A powermeter mystery

I’ve taken part in my first « Ironman » triathlon last sunday. I use and train with a Powertap G3 powermeter since I’ve started riding thanks to the advices of Alban Lorenzini from Cycles et forme coaching. I am highly convinced in the interest of powermeters. But, in the last year I have also learnt that I shall train my body to feel the way I ride.

I already had problems with powermeters during triathlons, during the Troyes triathlon last year I had power dropouts. Maybe because of the bad road. Someone told me that there might be an issue with Bluetooth and to turn the BT off on the Garmin devices before a race. I remembered this, but, on my Forerunner 935 it’s been a while since they have erased the Bluetooth menu in the settings 🙁 Also, I might have tried to turn the BT off on my old Garmin Edge 810 but I forgot to do it because I had to manage a flat tire (another mystery) right before the start on the triathlon.

Before the race, I knew that my FTP is somewhere around 260 watts (63 kg). Frankly I have not done a proper test in the last weeks but I have done severals 10 minutes tests and also Xert FTP estimation was in line with this and it’s a tool that I’ve find quite reliable during real strong effort. My weight is between 62 and 64 kg. Since, I would stand on the side of caution I told Best Bike Split that I would ride at 70% of my FTP during my race, so it gave me a NP of 182, which is ok according to the IM simulation rides I have done. With this NP, BBS gave me an average of 169 and according to the course, my bike, the CdA (0,2336) estimated from other races, my weight and some speed limits I set during climbs and descents, BBS told me that I could do the Roth bike leg in something like 5 hours and 15 minutes at 34,14 km/h. Frankly, I would not believe it ! So in my mind, the 5:30 mark was more achievable with a speed of 32,5 km/h which is already super high according to my results on other races. Anyway, I would race between 170 and 180 watts. That’s the idea.

So here are the datas according to my file recorded on my Forerunner 935 :

 

during the first hour, before the dropouts, I was a little bit higher than my goal of 169 power avg. I was at 172 watts avg and 180 NP. So quite close. My heart rate around 140 (correct according to the heat, the adrenaline and having a good swim before) and my RPE also on par. Average speed was 34,4 km/hBut after one hour, when I climbed Greding hill for the first time, I lost the power during several minutes on my watch and on my Edge device.

If I look at the power in the hour after riding Greding, it’s all 20-25 watts lower, but the speed and the heart rate are still in the same zone as during the first hour : avg 147, NP 162, HR 137, speed 34,7 km/h.

If I look at the data after this bug, my avg power is 137 watts for a NP of 152, and I still ride at 34 km/h ?!!? wtf

You see the issue ?

Also when I load my whole file in BBS, riding 178 km in 5:17 at 33,7 km/h and 144 average power calculates a CdA of 0,2041 which is impossible. So since time, distance, weight are good. I don’t think Rolling resistance would impact that much the speed, nor the air resistance. So I think there was a problem with the powermeter. And I am pretty sure of the other parameters since I have done multiple tests to understand BBS, see this note.

Now, I have to do the maths to know the power I have produced during the ride, maybe with the help of a physicist and a great athlete I know 🙂 Update, I have done this, and I think my average power was around 177 watts.

In conclusion, training with a powermeter is great, but you must know how to pace by yourself +++

(and there is something to dig with this BT/Ant+ dual connection, it’s a bummer)