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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?

3 Answers 3

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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
    Commented Aug 10, 2012 at 17:57
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I'd say that ideal is about 3 cm (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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    1.25" is about 30mm in non-freedom units, for those interested.
    – glennsl
    Commented Oct 18, 2021 at 14:26
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In the intervening time since the question was asked, my observation is that the basic tube in bike stores has settled on a 48mm stem. I think in the 2000s you had 30mm, 48mm, and I think 60mm and 90mm as options, but rims then were commonly 20mm high. If my memory is right, then a 30mm tube would leave about 10mm exposed on a standard rim.

My current observation is that the average stock basic wheelset is 30mm deep. The tube is nominally 48mm tube, that should be 18mm exposed. That should be enough for most chucks to seat. Also, I just measured the nominally 42mm stem on a 30mm wheel I have. 12mm of the brass stem the air flows through is exposed, and the presta valve core sticks a bit out of this. So there might be some wiggle room in the measurement. In any case, you don't need to keep a full 30mm of valve stem exposed.

Your wheel is rotating. The brass stem is a cylinder. So, the wheel's rotation is dragging a pretty un-aerodynamic shape through admittedly turbulent air. Nevertheless, in theory you could try to expose as little stem as possible. I have a bike with 50mm rims and what I suppose are 60mm tubeless valves, so only about 10mm of the stem is exposed. However, there aren't that many standard tube lengths available, and not all rims match up nicely. I think you can easily get tubes with 48, 60, and 90mm valves. I think people normally buy 48mm tubes with valve extenders, of which I've seen a number of variants but 40mm are common. I was under the impression that the total power required to spin the average wheel is small (NB, going off memory), so the potential savings from shortening the stem by a bit could be minute. Silca makes Speedshields, which are little airfoils shaped caps you glue to the rim around the stem. I think these have been asserted by the company to save something like 1W at 28-30 mph. Most people wouldn't bother.

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