In my years of racing I can think of one time I DNF’d. No, it was not a broken foot. Rather, I flatted and did not have the right valve extender and tube with me.
For those who do not know, a valve extender is used to “extend” the valve stem of your tube (or tubular). Valve extenders are necessary for use with deep dish wheels, especially those used in time trial and triathlon. Most tubes have 32mm or 48mm valve stems. This is fine for most box section road rims, which may only be 25 or 30mm, but a time trial wheel may have a 60 or 90mm rim. You need the valve extender in order to “extend” the valve stem enough to add or remove air from your tire. This is not the most exciting thing out there, but it can make the difference between a DNF and a finish and riding your bike to T2 versus walking a really long way.
Valve extenders come in two varieties. The first can be used with any kind of valve stem. In order to install it, loosen and lock out the tube’s presta valve. This sounds simple, but the valve on your tube can break, making it difficult to remove air from your tire. In the 2012 Ironman World Championship, Sebastian Kienle lost critical time, because a broken valve prevented him from letting enough air out of tire for him to be able to remove the tire and change it. The valve stem can also rattle loose and collapse. This problem can be addressed by using Loc-tite, although I have never been able to get this solution to work.
The latter type of valve extender can be used only with tubes that have a removable valve core (i.e. the valve can be removed from the stem). The valve extender looks like a hollow tube. They are much easier to install and you don’t have to worry about the valve rattling shut. You unscrew the presta valve from the tube’s valve stem, screw the extender onto the stem, and then screw the presta valve onto the extender (typically with some teflon plumber’s tape to ensure a good seal).
The only problem is that they only work with tubes with removable valve cores and these tubes are usually thinner and less durable. They are also harder to find at bike shops and your team mate is less likely to one he/she can loan you.
The alternative to valve extenders is to use relatively shallow race wheels (60mm) or less and tubes with long valve stems. My wife (Dr. G) uses Hed Jet 6 wheels, which have 60mm rims, and tubes with 80mm valve stems. This way she does not have to worry about valve extensions, although these tubes, much like those with removable valve cores, are also harder to find at bike shops and a tube a team mate is less likely to have on hand to loan you.
Which one do I use? I have three different race wheels – a Hed Jet 6 front, which has a 60mm rim, a Hed Jet 9 rear, which has a 90mm rim, and a Hed Jet Disc. I use tubes with 48mm valve stems, because they will work with all of my wheels, while I use a short valve extension for the Hed Jet 6 and a longer extension for the Hed Jet 9. On the day I DNF’d I only had a extender that could be used with a tube with a removable valve core, but I had forgotten to switch out my “training replacement tube” for a spare tube with a removable valve core. A could friends passed by and offered to toss me a tube, but none had one with a removable core. From that day on I have carried each type of valve extender for each of my race wheels in my flat kit, regardless of which wheel I have used, so that if I flat I have the right kind of valve extension, regardless of what kind of spare tube I can get my hands on. I know people who do not carry flat kits when they race. For me, even on that off chance that I flat and regardless of whether I remain competitive, I would rather not get stuck walking, because I know from my DNF experience that it is not fun. Neither does my wife need to worry about me more than she already does.
Whether you are training or racing, be prepared to fix a flat. A cell phone to call for a ride does not count. Carry a spare tube or tubular and carry enough valve extensions or enough variety so that you can fix your flat and get moving again, regardless of what kind of spare tube you can get your hands on.