high speed, although it is not likely.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.misc,rec.bicycles.tech
Subject: Re: Pressure Retension - Presta vs. Schraeder
Date: 18 Jan 2000 02:14:45 GMT

Joseph Geretz writes:

>> You're trolling!

> Hey Jobst,

> I resent that. you might not remember but we had an extensive I
> resent that. you midiscussion on this subject last summer. I had not
> previously used Presta's before and I was curious as to the
> difference. A number of suggestions were proposed, among them the
> fact that because Presta uses pressure, as opposed to a spring, to
> seal the valve, that Presta valves hold air better. Maybe so, but as
> it turns out, the wall of the tube leaks air at a faster rate than
> do either a Presta or Schraeder valve, so that consideration is
> moot.

I don't recall the issue but Presta valves are not held shut with air
pressure, at least not for people who don't gamble.  The presta valve
has a lock nut that should always be closed when it is to hold air.

There have been riders who thought it a good idea to break off the nut
so they wouldn't be bothered with opening it an closing it, but most
of these folks had leaky valves sooner of later, just as the guys who
did so for their Schrader valves.  There are two kinds of Schrader
valves, one with an internal spring and one with a long external
spring.  The latter type lends itself to spring removal.  Neither
Presta or Schrader valves, used properly lose air, and as you may have
read, the porosity of tubes of different kinds varies greatly.

Meanwhile, moot is moot (adj) because its misuse has made it,
according to Webster's dictionary, meaning both debatable and
undebatable, aka meaningless, much like your test.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jobst.brandt@stanfordalumni.org
Subject: Re: Specialized Floor Pump
Newsgroups: rec.bicycles.tech
Message-ID: <rglt8.15586$44.98364@typhoon.sonic.net>
Date: Thu, 11 Apr 2002 19:14:31 GMT

Henry GlaucMan writes:

> I bought a new Specialized Air Force floor pump to replace my old
> Avenir pump.  The problem I encounter when pumping up 700x23 tires to
> 120psi is that I must always let some air out of the tire prior to
> pumping air in.  Otherwise it is impossible to put air in the tire
> regards of the amount of force I can apply to the pump handle.  The
> pressure gauge will exceed the max reading and I must remove the head
> let out a small amount of air and only then will the pump function.
> My Avenir was never like this.  Note that the Specialized pump has a
> larger diameter tube assembly.  The LBS advised I must "break" the
> seal in the Presta valve for the pump to work.

Presta valves use a conical sealing element that is held shut by air
pressure enough to not leak but to make sure, the holding nut should
be tightened.  At high speed and road shock, an unsecured Presta valve
can open.  Therefore, the valve is generally "jammed" when properly
shut and must be broken open before pumping.

Losing a little air when freeing the valve is not a great loss because
the reason for pumping is to inflate a low tire.  If the valve is not
made free, it can resist more than the pump pressure gauge can take.
Several of my gauges were ruined that way by riders who didn't loosen
the valve.  That the gauge is shot is apparent by the dial needle not
returning to zero or against its stop when under no pressure.

Jobst Brandt  <jobst.brandt@stanfordalumni.org>  Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: Why the notches in a Presta nut?
Newsgroups: rec.bicycles.tech
Message-ID: <G%dH9.52557$Ik.1454469@typhoon.sonic.net>
Date: Wed, 04 Dec 2002 02:45:58 GMT

Andrew Muzi 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?

> 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.

Jobst Brandt  <jobst.brandt@stanfordalumni.org>  Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: Why the notches in a Presta nut?
Newsgroups: rec.bicycles.tech
Message-ID: <MciH9.52583$Ik.1458694@typhoon.sonic.net>
Date: Wed, 04 Dec 2002 07:33:00 GMT

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'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).

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.

Jobst Brandt  <jobst.brandt@stanfordalumni.org>  Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: What's the deal with that Presta valve nut?
Newsgroups: rec.bicycles.tech
Message-ID: <xtDma.5570$JX2.381881@typhoon.sonic.net>
Date: Mon, 14 Apr 2003 18:54:21 GMT

Michael Press writes:

> You know, the nut that screws on around the outside of the valve,
> and snugs up against the rim.

> Is it necessary?  Is there some benefit?  Should it be tight?
> Should it be loose?

It keeps the stem from wobbling around and sinking into the tire when
pumping with a hand pump.  That is its only purpose.

> I've had several (expensive) tubes fail at the base of the valve
> core; the rubber seems to have worn thin or been stretched thin.
> I'm wondering if this is due to making the Presta nut too tight.

