Inflating Presta threaded valve tubes - a tip.

[Alan O]

I started this thread, merely as a piece of advice for anyone using Vittoria tubes who was thinking of tightening the lock collar whilst the tube was flat. Readers can take it or leave it; I don't care one way or the other. I know what caused the rip in the tubes. I hadn't much intended to start a Q&A session.

No matter which rims I was using, I doubt that I would make the effort to unmount the wheel, tyre and tube to measure the hole with my Vernier caliper in order to satisfy your curiosity. As the rims are Mavic MA40s, which have a well deserved reputation for being monstrously hard to refit tyres on, my reluctance to do so is even stronger.
To the best of my knowledge, Mavic only made this rim with a Presta fitting; there would be no point in making such a narrow, light rim to take a Schrader valve. Looking at the assembled wheel, the hole is just about wide enough to take a Presta valve stem and no more.

Whatever the forensic outcome, you observed a potential problem and alerted us to it in good faith - and I thank you for that.

 

[T4tomo]

'Tis the way of the world that threads end up like this. And sometimes you learn some useful info...

Such as that @Ajax Bay considers a disc with a cross shaped hole in the middle to be "polo mint shaped". I now doubt his authority as a confectionery expert. (Edit: Yes I know they are topologically equivalent, but my polo mints have a round hole)

Would an actual.polo mint work just as well, I don't have and spare circular pieces of rubber, but I do have a pack of polos.
Also what ever happened to fruit polos?

 

[Alan O]

Also what ever happened to fruit polos?

They were cast into the outer darkness where they belong - polos are mints!

 

[Tim Hall]

'Tis the way of the world that threads end up like this. And sometimes you learn some useful info...

Such as that @Ajax Bay considers a disc with a cross shaped hole in the middle to be "polo mint shaped". I now doubt his authority as a confectionery expert. (Edit: Yes I know they are topologically equivalent, but my polo mints have a round hole)

You want topologically equivalent? I bought The Boy a mug labeled "Doughnut"

 

[mjr]

You want topologically equivalent? I bought The Boy a mug labeled "Doughnut"

I hate pure maths so my head hurts trying to decide if The Boy is topologically equivalent to a Doughnut but I think it's unlikely unless there's been surgery.

 

[mjr]

A bit of research reveals that the hole in a polo is approx 8mm and a presta valve is 6mm. So it's not impossible. However as polo mints are quite brittle they may not be ideal. Further research suggests that "Polo Gummies" may be just the thing. Unfortunately they have been discontinued (unlike fruit polos which appear to still be available).

Look, if you've that much time spare, I've some bikes that need repair...

 

[Tim Hall]

I hate pure maths so my head hurts trying to decide if The Boy is topologically equivalent to a Doughnut but I think it's unlikely unless there's been surgery.

A lifebuoy might be...

 

[Ajax Bay]

@Ajax Bay considers a disc with a cross shaped hole in the middle to be "polo mint shaped". I now doubt his authority as a confectionery expert. (Edit: Yes I know they are topologically equivalent, but my polo mints have a round hole)

I considered and discarded the notion of referring to this item as a torus in my post, on the basis that its use would decrease the readability of my post. too high on the Flesch–Kincaid Grade Level.
I know what shape I cut the rubber into and tried to share that. As another (was it the OP) said:" . . . merely as a piece of advice for anyone . . . Readers can take it or leave it; I don't care one way or the other. I hadn't much intended to start a Q&A session."

 

[youngoldbloke]

If the presta valve stem has a diameter of 6mm, one of these may prove useful in the manufacture of a rubber washer, similar in form to the well known mint-with-a-hole.

 

[mjr]

If the presta valve stem has a diameter of 6mm, one of these may prove useful in the manufacture of a rubber washer, similar in form to the well known mint-with-a-hole.

Or just use a 6mm hollow punch, you freaking nutters.

 

[Ian H]

I can't see how a tiny bit of force exerted by the lockring...

It isn't exerting force, it's constraining the tube so that it can't assume its proper shape and position within the tyre. With a wide rim/ tyre it probably wouldn't matter, but with a narrow rim the tube is probably pinched between the tyre beads and tries to expand under them.

 

[Ajax Bay]

the rims are Mavic MA40s

"480 grams for 700C. External width 20.5mm Internal width 13.5mm (between the lips)"
Normal bead width for a road tyre is just over 3mm so on a narrow rim like the MA40s, the rubber reinforced foundation of the valve (maybe 7+mm), would be very tight to the inside of (each of) the bead(s). The maths can support @Ian H 's hypothesis.
An interesting (to some) experiment would be to mount and inflate the tyre with the lockring screws finger tight; measure the length of valve protruding; unscrew (or even remove) the lockring; and measure again. This would show the extent (or not) of tube's "need" to "assume its proper shape and position within the tyre."