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I grant this is probably a silly question, but one never knows...

I was out riding and rain arrived earlier than expected. I had inflated my tires before setting out to a pretty high pressure for my gravel bike (70 psi)so I thought it would be a good idea to lower them a bit in order to avoid slipping.

I stopped in as much shelter as I could find and bled the presta valves a bit.

To my thinking the positive pressure of the air would prevent water getting in. Is this true? Is there zero percent chance water got in?

If any water did get in, is this likely to be a problem, and what could I do about it?

(For whatever it's worth, they are tubeless tires. A little sealant fluid escaped when I was letting the air out...)

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  • 7
    Unless you're using pre-dried, pre-compressed air to inflate your tires, you're putting moisture in them every single time you hook them up to the pump. There is always humidity in the air, and you're pumping that directly into the tires. Even if some rain water molecules were to manage to push past the escaping air and make it's way into the tire, it would only be joining friends that had been hanging out in there for weeks/months already.
    – FreeMan
    Sep 16, 2022 at 16:57
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    Considering you intentionally put like 100mL of water-based sealant in your tires, I don't think another 0.5mL or so is going to hurt. If anything, it's replacing the fluid lost ;)
    – MaplePanda
    Sep 16, 2022 at 22:39
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    I ran a tire with a regular butyl inner tube filled with water for about 5 months and no ill effects where perceived. The interesting point of this question is the fact that the O.P uses tubeless ant thus, sealant.
    – Jahaziel
    Sep 16, 2022 at 23:21
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    The sealant getting blown out of the valve is the only concern here. It tends to clog and sometimes foul the valve. Better turn the wheel to a position where sealant will flow out of the valve stem while the valve is not near the lowest point.
    – gschenk
    Sep 17, 2022 at 1:09
  • Yes, I always bring the valve to the top before adding air or letting it out, to avoid sealant leakage. Still, some came out this time...
    – Cerulean
    Sep 17, 2022 at 23:33

4 Answers 4

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No risk. 70psi is about 5 times the atmospheric pressure. Most sealants are also using water as main fluid (or water/ammonia), unlikely that the small quantity that may get in will have a significant impact.

If you really want to be on the safe side, just let the air out with the valve pointing down.

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The OP mentions tubeless systems, but here is an experience I had with tubes.

I once pumped my tyres up with a pump that had been standing outside in the rain for a few weeks. I ended up with what must have been a few hundred milliliters of water in each tyre.

Although the pump sounded odd when I used it, I didn't realise what had happened until a few weeks later when the weather got cold enough to freeze the water and I could hear the churned up ice sloshing around in the tubes.

In other words: even a considerable amount of water is unlikely to cause serious problems, at least in systems with inner tubes. Pneumatic tyres rely on having a compressible fluid (i.e., a gas) inside them. As long as the majority of the content of the tubes is gas, they should still function mostly as expected. In the hypothetical case of a drop or two of rainwater getting in, the effects will be negligible.

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See this video around 8:00.

The video deals with a car tire, but the essence is the same for a bicycles.

Note the fog that appears when the air is released.

Yes, this is water. It got inside as a vapor, together with the air.

In short, you already do have water inside. Depending on the temperature changes, some of it may condense and even freeze inside.

In a full-size gravel bike tire you may have as much as few grams of water.

This is a matter of fact, so the wheels are just engineered to be tolerant to this much water.

As @Dean answer states, you do allow some water molecules in. In the same time, you release a lot more with the air.

If the weather is colder than it was when you did inflate them, chances are that you already have condensate inside.

In this case, it is better to point the valve UP (i.e. get it to the lowest point). This way, the escaping air will have chance to blow some water droplets out and you will end up with less liquid water inside the tire.


For some use cases, it is important not to have much water inside the tire.

In these cases, the usual approach is to inflate tires with nitrogen.

Or at least have an air drying stage in the compressor.

Or, at least, drain your compressor tank once in a while.

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Just mentioning this as a point of interest - there is zero chance of this being of practical concern. I just think it's neat.

Water vapour, if present, would absolutely have flowed "upstream" through the valve into your tyre. Since the average transport speed of molecules in air is Mach 1, the flow from the tyre bleed would have to be supersonic to prevent this.

(I've dealt with scientific instruments that have a purged nitrogen atmosphere, and having a positive pressure gradient is simply not enough to keep those pesky oxygen molecules out!)

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    "Pesky oxygen molecules" Don't bite the hand that feeds you.
    – DKNguyen
    Sep 16, 2022 at 23:27
  • The flow from the tire valve IS supersonic. This is why one hears the hissing sound. Of course, this is also not enough - the water molecules can crawl near the surfaces.
    – fraxinus
    Sep 17, 2022 at 5:00
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    You hear a hiss because it's turbulent, not because it's supersonic.
    – Beanluc
    Sep 17, 2022 at 17:16

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