3

As you see in the image below, after factoring in that I lined up the ruler leaving 2 mm from the edge to allow for the tire, I am left with 76 mm. (We do measure without the caps on; no?)

presta stem length

Of those 76 mm, I subtract 23 mm for the inner tube and get ~53 mm for the length of the stem.

Which puts me right in the middle of the available stem lengths: 48 mm and 60 mm.

Which one should I be using?

And if it's the 60 mm one, what's wrong with using 48 mm inner tubes anyway (which I ordered before paying attention)? Since there is plenty of clearance when inserting the (presta tip of the) pump, what is the point of having those extra 12 mm?

2
  • 1
    Just checking, but it looks like you have rims that are about 30mm deep. Am I correct?
    – Weiwen Ng
    Aug 23 '21 at 22:31
  • 2
    @WeiwenNg Yes, the rims seem to be 29 mm (to be exact). I see now that I may have made a mistake (at least one, anyway). The 23 mm nominal diameter is that of the (outer) tire, not that of the inner tube.
    – Sam
    Aug 23 '21 at 22:37
4

Your rims look like they're about 30mm deep aluminum rims. I assume that you placed your ruler on the tire, such that the 0mm mark is probably a few mm to the right of the tread. Your tube sits on the rim's bead seat, so the presta valve probably starts around the 30mm mark on the ruler. That's around where the brake track ends, or maybe just after it.

You merely need enough space for your pump's chuck to seat onto the valve. On 30mm deep rims, I've typically used tubes with 48mm-long Presta valves. So, these are adequate. This page, which appears to be an online store, recommends 55mm Presta valves for 35mm and 40mm deep rims, and 60mm valves for 45 and 50mm deep rims. I'm not aware of tubes with 55mm valves right now.

I think that in the 2000s and earlier, rims used to be shallower, typically around 20-25mm. It's possible that tubes with 40mm valves, rather than 48mm, were more common then. 40mm long valves would be a very tight fit on 30mm deep rims. I think I did this a few times, and that it was tricky to seat my pump correctly. Some pumps have chucks that thread onto the valve, e.g. many Lezyne models, and it's possible these need less exposed valve to work.

Tubes with 60mm valves should be fine for up to about a 45mm deep rim. I know that there may be tubes that are even longer than that, but I believe they are rare. Another option is to use a valve extender: you'd unscrew the valve core, screw the extender on, and then screw the valve core into the extender. A couple of the answers to this question discuss valve core removal tools. Anyway, using valve extenders with deeper wheels enables you not to worry about what sort of spare tube you carry - although if you have a tubeless setup and deep wheels, you will want to ensure that your spare kit has a valve extender or a tube with a long enough valve.

Naturally, you can use a long valve stem on a shallower rim. Logically, there has to be a non-zero aerodynamic penalty, but this is probably small in most amateur cycling contexts.

1
  • 4
    Aside - some pump heads need to go further onto the valve stem than others. That has caught me out in the past.
    – Criggie
    Aug 24 '21 at 0:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.