Hi all
I was hoping for some advice surrounding a recent FTP measurement.
I have a PowerTap and a Fluid 2 trainer, and I use TrainerRoad (just finished Sweet Spot Base I). Yesterday evening I performed another FTP test (20 minute one) to help reassess my fitness for my next TrainerRoad program.
I was very happy with my execution during the test. I knew I had made some good gains in the last couple of months (I am reasonably untrained), but where I was expecting a new FTP of perhaps 240 (best case scenario), I instead pulled out an NP of 275 for the 20 minute interval, with TrainerRoad suggesting a new FTP of 260.
As happy as I was with that, I felt it was a little on the high side. Now I have a feeling that this may be due to the temperature in the room. I calibrated the PowerTap at the start of the workout (where the room will have been around 15 degrees), but god knows how hot the room got by the time I finished the 20 minute interval (its only a small study-sized room). I have a big fan (albeit blowing more on me than down near the hub) and I had the window cracked open about 6 inches (it was about 3 degrees outside), but the room was definitely quite hot by the time I finished.
Here is a shot of the test:
Now there is obvious upward drift in my power during the interval. I feel this can be explained to a large extent by my execution, as I started a bit too conservatively and slowly kept pushing harder and harder according to what I thought I could manage for the whole interval. I just have a sinking suspicion that a certain degree of that slope can be explained by a rising room temperature, and consequently an inaccurate power measurement from the PowerTap. You will notice my cadence was very stable over the interval, yet power was rising. This is a known issue with the Fluid 2 trainer, as the resistance increases as its temperature rises. This is fine and does not affect my PowerTap measurements in absolute terms.
I have read this study…
When calibrating a PowerTap at an ambient temperature of 8 degrees, the wattage error was discovered to be around 8.4% over an hour. However, I can only assume this is due to the internal temperature of the PowerTap increasing as the wheel is spun. What I need to estimate, is the effect that the ambient room temperature has on the PowerTap readings, which is not reported in their study.
I have no idea how to quantify this, and I don’t want to waste my FTP test.
I think I have two options:
I rest up a little and redo the test, this time re-zeroing the PowerTap right before the 20 minute interval (although this will not control for a temperature rise during the interval, of which there will be). I will open the window wider as well :)I use the above test, but with help from the hivemind apply an adjustment downwards to try and control for this phenomenon
I’m not sure what to do about it. My objective is just to get a reasonably reliable FTP measurement to help guide my training over the next few weeks, and also to be able to remove these biases for future tests. I am very sure 260 is too high, as my previous FTP was 215 and I haven’t exactly been finding the workouts easy.
Thanks a lot for reading!
Benno