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On my road bicycle with tubed tires ridden only on paved roads, I never used the caps that covered the tube stems. Now that I have switched to road tubeless tires, do I need to use the caps to reduce any air leakage?

2 Answers 2

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I'm not opening up the caps vs. no caps discussion but I think the main purpose is to protect the valves from dirt/contamination and damage rather than sealing it. I never tried but I'm pretty certain a valve cap wouldn't hold in several bars of pressure when the valve leaks or gets loose.

So you may use it but if you haven't seen any benefit with tubes and your riding profile hasn't changed (more dirt riding), I think you can leave them off, it's a Presta valve as used on every tube.

You could make an argument that tubeless valves stay longer on the bike because you don't swap them like tubes and thus, protection matters more than on "throwaway" tubes where you tend to replace them due to punctures from time to time. Also, some premium valves have nice metal caps you might want to keep on for the looks, but that's rather subjective. enter image description here Related: Does the plastic cap on a Presta valve serve any purpose once fitted to a rim?

(I am team caps off and haven't had any issues whatsoever, but you do you...)

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    In fact, external contamination shouldn't really get in to a closed Presta valve anyway.
    – Weiwen Ng
    Feb 21 at 18:17
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    @WeiwenNg I never had an issue in years without but I tried to keep it neutral, it is still the internet and a divisive topic.
    – DoNuT
    Feb 21 at 18:20
  • My main use of the caps is to protect the tube from the valve while its all folded up in my bag, before installation.
    – Criggie
    Feb 21 at 18:30
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    @Criggie The only reason I could see is that a tubeless valve is less of a throwaway item than a tube and thus longer exposed to the elements but from what I've heard the main problem is still valves clogged with sealant rather than aging on the bike and failing due to lack of caps. Another might be that premium valves come with metal/alloy caps rather than cheap plastic and there is an aesthetic aspect of keeping them on.
    – DoNuT
    Feb 21 at 18:37
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Valve stem caps do not (directly) have anything to do with air retention, they exist to protect the valve stem and possibly valve core from damage and debris, and also to protect any attached tube during storage.

For a Presta valve, as long as you have the nut on the valve core tightened down properly and the valve itself is still good, no air is going to leak out of the valve irrespective of whether the cap is there or not. External contamination is unlikely to be an issue for properly closed Presta valves as well, irrespective of the presence or absence of a valve stem cap, though damage from debris impacts is still possible (albeit relatively unlikely).

For a Schrader valve, if the cap is not there a debris impact from just the right angle might have sufficient force to depress the poppet in the valve core and allow some air to escape, though this is not particularly likely, and the more likely issue due to the lack of a valve stem cap is debris fouling up the area in the valve stem above the valve core.

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