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Old 08-18-12, 02:25 PM   #1
jim p
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Educate me on the presta valve

It appears that there is a nut on the valve pin which needs to be loosened before air can be added or released. I am wondering if it is necessary to depress the valve pin before air can be added to the tube. With the nut on the valve pin loosened, will the pump push air into the tube with out having to depress the valve pin.

I am trying to come up with a system so that I can be confident that the air pressure in the tube is the pressure that was indicated on the pump after the inflator is disconnected. It seems that some air escapes when the inflator is disconnected.

So if someone can explain exactly how these air valves work, then maybe I can put my mind at ease.
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Old 08-18-12, 03:03 PM   #2
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Watch this video:

http://www.youtube.com/watch?v=9Ux08bz43qY

Don't worry about the sound of a tiny bit of air escaping when the pump head is removed. Normally its an insignificant amount of air.
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Old 08-18-12, 03:05 PM   #3
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Yes, loosen the little nut. I usually depress the valve pin once before pumping because it often gets stuck so that air can't get in. Once it's loose, you should be able to pump easily. If you forget to depress it first, and it is stuck, you will usually see the psi read very high immediately because the air in the pump hose has no where to go. That should indicate it's stuck and needs to be tapped with your fingertip.
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Old 08-18-12, 03:33 PM   #4
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Yes, loosen the little nut. I usually depress the valve pin once before pumping because it often gets stuck so that air can't get it. Once it's loose, you should be able to pump easily. If you forget to depress it first, and it is stuck, you will usually see the psi read very high immediately because the air in the pump hose has no where to go. That should indicate it's stuck and needs to be tapped with your fingertip.
+1 on all the points. It didn't used to be so, but modern PVs need to be burped once to unstick them. Once that's done the valve will act as a simple one way (check) valve and the pressure in the tube will always be equal to the hose pressure, until you reduce the hose pressure when disconnection.

With a typical pump, you'll see a spike in pressure with every stroke, which then settles as the air flows into the tire. It's that lower (settled) pressure that is equal to the tire pressure.
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Old 08-18-12, 04:12 PM   #5
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Presta valves work best if they are positioned near the 12-O-clock position on the wheel, which helps the un-sprung valve core more redily fall into the closed position. This way, all the air that you hear escaping when you detach the hose will be from the hose, not from the tire.

Having the pump head fitted straight-in-line on the Presta valve is critical to allowing the valve's core to open and shut freely. You may have to twist the hose at one of it's ends to allow the head to sit more straightly on the valve stem.

If the pump goes over your pressure target before the valve pops open, be careful as to how high the guage reads, since very-high, maximal gage readings tend to destroy the guage's calibration, causing it to forever read "high".
Also, with the pump and hose moderately over-pressurized, and with the Presta valve still stuck shut, you can rock the pump head sideways, which usually assists the valve popping open and immediately shows your actual tire pressure on the pump's guage without feeding in more than one or two psi to the tire's pressure. This will allow you to precisely monitor your tire's actual rate of pressure decline over one or more day's time.

Lastly, all Presta pump heads are intended to only engage the smaller-diameter threaded end of the Presta stem. Forcing the pump head gasket-packing over the major diameter of the stem will result in the pump head's gasket becoming leaky and permanently distended as well as requiring much more force to pull free of the valve, which can tear the valve away from the inner tube.
I will thus happily loan my expensive floor pump to kids with Shraeder valves on their wheels, but always hesitate to loan out my pump to experienced cyclists with Presta valves, as so few seem to know the proper way to fit the pump head!

Last edited by dddd; 08-18-12 at 04:19 PM.
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Old 08-18-12, 04:15 PM   #6
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Thanks for the explanations. I now feel better about the final pressure in my tube. I am going to see if I can attach my inflator such that no air escapes. With my current setup I always lose some air when I attach the inflator. This lets me know that I am also losing some air while I detach the inflator.

Since the presta valve is a one way valve or check valve unless you depress the pin, I can't see any reason to have my inflator depress the pin. A single burp of the pin as described should make sure that the valve is not stuck.

