I know in times of ML and AT FTP tests are out, but here goes anyway. While my brain was frantically looking for a way out of the self-inflicted 20 min hell, I realised that you should be able to calculate if the 20 min test is actually possible to finish if you have previous power data.
The thing with all short-form FTP tests is that you have to ride over FTP for at least some of the test time. If you ride over FTP, you are burning through your anaerobic capacity (FRC). If this isn’t enough to last you for the duration of the FTP test, you can’t finish, or at least you will fall somewhat short of your real FTP.
In the case of the 20-min test, this is fairly easy to calculate. The resulting FTP value is calculated as 95% of the average over over 20 minutes. Thus, you have to ride at just over 105% of you FTP to keep it at the same value (and obviously more if you want an FTP increase). Which means that you need to have an FRC (in kJ = 1000 Ws) of at least 5% of your FTP for 20 minutes, which is 1200 seconds. Or, in maths: FTP x 0.05 x (20 x 60)s 7 1000 = FTP(in W) x 0.06s. The factor 1/1000 comes in to convert from J to kJ.
For an FTP of 200W, this means an FRC of at least 12kJ, if your FTP is 300W, you already need 18kJ.
Similarly for the 8-minute test. Typically 90% of the average power are taken as the FTP, which means you need to ride 11% over FTP, but only for 480 seconds. Thus: FTP x 0.11 x 480s/1000. Interestingly this results in 10.56 kJ for a 200W FTP, and 15.84 kJ at 300W FTP.
The FRC needed is considerably less for the 8-minute test, which is likely why that test has a reputation for being unreliable (strong riders overtest), and variations wit “anaerobic blowouts” exist. However, for those with relatively low anaerobic capacity, it might actually be a good choice?
The ramp test is a bit more complicated because the percentage over FTP changes with every step, and the total FRC needed is a cumulative product, so I*ve made a table:
Min - is the minute step in the TR ramp test (might be off by one?)
Step %FTP - is the power of that step as a factor of FTP
result FTP% - the resulting FTP (as factor of current FTP), if only that minute is completed
factor t - the multiplicand to calculate work over FTP
cFRC (200W) and cFRC (300W) are examples of the cumulative FRC needed to complete that step, for example FTPs of 200W and 300W
As you can see, the FRC needed to complete the magic 19.5 minute is roughly similar to that of the 20 minute test (coincidence, or by design?). However, with the ramp test, each extra minute needs progressively more work over FTP. With the 20 minute test, each extra minute takes exactly the same amount of energy. This is probably why the ramp test seems to give more anaerobic riders too high an FTP, and those with less FRC can’t seem to reach their actual FTP. We knew that beforehand obviously, but I found it interesting to work out some numbers which you can use to get an estimate of what you should be able to do.
I do think that there is more that is going on, and more that is needed to complete a successful FTP test. For example, some sort of aerobic capability and muscular endurance is needed, otherwise you won’t make it to those last minutes. And track sprinters would have amazing FTPs.
…how we spend our Friday nights, eh…