Unless you use more than your finger tips to screw it on, I don't
believe this is the cause of your stem separations.  Some tubes are
more susceptible to this than others.  I have had a stem separate on a
wheel where I did not have a stem nut.

Meanwhile, this can be fixed.  I carry a spare stem from a latex tubed
tubular tire in my patch kit.  In the event of a stem separation, I
remove the failed stem, insert the tubular stem, tighten its clamp nut
and inflate the tire.  I'm on my way.

Jobst Brandt
jobst.brandt@stanfordalumni.org
Palo Alto CA

From: jobst.brandt@stanfordalumni.org
Subject: Re: Great ride today but what's with other cyclists?
Newsgroups: rec.bicycles.misc,rec.bicycles.rides
Message-ID: <5bxXb.1420$_3.21171@typhoon.sonic.net>
Date: Sat, 14 Feb 2004 22:20:49 GMT

Rick Onanian writes:

>>> Recently, I went to unscrew a Presta to inflate it, and unscrewed
>>> it right out; and it didn't want to screw back in properly...

>> Removable Presta cores are not all that common in most brands but
>> they are detectable because the coarse cap threads have two flats
>> for tightening the core and loosening it.  In that respect they are

> Err...I mean, no cap on it, I unscrewed the core too much, and it
> wouldn't go back together. I guess I have to start keeping better
> track of this stuff, this stuff just never concerned me much...I
> just try to bear in mind that Presta valves are, IME, delicate.

>> identical in function to Schrader valves that also leak if not
>> screwed in firmly, the difference being that for the Schrader a
>> special wrench is required, one that comes on the tip of some old
>> metal valve caps.

> You and David Kerber have mentioned Schrader valves that can be
> screwed and unscrewed, but I don't think I've ever _seen_ such a
> thing.

All Schrader valve cores are removable.  You can buy them at auto
parts stores.  Next time you pass a auto tire shop, see if they have
the tool handy.  Tires on cars are always mounted without valve cores
installed so that tires can be rapidly inflated and deflated.  The
last thing is to install the valve cores and inflate to user pressure.

> Well I'll be a monkey's bare-assed uncle. I just looked in a
> Schrader valve, and sure enough, it appears there's a mechanism by
> which you could stick an appropriate tool in there and do stuff.

You'll notice the stem has internal threads as well.  That's the clue.

> With a Presta valve, you always screw and unscrew it.  I've _never_
> heard of anybody actually screwing and unscrewing a Schrader's
> innards like that.

The Presta, not having a closure spring, relies on a conical seal on
the inner end of a threaded secure/release pin on which a lock nut is
located.  If this nut is not screwed down, the valve might leak but
what's more important is that it can centrifugally open or open when
hitting a "curb"... ta-da!

>>> While people should stop to help, anybody who needs help should

>> people cannot see when they are not needed.  Just a "Thanks" after
>> the first question seems not to be enough.

> Maybe "Thanks, I've got everything I need" will work.

Yes, the kind of voluteerism of these folks is not help I prefer.  If
they cannot see that someone is well in charge, sand papering the
patch area, and isn't glancing around for clues, they probably don't
understand the process itself.  It's like riders who give obviously
experienced old time riders advice on how to ride not noticing that
this guy is on a well used 1960's bicycle just cruising along.  They
wouldn't recognize a professional racer either if the met one.

Jobst Brandt
jobst.brandt@stanfordalumni.org

From: jobst.brandt@stanfordalumni.org
Subject: Re: valve nut ?
Newsgroups: rec.bicycles.tech
Message-ID: <Qr06c.204$Fo4.1666@typhoon.sonic.net>
Date: Wed, 17 Mar 2004 18:05:36 GMT

Mathias Koerber writes:

> A quick question regarding the nut which is used to lock the valve
> against the rim.

> The other day I observed that a local LBS mech installed such a nut
> onto a wheel which had none and tightened it before pumping the
> tire, then removed the nut again.

> I also remember to have read elsewhere that that nut being tight
> during riding might result in a tube tearing at the valve-stem.

> So, what the deal?  Is that nut required/beneficial or a problem for
> a) normal riding and b) during pumping?

I don't know what this mechanic had in mind, but the stem nut has been
made the scapegoat for a manufacturing flaw of tubes from which their
brass stems separate from the tube and leak.  Many riders are loathe
to place blame where it belongs on the tube and believe they caused
the leak by tightening the stem nut too tightly.  Therefore, how tight
should it be?  The result is that the nut gets removed entirely.