Thanks again.
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Old 08-18-12, 04:23 PM   #7
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Originally Posted by jim p View Post
Thanks for the explanations. I now feel better about the final pressure in my tube. I am going to see if I can attach my inflator such that no air escapes. With my current setup I always lose some air when I attach the inflator. This lets me know that I am also losing some air while I detach the inflator.

Since the presta valve is a one way valve or check valve unless you depress the pin, I can't see any reason to have my inflator depress the pin. A single burp of the pin as described should make sure that the valve is not stuck.

Thanks again.
Yes, chucks with valve pins on PV can be a nuisance. Try to fiddle with how far you fit the head, so the valve doesn't depress when you engage the thumb lock. This is why many old timers prefer the simple push-on chuck with a gasket shaped like a volcano.

As the pressure in the head increases it presses the sides of the cone gripping harder, making a simple, reliable seal. The drawback is that it's very easy to break the nut off with this type of head, so the better ones have some kind of pressure release button, to bleed out the hose before you yank off the head.

BTW- valve orientation while pumping doesn't matter because the force from pressure behind the valve dwarfs the effects of gravity on the 2 gram (if that) moving part.
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Old 08-18-12, 04:50 PM   #8
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In my experience pressure guages built into the pump give inaccurate and inconsistent pressure readings. A hand held guage will give far more useful readings.
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Old 08-18-12, 07:19 PM   #9
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Thanks for starting a topic on such a simple question Jim. I'm mostly new to presta too and learned a couple of things. Maybe I'll start taking presta valve classes. Man, this is a lot more intricate than Schrader.
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Old 08-18-12, 08:03 PM   #10
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I prefer Schraders because they will take more abuse. However, Prestas, handled carefully, will give you good service. bk
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Old 08-18-12, 08:04 PM   #11
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In my experience pressure guages built into the pump give inaccurate and inconsistent pressure readings. A hand held guage will give far more useful readings.
It's possible to calibrate even the inexpensive guages on most floor pumps, if you have a reference-standard guage to compare to.

I've rotated the needle, wound it quite a ways, to effect a permanent change in the 0-psi rest position (clear of the stop post for comparitive readings.
In certain cases, I moved the needle until the gear drive actually skipped a tooth, then advanced to an exact position in the other direction.
You can also get the needle to move on the drive post in some cases, but either way can give extremely accurate readings within a useful pressure range that's used for your tires.

It's still handy to have the hand-held guage around, to simply check the pressure, but my floor pump is handy too and it gets the pressure-check done in no time while I top up any pressure losses.

Last edited by dddd; 08-18-12 at 08:17 PM.
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Old 08-18-12, 08:14 PM   #12
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BTW- valve orientation while pumping doesn't matter because the force from pressure behind the valve dwarfs the effects of gravity on the 2 gram (if that) moving part.
It might be stated more precisely to say that *air movement* through the valve dwarfs the effects of gravity on the ~2 gram part.

True in most cases, but not always true. If the leakage rate of the presta head seal is moderate (i.e. average, somewhat normal), there is pressure loss the moment that pumping effort ceases, and until the pumphead is perhaps sharply pulled off the valve stem, this lost air's effect on pressure is cumulative and unknown.
With the valve inverted at 12-O-clock, the air flow (however slight) can become strictly one-way, as long as the chuck isn't so sideways as to hold the valve open.

Riders in the old days knew that the valving action when using valve-less frame pumps worked better with the valve up top of the wheel, the better to reduce the incidence of a blown-back pump handle, but it was perhaps even more important to hold the pump's head squarely with the axis of the valve stem.
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Old 08-18-12, 08:28 PM   #13
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Originally Posted by jim p View Post
Thanks for the explanations. I now feel better about the final pressure in my tube. I am going to see if I can attach my inflator such that no air escapes. With my current setup I always lose some air when I attach the inflator. This lets me know that I am also losing some air while I detach the inflator.
Some inflator heads are easier to use than others.

