Bicycle Mechanics - Air escape from presta valve
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This is probably a basic question but I have searched a bit and cannot find a great answer.
This is my first set of tires with Presta valves, and I think I am using them incorrectly.
After I pump up a tire using a floor pump with a presta valve I seem to get a lot of air escaping when I remove the chuck from the valve. I experimented by pumping it up to 120, removing the chuck (and hearing the "woosh" of air when I pulled it off), then re-attaching the chuck. The reading was almost 105. So I lost 15 psi from removing the chuck? Is that normal? Is the chuck broken (brand new Specialized Airtool Pro)? Am I doing something wrong (likely!!)? When the guy in the LBS showed me how to use the pump he got the same air escaping sound when he pulled the chuck off, so I assume it is normal.
Thanks to anyone who can help me here...
08-13-09, 09:42 PM
You worry too much.
A Presta valve is a simple poppet valve. As long as the air in the inner tube exceeds the outside air, the valve is pushed closed. When you pull the chuck off of the inner tube, the air pressure in the pump hose drops and the presta valve snaps closed. Now you still have high pressure air in your pump hose with nothing to hold it in so the "woosh" that you hear is coming from your pump, not your tire.
So now you put the hose back onto your tire. Assuming that the process of replacing your pump chuck forced the presta valve open, the tire air would have to fill your pump hose before the gauge would register any pressure. Since you let some air out of your tire of course you are going to get a lower pressure reading.
Ah, that makes a lot of sense.
08-13-09, 11:19 PM
When you remove the chuck, screw the little brass top down on the Presta-Valve. This shuts it off to losing pressure from bumps, etc. And try to remember to un-screw the brass top before putting the chuck on the valve. :D
08-14-09, 09:13 AM
My experience is that the most minor of deflection to the side of the "lock nut on threaded rod (what should this be called?) will open the valve and air will escape. Pulling the chuck off could cause this, no? I DO get a very short bit of air escape when I pull a pressure gage off... AND I'm being very careful to pull it off as straight as I can.
08-14-09, 09:22 AM
Use a pressure gauge to check the air pressure in your tires. Use the pressure gauge on your pump only when you are inflating your tires. It doesn't take much to let 15 psi out of a bike tire.
08-14-09, 09:23 AM
I am not fond of the flip lever / shrader-presta valve hose ends found on most floor pumps now. I cut mine off and put a dedicated brass silca presta valve fitting on. I used a small hose clamp from the hardware store to secure it. It pulls right off with little air loss. It's easier to pull off the smooth presta valves, sometimes pulling it off threaded presta valves is a struggle.
08-14-09, 09:46 AM
This is the type of pump you need, an old frame pump with a flexible connector. You don't lose any pressure when you unscrew the connector.
08-14-09, 10:26 AM
Proofide, you're kidding, right?
08-14-09, 11:15 AM
Not at all. I picked up that pump in the late seventies. It probably cost about twenty-five bob. I no longer have a frame which it clips on to (they seem to space the braze-ons a bewildering variety of distances apart), but it's always been fine for inflating Presta tyres. Actually, I cheated a bit. The connector in the shot is my Woods one, for really old bikes. The Presta connector looks virtually identical, but it's so well-used that the cotton covering has got a bit hairy, and I'd be mortified for too many people to see it. Consider: a pump which has rendered sterling service for thirty years, and still works fine. It gets high-pressure tyres as hard as you could possibly wish for, but it has an added plus. When the tyre is fully inflated, using this pump becomes so difficult that anybody with any sensitivity would stop right there. I'd imagine that, with the modern super-pumps, it wouldn't be out of the question to over-inflate a tyre. So you start worrying about pressure gauges. Phooey. Stop about two pumps before it busts, that's my motto. Never failed me yet.
08-14-09, 02:57 PM
OK then. You're a funny guy, even if you aren't joking.
The pumps I had that looked like that worked poorly. The Zefal HP was a breakthrough when it came out. Now I usually use a floor pump with built-in gauge. It takes very little effort to get to 120 psi.
Long ago, I took a three-month tour on bike through France and the British Isles (so I now know what twenty-five bob means). The valve pin broke off one one of my tires. I was using tubulars, like the idiot I was. Since I couldn't measure the pressure with my guage, I just added a little air every day, to be safe. Well, I was adding more than I was losing, so eventually, the tire blew out from overinflation. Awesome: how much pressure does a tubular need to blow out? A lot, I think.
08-14-09, 03:12 PM
One other thing you should be aware of, although I doubt this is what happened to you. On some (but I don't believe all) presta valves, it is possible to unscrew the entire valve core from the stem. I have had the core come slightly loose - enough to cause the sort of slow leak that emptied the tire overnight.
It's a good thing to check before yanking the tire off the rim, or worse, making an incision on a tubular.
08-14-09, 03:56 PM
If your rims aren't too deep a Vee, get schraders. Drill your stem holes to 11/32". bk
The real beauty of a presta valve is that the hole in the rim is smaller for more strength.
08-14-09, 05:31 PM
Big increase in strength there? Don't think so. bk
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