I would try to have both tubes fully assembled with the correct valve length and put it under the saddle.

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man, you seem to be complicating the hell out of this. why would you even consider messing with (removing and reinstalling, requiring an extra tool ??) a valve core during your race ? if you are limited to a single spare tube, why not carry a 48mm stem tube (for your disc) and carry a valve extender that doesn't require the installation of the valve core, one that simply threads onto the tube stem (with the valve open) ? pre-install some teflon tape onto the valve stem threads and that should do it.

personally, i'd find a way to carry 2 tubes (both 48mm stem, pre teflon-taped as described above). can't imagine your rig can't accommodate that in some way .......

Similar set up (Zipp 808 and disc) and I race with one tube. My race tubes (latex in the wheels) use the internal valve extenders and removable core valves. My spare tubes (butyl) use the external extender that work with removable and non removable cores. I wouldn't want to mess with removable cores in a race.

However the key is to prep the spare tube as follows:

1. Use a needle nose pliers to lock the threaded lock nut on the valve opening. An eigthth twist past what you can do with your fingers will lock it open.
2. Use teflon tape on the valve.

This article explains it well.

I keep the screw on the external extender on the tube and don't tighten it more the finger tight.

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Suffer Well.

Yes, just carry on short valve tube and one simple extender. With a valve extender where you don't move the valve, you don't need any teflon. Just screw it on hand tight and pump it up and off you go.

Keep it simple

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''Sweeney - you can both crush your AG *and* cruise in dead last!! 😂 '' Murphy's Law

Thanks for the recommendations. I only have experience with extenders that require you to remove the valve core. Didn't even cross my mind that there was an alternative.

I have the same setup. I use latex, have a spare Michelin 36mm stem latex with the old Zipp non-removable valve extenders. Haven't needed them yet in a race, knock on wood.

Stupid question but once it's locked open with pliers if I needed to use it without the valve extender would my air chuck just plug right into it? Right now I have a 48mm tube with teflon tape on the threads and carry a Mavic valve extender. I've tested it with just opening the valve and screwing on the extender. It held air fine but I didn't ride on it.

It's not about holding air, it is about making sure the valve doesn't tighten and close off when you screw on external extender or vibrate closed when riding and not allowing you to pump up the tires. It is pretty hard to unscrew if it tightens once you have the tire and tube mounted with the external extender in place.

The spring the valve seals the presta valve closed. Tightening the nut just the suspenders to the spring's belt.

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Suffer Well.

I think I am going with butyl spare(s). I haven't tried doing latex during a race, but it seems risk (of getting the latex install wrong and blowing out the tube) versus reward (better Crr for remainder of the ride) isn't worth it.

a latex tube is much more pliable/ easy to install than a buty tube, especially when rushed during a race.

If I'm using it for a spare during the race does it matter much if the valve closes?

Not much, but I've never had one close on me. You can also wrap the plumber's tape over the threads + a little higher and it'll help avoid that.

^^^^^This.

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

No- once you have aired up your tire. If you need to add air later, than it could.

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Suffer Well.

How's the latex tube work with CO2? How about after 4 hours if you flat early in the race?

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Suffer Well.

I remember reading you can loose 10psi/hr with CO2 in latex. Most CO2 will inflate you to 120psi, so you're be around 80psi after 4 hrs...unless of course you deflate a bit to a desired riding psi like 90 or 100.

The 120 psi for a 16 gram cartridge has been touted by folks like Genuine Inflation for 15 plus years. Therefore, I assume that was measured on narrow width rims. With wide rims and thus higher volume tires, that you are going to start out quite a bit lower initial psi. And yes, latex loses CO2 air (like it does with atmospheric air) quicker then butyl. Several discussions here over the last decade.

I'm sticking with butyl tubes as spares.

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Suffer Well.

actually the opposite when using a wider rim

I have the same dilemma. Just pack one of the old school extensions that doesn't allow you change the valve out and where you simply leave it open. They aren't as good, but they work well enough. I pack one of those AND one of the replaceable valve extensions with a valve core currently mounted plus two of the little wrenches. The "over the valve core" option is important in case you get another flat and need to rely on neutral support as their tubes may not have replaceable valve cores.

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"Non est ad astra mollis e terris via." - Seneca | rappstar.com | FB - Rappstar Racing | IG - @jordanrapp

Not following your comment.

Given the same tire, when mounted on a wider rimmed wheel, it has a larger volume than when mounted on a narrow rim.

A CO2 canister has a set amount a compressed gas. In a higher volume set up (same tire on a wide rim) it will yield a lower pressure than on a lower volume set up (same tire on narrow rim). PV=NRT or something like that.

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Suffer Well.

I see what you are saying, on a 19mm rim a 16gram would fill to 120psi, on a 23mm wide, 105psi or there about. Although I have not measured and is really low on my list of things to worry about come race day. Flat, fix, fill, ride.