Valve extender question

Couple things - you can get extenders with a removable core in them, which is what I'd recommend.

But also, air will only leak out slowly if your core isn't tighten down, it comes out fast only when the core is pressed. So, if you pump them up before a race, you'll be fine

It sounds to me like you are talking about the Zipp type extender without the removable core.  When you open the thread to the top you are not locking the valve tube open.  You are just locking the nut from threading back down the valve.  So the tubes valve is still in place, doing its job...ie allowing air to go into the tire but not letting in escape.  Just think about it for a second.  If you want to fill your tire with are without the extender what do you do?  Open the valve by unscrewing it.  You're doing the exact same thing.  You're just locking the nut so it doesn't block the flow of air into the tube, ensuring that it isn't in the closed position.

If you are talking about a tube/extender with a removable core (Vittoria, Tufo), then you would remove the core from the tubes valve and install it onto the extender(if it doesn't already have one).  So the valve has just been moved from the tube valve position, to the extender valve position still doing its job.  

Hope this helps.
 

Edited by mktoson 2011-02-01 1:06 PM

I have used two types. The hollow tubes such as Zipp's and the ones that will tighten the valve like Topeaks. With the Zipp's you leave the valve open but the outward pressure from the air in the tube will kept them from deflating. To release air you stick a pin in the extender to push down on the valve. They will leak over time but I find these to be simpler. The Topeaks just didn't work well for me.

Thanks for asking the question. I had the same one.

When I inflate the tire, it doesn't inflate quit the same as when no extender is used. It's harder to get up to 120psi. I eventually do, but requires pumping harder.

Is this normal ?

how long do the valve stems have to be for your race wheels ?

I have zipp 404's and use tubes w 80mm valve stems. thus no extender.

That's about the limit (404) before you need an extender.

With an 808, I use 48mm valve stems + 72mm extender.

If you can get away with no valve extenders advice is to go with longer tube stem.   My life has been much happier since  Mine used to leak all the time and just created another point of potential failure.   

Great idea. I'm stopping by the LBS tomorrow. That'll at least take care of the front.

Can't get latex tubes in 80mm so you'd be using extenders as well.

I use Topeak valve extenders. They have an o-ring in them so you don't need teflon tape and their setup is made so you do not need removable cores or anything special. They just grab onto the existing valve and you turn it open and closed just as if you were using the original valve, just 30, 40 or 70mm longer.

To answer the original question, the air pressure inside the tube pushes against the valve and that keeps the valve sealed good enough for a day for a race as a worst case. I still prefer the Topeak way of doing it.