Flats. No fun. Still, best to be ready. Think strategically about how you're going to deal with this. If you have race wheels, you want to make sure that you have tires/tubes and valve extenders that are compatible with each other.

There are two kinds of presta valve styles, removable versus fixed cores. A removable core means you can unscrew the presta valve from the valve stem. Why would you want to do this? If you want to expose the presta valve, outside of a deep dish rim, you need a valve extender that sits in between your valve and your tube or tire's valve stem – the case in the image adjacent.

You don't want to have a whole bunch of valve stem sticking out there, so think about what stem length comes on the tire or tube you're using or intend to use and marry that with the right length extender. Below are two lists: common clincher tubes and their lengths, and commonly used valve extenders. So, for example, Mavic makes a tube with a 48mm valve stem and they make a 70mm extender, totaling about 120mm of length. If you stuck this combo through a Zipp 808 you'd have about 40mm of this construct sticking outside the rim and that's certainly enough and maybe slightly more than ideal. It would be an awful lot of stem sticking up and out of a 40mm rim. But a Continental 42mm valve stem along with Conti's 30mm extender would give you about 30mm above a 40mm-deep rim. Better.

Zipp makes 4 extenders for removable valve cores, and it recommends using the 27mm for its 303 model wheels, 41mm for 404s, 65mm for the 808, and 91mm for 1080 models. But it depends on what the valve stem length is on the tube you're using. For these kinds of extenders that work with removable cores Zipp has a nice how-to. I'm borrowing from their list of images. Still, where they recommend grease I'm a Teflon tape fan, as is former SRAM/Zipp employee (and now Slowtwtich tech editor) Greg Kopecky (I link to his how-to here on Slowtwitch a little further below).

Are you riding a disc in the rear? If so, the length of the stem is important. If you buy one of those Continental tubes with an 80mm valve stem, thinking you won't need a valve extender for your 40mm or 60mm rim, how is that tube going to work on a HED3 or a Zipp disc if that's your other wheel? You might not have enough room to get a pump head or even a crack pipe on there, once the tube is mounted. Best to choose your tire or tube valve length with your disc in mind, then choose an adapter to lengthen the valve as needed so that it protrudes the proper amount from your front race rim.

If you'll race your disc, this is where you might want to consider the style of valve extender. One kind of extender sits in between your valve stem and your valve core, that is, you remove the core (if you have a tire or tube with a removable core), thread the valve extender onto the valve stem, then thread the core onto the extender. Me, I always put Teflon tape on the threads of the extender and then again on the valve core, so that there are no leaks. I don't want to twist the valve stem and compromise the junction of the valve stem and the tube, so I grab the valve stem with a pair of pliers, likewise the extender, and tighten them. No need to go overboard here. Use the least force necessary. Then replace the valve core, threading it into the extender. There's plenty of new little tools and wrenches that have become available and ships with a lot of tubes, wheels and tires these days, or you can use a 4mm open end wrench, or an 11 gauge spoke wrench (if you have one of those circular spoke wrenches with every gauge known to man around its perimeter, you'll find an 11 gauge on there).

If you want a brilliant explanation of how to do this, with pics, from a man after my own heart when it comes to the attention to detail that attends topics like this, read Greg Kopecky's how-to of valve extender use.

Here's a few clincher tube models, just for reference, with valve stem lengths and whether they have removable cores:

Michelin – 40mm, 52mm, 60mm valve lengths – removable cores
Q tubes – removable cores
Continental – 42mm, 60mm, 80mm lengths – removable cores
Vittoria latex – 51mm – removable cores
Challenge latex – 48mm – removable cores
Schwalbe – 40mm, 60m, 80mm – removable cores
Bontrager standard butyl tubes – cores not removable
Bontrager Race XXX Lite – 48mm and 60mm – removable cores
Bontrager Race Lite – 36mm, 48mm, 60mm, 80mm – removable cores
Bontrager latex tubes – removable cores
Mavic – 48mm, 60mm – cores not removable.

Should you pre-mount extenders on your spare tubes? Yes, I think so. If I race without a disc, the extenders are on my spare tubes already. In other words, if I've got 2 spares, I've got 4 extenders with me during the ride: the 2 that are on my wheels, and the 2 that are on my spares. Just make sure your spares have extenders that'll work with both your wheels.