When a tire is inflated to more than 100 psi, the nut is usually loose
even if it was tight when the tire was flat.  It's main purpose is to
keep the stem from submerging into the rim when the tire is flat.  For
users of pumps that clamp onto the stem, this may not be a problem but
racing pumps (that are becoming rare) with push on pump chucks, the
stem nut is a great assist in tire inflation.

Although there have been episodes of stem separation, the stem nut
does not them although that is most likely what is behind the action
you saw.  Those who have had a few years of experience with these
things will recall that no such failures occurred over many years,
even though the stem nuts were always used (push-on pump chucks) and
then suddenly there were some stem separations.  Currently there are
none, the bad ones having moved through the market.

Should we then, forever after, shun the dread valve stem nut?  I still
use a pump with a Campagnolo pump chuck and don't plan on tossing the
nut.

Jobst Brandt
jobst.brandt@stanfordalumni.org

From: jobst.brandt@stanfordalumni.org
Subject: Re: Sticking Presta valves
Newsgroups: rec.bicycles.tech
Message-ID: <1rUqd.8501$_3.101463@typhoon.sonic.net>
Date: Tue, 30 Nov 2004 06:29:17 GMT

Joe Riel writes:

 http://www.geocities.com/cyqlist/valve.html
 http://www.geocities.com/cyqlist/pcore.jpg

> In the picture of the removable Presta core, there is dark region
> around the circumference of the inner end of the brass "piston" (the
> movable valve).  Is that a thin rubber(ish) seal?  If so,
> presumably that is what causes (with the taper) the high friction.

The black seal at the end of the plunger is normally a rectangular
cross section rubber ring.  It gets pressed into a tapered bore to
give a reliable seal even without screwing down the lock nut.
However, at high speeds, centrifugal force can cause an unsecured
valve to leak.  This has been tested on a fast descent at over 50mph
by Tom Ritchey in the days when he routinely did not secure the lock
nut.  You can imagine how much "I told you so" he got from other
riders on that Sierra tour.

Jobst Brandt
jobst.brandt@stanfordalumni.org

From: jobst.brandt@stanfordalumni.org
Subject: Re: Sticking Presta valves
Newsgroups: rec.bicycles.tech
Message-ID: <RM8rd.8628$_3.103787@typhoon.sonic.net>
Date: Wed, 01 Dec 2004 01:04:49 GMT

Leo Lichtman writes:

> What puzzles me is why this even matters.  I have always understood
> that on Shraeder valve stems, it is the cap that really holds the
> pressure--the valve core is there just to hold until the cap is on.
> Isn't the cap on Presta valve stems able to hold pressure?

Schrader valves have a spring that closes the valve securely, even
against accelerations that might occur.  Many construction equipment
tires run without a cap as do many cars.  That the cap holds air is
not true, most caps today being leaky plastic, just enough to keep
dirt out of the valve orifice so that it will not foul the valve seat
on addition of air.

Jobst Brandt
jobst.brandt@stanfordalumni.org

From: jobst.brandt@stanfordalumni.org
Subject: Re: Sticking Presta valves
Newsgroups: rec.bicycles.tech
Message-ID: <WH8rd.8624$_3.104023@typhoon.sonic.net>
Date: Wed, 01 Dec 2004 00:59:34 GMT

Dave Vt? writes:

>>> However, at high speeds, centrifugal force can cause an unsecured
>>> valve to leak.  This has been tested on a fast descent at over
>>> 50mph by Tom Ritchey in the days when he routinely did not secure
>>> the lock nut.  You can imagine how much "I told you so" he got
>>> from other riders on that Sierra tour.

>> Hmm.  I have to wonder how much of that was centrifugal force and
>> how much was vibration causing the valve stem to shake and walk the
>> seal down the taper.  If spinning smoothly, the revs required to
>> overcome the tire pressure should be *huge*.

> Someone did the calculations in this newsgroup way back when:
> http://tinyurl.com/4f5l9 (points to Google archives)

> Summary: on a smooth road, you'd have to go around 120 mph to make a
> presta valve leak. Read the original post to find all of the
> assumptions behind that number.  I'm pretty sure that you need a
> rough road in addition to high speeds to make a presta valve leak.
> In other words, centrifugal force alone didn't make Tom's tire leak.

Don't leap to conclusions that were not stated.  Obviously a mountain
road has bumps and pavement irregularities and at more then 50mph
these become sharp accelerations.  The centrifugal force must have
been part of the problem because he never had a tire lose air like
that before.  He pumped it up and closed the valve.  It was not a
puncture.

Jobst Brandt
jobst.brandt@stanfordalumni.org