A while back I received an email from a triathlete who explained that he got different FTP results if he did a 20-minute test on his Watt Bike indoor trainer versus doing a 40km time trial on a tri bike that took about an hour versus doing a 20-minute test on the road on a road bike. He pointed out that the results don’t all agree, and, in fact, there are considerable differences. Should they agree? And if not what should he do to set his FTP? These are all good questions I’m often asked by athletes and their coaches. Here’s my reply.
There are many different ways of determining FTP. They won’t all agree based on unique characteristics that show up in each. It’s not a perfect system no matter how you choose to do it. For example, when you did the 40k TT was it a race or a hard workout done solo? How tired were you? What was the terrain like? How hot or cold was it? All of these things (and more) could affect the calculation of your FTP from a racelike event, which is, by the way, a good option for determining FTP.
Also, indoor tests give different results than outdoor tests. Different power devices (Watt Bike trainer versus whatever power meters you have on your tri and road bikes) give different results. Your positions on the bikes also affect power output. For example, everything else being equal, an extreme aero time trial/triathlon position will typically produce lower power numbers than a typical road bike position. The terrain and steepness of an outdoor test course has a significant impact on results also. As does the temperature and humidity, especially when testing indoors. Different indoor trainers will also give you different results.
And there’s a lot more that we could get into here, but it starts becoming a bit techie and detailed. The bottom line is you need to test on the bike you will be racing on, have one primary method (and test course if on the road) to determine your FTP and then stick to it. That will give you more meaningful data to work from so you can more accurately set training zones and gauge changes throughout the season.