Before you race an actual TT, practice on your bike while out riding. After a good warm-up, find a stretch of road with no stoplights, or a route with minimal stopping. Ride with the chain shifted to the big chain ring and the middle of the rear cog set. If you have a cyclo computer with cadence, try to hold 75 to 80 rpm (revolutions per minute) and ride without stopping for 15 to 20 minutes.
This can also be done on an indoor trainer—you simply do not have wind and gravity to fight you. Ride in your handlebar drops or install aero bar extensions if you have them.
During your practice session, monitor your heart rate. If you don't have a heart rate monitor, pay special attention to how you feel. Your perceived rate of exertion (RPE) for the duration of your effort (on a scale of 1 to 10 with 10 being all-out) should be at an RPE of around 7 to 8.
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You shouldn't be able to speak in more than one or two word sentences. Your breathing will be labored and you should feel like you want to stop, but with some fortitude you can sustain the effort for 15 to 20 minutes.
When the time is up, shift to an easy gear and cool down. Consider how the effort felt. Was it so hard that you are about to throw up? Or was it an effort that felt hard, but you were able to hold the intensity level? You should aim for the latter.
If you have a heart rate monitor, what was your average HR for the duration? This is an important number to know because this is a sample of your functional threshold heart rate, or basically the heart rate at which you can sustain an upper intensity effort for longer than just a few minutes. You can now use that number to help measure your efforts as you get more into racing TT's (and road races too).
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If you sometimes train with groups, practice your TT by riding at the front and taking long pulls. But don't use your aero bars! Simply go to the front and pull for as long as you can. Watch your heart rate and your cadence. Pull off when you can't maintain the speed. Once you recover go back to the front and repeat.
During an outdoor solo or group ride, you might also want to monitor your speeds during the effort. The magic number is 25 mph. That is the speed you need to be able to average to be a competitive time trialist.
Why? Because an average speed of 25 mph will allow you to ride a 40K TT in less than one hour. A sub-hour 40K is the mark of a strong rider. If you find that you can maintain 25mph for a 15- to 20-minute outdoor solo effort, you're on your way and ready to race. If you can't maintain that speed, you have some work to do.
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