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I can see the use of the valve cap on a Presta valve when the tube is still packaged (it obviously stops the valve puncturing the tube), but does it do anything except look pretty (or not) once you've fitted the tube to a rim?

I've kept them on out of habit, because I've had problems with sticking Schrader valves in the past when I've lost the caps and dirt/grime has got in there - but the design of Presta valves suggests to me that they wouldn't suffer this fate.

Given that I really need to inflate tyres on a road bike every day, is replacing this cap something I can leave out of my daily chores?

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One side comment that maybe should be a separate thread: The nut on a Presta valve should not be wrenched down tight. Tighten just enough that it resists moderate finger pressure. Any tighter and you damage the rubber valve seat. –  Daniel R Hicks Aug 3 '11 at 15:05
    
I'm sure I've seen push-fit valve covers (though they might have been Schrader I guess) made from a rubbery plastic - that would save you the few seconds twisting. –  Chris H May 21 at 9:46

7 Answers 7

up vote 30 down vote accepted

The cap keeps dirt and rocks away from the valve, particularly the fragile release mechanism. It's not the end of the world if the caps are missing, but I suggest leaving them on. They don't take that long to remove and replace.

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I am pretty lazy... but you make a good point about the fragile top part of the valve. I guess if it were a bike that I only ever used on the road in nice dry conditions I wouldn't need them, but I'd also have more time to worry about the little things like that! I guess I'll leave them on and stop being so lazy. :) –  Will Sep 23 '10 at 19:34
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It's still good question. I'd like to add that, if anything, Presta valves are more vulnerable to damage than Schrader valves. –  Neil Fein Sep 24 '10 at 0:45
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It's not like they last very long anyways. I use them until they break, but don't bother buying replacement caps after that. I'd say that I average only a couple months of light riding before the caps crack and won't stay in place. –  Brian Knoblauch Dec 7 '10 at 13:30

Keep the cap on to protect the valve from accidental damage, dirt, salt and oxygen.

There is a bit of rubber inside the Presta valve:

enter image description here

Leave the valve cap off and oxygen in the air gets to the rubber causing it to perish. This problem will take time to develop, however it will do regardless of where you live, where you ride and how often you clean your bike. If that bit of rubber goes then you need a new tube or a new valve insert.

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I'm not sure about the time scale required for this to be significant (sounds a bit like putting nitrogen in your tires - in theory, yes, but likely doesn't make a difference). –  Batman May 21 at 15:15

Nobody has noted this yet, so maybe I'm particularly clumsy.

The caps prevent you from bending the somewhat delicate presta valve stem if, like me, you're fitting a fairly beefy lock between your spokes every day. I smacked mine pretty good after a groggy morning commute, and it bent the valve stem pretty severely. They're somewhat known for snapping off when you try to bend them back, but I was lucky in that regard.

Ever since I've kept valve caps on.

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Aha, yes, you're not the only one. –  Sk606 Dec 30 '10 at 7:29
    
Agreed. I've always put them on on my bikes, but I've seen several people with problems when the threaded shaft got bent. –  Daniel R Hicks Aug 3 '11 at 15:03
    
Done this, also broke an entire valve right in half trying to pump it up. They're kind of brittle. –  JFA Mar 9 at 22:22

They have one big benefit on road / commuting bikes - they stop the valve corroding and seizing due to the salt and crap they get exposed to. On a mountain bike, this isn't such a problem, as the salt gets washed off by puddles and mud (and you tend to wash your mountain bike more).

That's been my experience in the muddy, wet UK, when the roads are salted for over half the year. I leave them on the road and commuting bikes, and leave them off the MTB for speed of tyre changing.

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One problem that is rather rare is that the caps can add to the weight of the valve and contribute to wheel wobble when going at very high speeds (> 80km/hr)

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Wouldn't the large metal valve contribute a lot more to wheel wobble than a tiny plastic valve cover? –  Kibbee Sep 30 '10 at 2:21
    
Yeah, the valve is a bigger problem, but the cap just adds to it. –  Anthony K Sep 30 '10 at 8:42
    
I'm also not sure when cyclists hit 50 mph... –  Batman May 21 at 15:16

I don't use them is if someone is into racing and we are looking at changing a flat being important down to the second. In that case it is just another impediment to speed.

Otherwise it keeps the system (whether presta, schrader or dunlop) clean and easy.

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On my MTB I leave them on to keep mud from crudding up the works. On road and commuter bikes they are useless and I toss them.

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they're not useless per the answer above. They do keep dirt out of the valve and keep the screw of jamming. I've had the little screw jammed on a bike when I worked in a shop. It's not a huge deal but if your out on a cold/rainy ride it would suck. –  curtismchale Sep 23 '10 at 18:33
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LOL yeah and I've never had a problem with them in years. I'll go with the 99% rule. He said it was a PITA, and it is for 1%. –  Todd Sep 23 '10 at 18:51

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