If you use a floor pump, you can use an after market head like these ones:

http://www.bikepartsplace.com/discount/pump-end-40/

http://www.flickr.com/photos/28270886@N02/2650182576/
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Old 08-20-12, 07:00 AM   #14
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I keep a Presta valve adapter on my tubes in place of a cap and fill them with a Schrader pump and check them with a Schrader gage. I find it only takes a second to take adapter off turn it around and open the Presta nut and fill and check. Remove it tighten nut flip it around and replace it as a cap.

http://compare.ebay.com/like/2007845...Types&var=sbar
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Old 08-20-12, 11:46 AM   #15
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I keep a Presta valve adapter on my tubes in place of a cap and fill them with a Schrader pump and check them with a Schrader gage. I find it only takes a second to take adapter off turn it around and open the Presta nut and fill and check. Remove it tighten nut flip it around and replace it as a cap.

http://compare.ebay.com/like/2007845...Types&var=sbar
I used to use these with a Shraeder head in the old days, and found that with the right lenght of adapter, that the Shraeded head valve-depressing plunger would effectively open (and allow to close) the valve as the lever-lock was applied.
This also worked extremely well with the good Shraeder guage that I had, but again, the adapter's length was critical.
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Old 08-20-12, 11:53 AM   #16
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I prefer Schraders because they will take more abuse.

I can't see any advantage whatsoever in the Prestas. Greatest scam perpetrated on the American public since 1 hour Martinizing.
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Old 08-20-12, 12:12 PM   #17
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It appears that there is a nut on the valve pin which needs to be loosened before air can be added or released. I am wondering if it is necessary to depress the valve pin before air can be added to the tube. With the nut on the valve pin loosened, will the pump push air into the tube with out having to depress the valve pin.
What exactly do you mean by "depress"?

It is a good idea to tap the Presta valve a few times with your finger before attaching the pump hose - it helps the valve to get "unstuck".

But if you are asking whether the pump head is supposed to keep the valve tip pressed down when attached, the the answer is no. A typical Presta pump head does not depress Presta valves when attached and is not supposed to do so. The valve is opened by the incoming air pressure and only by the incoming air pressure. After the first pump that opens the valve, the pressure in the pump hose will equalize with the pressure inside the tube, meaning that the Presta valve, once opened, will no longer close (until you detach the pump head that is).

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I am trying to come up with a system so that I can be confident that the air pressure in the tube is the pressure that was indicated on the pump after the inflator is disconnected.
Huh? Disconnected? After the inflator (pump head?) is disconnected the pump pressure will show exactly zero. What are you talking about?

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It seems that some air escapes when the inflator is disconnected.
When you disconnect the standard pump head from the valve, the Presta valve closes instantly. The air does not escape from the tube when you detach the head (only a very very very tiny amount escapes). The short burst of escaping air that you hear when you detach the pump is the residual pressure from the pump hose. I.e. it is the air from the pump, not the air from the tube that makes that sound.

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So if someone can explain exactly how these air valves work, then maybe I can put my mind at ease.
Your pump has a built-in one-way internal valve. It is typically located at the base of the pump (for floor pumps). This built-in valve is what is responsible for keeping the air in the tube when the pump head is attached. The Presta valve in your tube does not really "work" when you are pumping up the tube: since the pressure in the pump hose and inside the tube is identical, the Presta valve simply stays permanently open. However, when you begin detaching the pump head from Presta valve, the seal gets broken, the pressure in the pump hose quickly drops and the Presta valve instantly closes, sealing off the tube.

There are "alternative" pump head designs out there that contain a pin, which is intended to mechanically open the Presta valve when you attach the head to the valve. Such designs work better with "sticky" valves, but at the same time they lose more air when detached.

