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I have noticed that presta tube valve stems come in various lengths. I always assumed the longer valve stems were to allow the use of deeper aero wheels. Why is it that many wheels in print ads seem to have what I would consider an excessive amount of stem exposed? Is there an advantage or disadvantage to usng a stem that is just long enough?

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I think the valve should be as short as possible while still allowing comfortable inflation of the tire.

On the other hand, a longer valve usually "won't hurt", specially if you have more than one bike or your group of fellows use to lend spare tubes to one another during rides (happens a lot to me, but in shcraeder-equiped mountainbikes).

Hope this helps!

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    Agree, make sure "comfortable inflation" is with all your pumps (some need more stem). Roll with a stem sized for your rim, carry a spare that will work for you and your buddy. If you are really nice, also carry a valve extender. – Ken Hiatt Aug 10 '12 at 17:57
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I'd say that ideal is about 1.25" sticking out of the rim. That's enough to accommodate most pump chucks without being so long that it increases the risk of damage.

But mostly an over-long stem just looks ugly.

I've seen a tendency in the last few years for the stems on the tubes at your LBS to get (much) longer (they generally only stock one or two lengths), presumably because of aero rims. Manufacturers make more lengths, of course, but you'd have to special-order.

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I have the same question.

In my case: A friend is about to buy a road-bike that comes with "semi-aerodynamic" (I don't know what else to call them) carbon "clincher"-type--tire rims. (The rims themselves are for rim-brakes, by the way). And as far as I can tell, from the specs that I had read online, the depth of the rims is 42mm.

My friend's local-bike-shop stocks inner-rubes that come in 42mm length, 45mm length, 60mm length, 65mm length, 80mm length, and 82mm length. I was wondering: Which would be the correct size of length to get for his spare inner-tubes, for his 42mm-depth bicycle-rims ?

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    Welcome to SE - This is literally a "me too", so try reading the other answers and applying them to your situation. As per the other answers, you want enough valve to allow any of your pumps to work. And anything more than that is unnecessary. At a guess - 65mm is about your minimum but it depends on the rim and the tube valve stem, and your pumps. Wait till the wheel turns up and try whatever tube you have, then decide. – Criggie yesterday

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