Thats about it. As it gets chunks of history more recent “FTP up” achievements are invalidated by new “older” ones that result in a higher eFTP. Thats why they say “+0w” and the power doesn’t make sense. If you go to one of those rides and click “Reset” it is re-analysed and the bogus FTP up achievement removed. Its an outstanding bug that this isn’t done automatically. I did that to this one:
(I need to get back to the French Alps again!)
The algorithm uses a max effort of at least 3 minutes (configurable, if your anaerobic power is good make it longer like 5 mins) to place you on one of many pre-defined power curves. The curve parameters (CP, W’, pMax) are uses to extrapolate you 60m power for FTP. The curves were modelled from real power curve data from FastfitnessTips using the first 20 mins of each set. There is a steep drop off after 20mins because not many people do max efforts longer than that so that data was not used.
It is effectively a generalisation of the 95% of 20m power rule of thumb to any point on the assumed power curve.
The decay is a bit hit and miss. If you reduce your training load for 2 weeks compared to the previous 2 weeks your eFTP decays slowly.
If you know it has dropped much more you can add a calendar event of category “Set eFTP” to reset it.