Last edited by AndreyT; 08-20-12 at 12:53 PM.
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Old 08-20-12, 12:20 PM   #18
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Andrey, thanks for the additional information. I think that I now know exactly what I need to make sure that I am airing up my tires correctly.
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Old 08-20-12, 12:36 PM   #19
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The pressure differential , higher in the tube than the outside air closes the check valve.
the little nut is a positive closure back -up.

floor pumps also have a check valve , so the hose holds pressure..

venting the hose, does not let air out of the tire.

Schrader valves, each has a spring, so the pump-head has to open the valve
by pressing in the center pin.

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Old 08-20-12, 04:11 PM   #20
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I can't see any advantage whatsoever in the Prestas. Greatest scam perpetrated on the American public since 1 hour Martinizing.
Ah, those French. Developing a simple lightweight technology that takes a little finesse, when they know Americans are klutzy brutes.
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Old 08-20-12, 04:32 PM   #21
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The Presta valve is better when the rim is very narrow. The thicker Shraeder stem requires a bigger hole in the rim, and the thick rubber pad at the base of a typical Shraeder tube's valve stem is barely accomodated between the tire's beads.

Presta valves more easily separate from the tube, but Shraeder valves sometimes get a pressurized blister in the rubber coating, leading to rapid air loss.

A pocket-type air pump can have a lighter attachment head with Presta, no need for a valve-depressing plunger or the bulk to accomodate any moving parts.
The Presta valve stem appears to be lighter and more aerodynamic.

Shraeder valves require a cap to keep dirt from settling in the stem, which gets blown into the pump head as soon as the leverlock is actuated. This can foul the mechanism on some pump heads.

For racing bikes, Presta gets the nod. But for most others, Shraeder is easier overall.
Presta is the standard for road bikes, and certain pocket pumps are dedicated Presta-only.

Lastly, very narrow Shraeder tubes end up with the thick pad wrapping at least half-way around the tube, which confines all of the tube's elastic expansion to the strip of thin rubber opposite the thicker rubber pad. This can more readily lead to rupture if the tube is used in a larger tire.

I believe that Presta valves (more specifically their attachment to the tube) would prove more durable if users only knew that their Presta pump head is only supposed to by fitted to the narrow tip of the valve stem. This would greatly reduce the forces that typically lead to failure.
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Old 08-20-12, 07:27 PM   #22
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I believe that Presta valves (more specifically their attachment to the tube) would prove more durable if users only knew that their Presta pump head is only supposed to by fitted to the narrow tip of the valve stem. This would greatly reduce the forces that typically lead to failure.
None of the pumps I've ever owned have fit to that area. It's always been on the thicker part of the stem, and I've never had a problem with presta durability. You must have a different type of pump head than Park, Topeak, etc.
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Old 08-25-12, 09:12 AM   #23
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Thanks for all the information. I can now air up my tires like a professional.

My current set up is a compressor that I can set the pressure on. I have a schrader inflator with a trigger at the end of the compressor hose. I use a presta to schrader adapter on the tube. In the past I would screw the adapter all the way down on the presta stem. This caused air to escape when I attached the inflator. Now I only screw the adapter onto the presta stem about 3 turns and when I attach the inflator no air escapes. I can trigger the inflator and the air enters the tube and when I release the trigger only the air trapped in the inflator escapes. Seems to be a perfect operation to me.

Thanks again.
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Old 08-25-12, 11:00 AM   #24
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No that this hasn't already been answered, but this is a good guide too;

http://www.madegood.org/bikes/librar...-sports-valve/
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Old 08-25-12, 11:16 AM   #25
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But if you are asking whether the pump head is supposed to keep the valve tip pressed down when attached, the the answer is no. A typical Presta pump head does not depress Presta valves when attached and is not supposed to do so.
It works well though; I've tried filing an adapter down to mimic a Shrader, and it gives a better pressure reading. Pumping a Presta in the usual way, you're only reading the pressure in the hose, because the valve is normally shut except in the lower part of your downstroke. If you pay attention, you can hear and actually feel it opening and closing through the pump.

Although if you're going to calibrate your gauge, there's no point, since it should be a consistent bias.

Last edited by Kimmo; 08-25-12 at 11:21 AM.
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