Very long post coming at you:
I’ve been meaning to do this for a while, but I finally got around to comparing the data from my left crank 4iiii PM to my Suito. In the past, I used the 4iiii to control the trainer via Power Match, however this season I have moved to having a dedicated trainer bike, and the 4iiii mounted to my outdoor only bike.
My expectation based on RPE was that the Suito was over reporting power compared to the PM, however, that isn’t the case…mostly.
The setup for this test:
Trainer Road iPhone app, latest bata controlling the Suito in ERG mode via bluetooth.
4iiii Left crank based PM recorded on Wahoo Elemnt Bolt via ANT+
Data uploaded and analysed in DCR Analyzer Tool
The first screenshot shows the stats:
Interestingly, there are some variations in the heart rate data when zoomed. I guess this has something to do with the difference between Bluetooth and ANT+ recording?
So, here is the full power graph:
As you can see, there is a substantial gap between the two power meters. It seems that the gap is most extreme at low power outputs around Z1 and low Z2 (for me). At these levels, the smart trainer to my surprise reports lower numbers than the crank based PM. The difference is as much as 20W which seems extreme. The average is 10W off.
At my Z2ish numbers things sync up a bit better, with the crank based meter now only reporting an average of 5w higher.
As power continues to rise, the difference narrows further, however at this point, the 4iiii is now reporting slightly lower numbers. Close enough to pace a TT based on my indoor training I hope.
I threw in a form sprint, and you can see how drastically the Suito over reports power vs the crank:
What does all this tell us? I don’t know… It certainly raises some questions for how I approach training and pacing. I ride indoors all week, all year, and outdoors on the weekends basically April-November. My key workouts are indoors, and my outdoor rides are largely by feel, so I’ll continue to base my training on the Suito. However, I wonder how the data reported from my outdoor rides will under-report TSS. I prefer very long days, so a difference of 8 hours averaging 180w, but with the data reporting 170 w could make a substantial TSS difference. The most interesting question to me though is, if my 4iiii is over reporting Z2 relative to the trainer, why do I feel so much more fatigued after 2 hours outside than I do after 2 hours inside? Cognitive load? Full-body engagement? Why is my heart rate at 160w outside always a good 10-15bpm higher than it is inside, when the power appears to actually be lower? Because motorists are terrifying? Perhaps.