Presta Valves are Complicated

In January of this year I wrote about Presta valves, prompted by the frequency with which people bend or break the little threaded stem on top of one. I did a little research and testing, some dissection, some math, and I prattled on for a few pages, thinking I had pretty well exhausted the topic.

The other day, sitting outside Coco Crepes eating a Bananas Foster Crepe, watching my friend “Pablo” wrestle with a slow leaking tube, I realized I had left a lot unsaid!

Pablo’s immediate problem was that when he unscrewed his Lezyne Road Drive mini-pump, the Presta valve core backed out with it, releasing all the air (and increasing the entropy of the universe a tiny amount.)

Understand, I find the Lezyne to be a great pump for high pressure tires. (Get the longest one.) And the thread-on hose works great – on non-removable valve cores. The pressure relief button is a nice feature too. Interestingly I “invented” the pressure relief button many years ago, to make it easier to pull my Silca Classic pump head (It wasn’t called Silca Classic then. It was just called Silca) off of threaded valve stems.

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But I digress. The Lezyne thread-on hose is essential for my friend Ironman, who runs Zipp 808s on his tri-bike. An 80mm valve stem barely protrudes from the rim, making it impossible to stab with any other pump head except the Lezyne, because the Lezyne screws onto the core only. Thus the problem with removable cores. I personally have unscrewed a few valve cores with a Lezyne pump by mistake. Others have too apparently. This is one of the eight reasons for which Russell Eich advocated elimination of Presta Valves in BikeRadar.

If you do use a tube with a removable core, it might have wrench flats on the main body of the stem so you can tighten the core with a couple of cute little wrenches (spanners for my British friends) or one cute little wrench on the core and a pair of pliers on the stem body if it has no wrench flats.

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On the left is a Zipp tube with an extender. On the right is a tube with a threaded valve stem body. These are the spares I carry on my road bike.

So what good is a removable core? It seems like a liability, not an asset. One benefit of a removable core is the ability to add a proper valve extender. By proper I mean one that screws in between the stem body and the core, and specifically not one that is just a hollow tube. But if you have the proper length stem, this isn’t an issue. As the guy on group rides that is expected to be able to fix anything, I carry two tubes and I make sure one of them is an 80mm… or a shorter stem with a proper extender. (While I was taking the pictures above, I made sure the cores were good and tight.) Oh, and I also carry a cheap “hollow tube” extender.

Brief digression on hollow tube extenders: Before you screw on a hollow tube extender, unscrew the nut on the valve stem core and tighten it against the backstop in the open position. Otherwise the extender might drag on the nut, rotating it and closing the valve stem –no air gets in the tire.

Sudden realization – this can happen when you attach a screw-on pump head! I think this is what happened when trying to pump up a tire on a recent ride. I thought my pump was malfunctioning. Forehead slap!

Moral of the story: Be careful using a Lezyne or other thread-on pump head on a removable valve core.

1. Unscrew the nut on the valve stem core and tighten it against the backstop in the open position

2. Push the head on as far as it will go before rotating it. This may prevent the O-ring from grabbing the nut and closing the valve while screwing on the head

3. Don’t tighten the pump head too much (the pressure seal is made by the o-ring; the threads don’t need to be tight)

4. Release the pressure in the hose before unscrewing it, and

5. Hope the valve core is tightly threaded into the valve body.

What About Tubeless? Valve stems for tubeless setups will always have removable cores for the addition of sealant, so be sure to tighten the core well when you put it together. Apply backup to the body of the valve stem while tightening the core so you don’t twist the stem and get your tubeless tape in a wad. This will cause a leak.

To add a layer of complexity, some tubeless valve stems have a round base, and some are rectangular. I prefer round. I think it is less disastrous if it gets twisted, based on a limited dataset of a few problems with rectangular base stems. 

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Next decision, threaded or smooth body? I dunno, I kinda like smooth body valve stems. It’s easier to get your pump head on and off. And they don’t tear up the rubber seal on your pump as much as threaded stems. Not an issue with a thread-on pump head, as it doesn’t engage the valve body. I once ripped a stem out of a tube while wrestling a pump head off of a threaded valve stem.

If you do use a threaded valve stem, do you screw the nut on it? To me, it just slows down a roadside flat repair for no reason. I have a tray full of valve stem nuts on my workbench if you want some. Ah, but Killa, what about when the valve stem rattles inside the rim? This is a common problem on deep carbon rims. It can sound like rocks rattling around in your frame. The nut can prevent that. OK, but I prefer a piece of electrical tape with an X slit in it punched over the valve stem. Bonus tip: Use a contrasting color of tape to make spotting the valve hole easier. (See the picture in the header of this post)

 Of course threaded stems are essential for tubeless setups. My ideal tubeless valve stem is threaded on the lower half of the body and smooth on the top where the pump engages it. See the stem on the right in the picture above.

One last thought on valve stems – I thought that Presta and Schrader were the only options for bicycle usage, but in researching for this post I learned that there is also something called a Dunlop valve. Who knew?

One more last thought – And I’ve said this before. I’ll say it again. In this age of Unobtainium bikes and Upsidaisium components, why are valvestems still made of brass?