But what if you're riding a disc? That changes things a little. If you pre-mount this extender with a removable core onto your spare tube or tire, that's great for the front, but what about a rear flat? You can't use an extender for most discs. This means you're better off with the other kind of extender, and this just threads over the presta valve. So, that list above of tubes with removable versus fixed valve cores, you can use either kind of extender on every kind of valve. But if you want your presta valve protruding outside your race wheel you must choose a tube with a removable core.

Again, were I not riding a disc, and if I was using that kind of extender that just threads onto the valve core, I'd thread it on with Teflon tape and I'd make sure it's on there pretty tight. In this case, the presta valve is open. It stays open. And Greg, in his article I reference above, talks about how to make certain the presta valve stays open. Yes, many of these kinds of extenders are made so that you can use the other end – the non-threaded end – to screw down the presta valve. Screw that. Or more precisely, do not screw that. Just leave the presta valve open. Nothing bad's going to happen to it. One value in leaving it open, with the extender on the valve during your bike ride, is that if you just have a slow leak you stop, get out your CO2, give it a cartridge worth, keep on going.

I have occasionally found presta valves to be sticky – you have to depress the valve, let out a little air, and then you commence filling the tube or tire. If you've got an extender on top of the valve, that is, threaded over the core, and you get this sticky valve problem, you've got to stick a spoke down inside the extender and bump the valve. A good pump head, like those made by Silca, will almost always overcome the resistance of a sticky valve, but this is still a possible problem. Topeak makes an extender that fixes that problem – it's got a little valve bumper-thinger built into the extender. After inflation you can thread the presta valve down by turning this bumper-thinger (I originally did not know about the ability to thread the valve shut with this extender, and Facebook commenters below disabused me of my ignorance).

If you've got a disc on the back, or a HED3, your spare tube can't have an extender tightened down. It's got to just be finger tight so that you can remove it if you need it for your rear wheel. I'd Teflon tape the valve stem, thread on the extender, but just go finger tight. Make sure your valve stem length, on your spare tube, is appropriate for your disc! Make sure your valve stem length, your inflator system, everything is disc compatible.

If you're riding a pair of deep wheels, just choose a valve and extender combo for your spare tubes/tires that works for the deepest of your rims, you can use either the removable or fixed core valve and extenders, you're good. Here's a short, inexhaustive, list of valve extenders:

Lezyne – atop presta valve – 70mm
Mavic – atop presta valve – 70mm
Zipp – atop presta valve – 23mm, 33mm, 48mm, 74mm, 98mm
Topeak – atop presta valve – 43mm, 75mm
Wheels Manufacturing – atop presta valve – 50mm

Zipp – for removable core – 27mm, 41mm, 65mm, 91mm
Vittoria – for removable core – 42mm, 60mm, 80mm, 110mm
Stans – for removable core – 40mm (image adjacent)
Continental – for removable core – 30mm
Problem solvers – for removable – 70mm, 50mm, 30mm

Note on the Lezyne: its valve extender sits atop the presta valve, but it's threaded, and it's made to accept its own threaded presta valve pump and inflator heads. You can see Lezyne's founder, Micki Kozuschek, demonstrating this system in a short video, which is in German but you'll get the gist.

You might think I'm overly anal about getting all this right and tight and good to go, but I refer back to the 1992 Ironman in Hawaii, top Finnish pro Teemu Vesala was filling up his tires with some brand of Fix-a-Flat like stuff that was available back then. He had on HED 90mm deep wheels and this was in transition, not long before the start of the race. The valve extender did not make a good seal onto the valve – probably no Teflon tape. But this was not immediately apparent because the joint between the valve and extender was inside the rim and out of view. The problem was discovered when flat sealant starting foaming out each of the rim's 24 spoke holes. Too late to do anything about it. Off Teemu went to commence the swim. Once on the bike, he shot off up Palani Road, and for miles he spewed a Fix-a-Flat rooster tail behind him like he was piloting a drag boat. On the plus side, it was a great deterrent to would-be cheaters – no one drafted him that day.