I have seen many posts regarding the inaccuracy of Polar's Power Option. I have two of these units on two different bikes. I have been able to reproduce the erroneous readings that people have reported to occur on a trainer with cog to cog variation of up to 25-30%.
I therefore decided to take my main road bike on two different road tests to examine the accuracy and reproducibility on a 1.2 mile stretch of road with a steady incline of 0.4%. I repeated the test on 8 of the nine rear cogs, i.e. the 12...23. I am using an FSA compact carbon crank with a 50/34 chainring in combination with a 11..23 cog set. I was monitoring boith speed and cadence throughout the trial. Here are the results of the readings from the large chain ring:
Gear... watt @ 19mph...% from average
50_23... 226... -3.48
50_21... 230... -1.77
50_19... 237... 1.22
50_17... 234... 0.04
50_15... 232... -0.92
50_14... 235... 0.27
50_13... 230... -1.77
50_12... 241... 2.93
Average Power...234...
The errors are pretty tight and resemble a combination of riding variations and reading errors. Regardless, the error was within 3.5%. In fact, ignoring the 50_23 combination which shouldn't be used anyway and the 50_12, the error is less than 2%. The same bike gave me a 25-30% variability on the trainer.
So I am quite satisfied with the performance and reproducibility from the road test and can second the opinion posted by others before that this device works well on the road and poorly on a trainer. I should also note that I calculated the power that would be expected given my weight, bike weight, incline and so forth at 237 Watt for the above trial. My average came in at just 1% below that. Again, that would suggest that the unit also reports absolute values correctly, unlike one of the websites that suggested that the unit over-reports by 20%. I have repeated this test twice with nearly identical results. The second test was done at 20mph with a slight steady headwind requiring a ~285 Watt power output. The differences between the cogs was within 3.5%.
Hope this is useful for someone.
Harry
I therefore decided to take my main road bike on two different road tests to examine the accuracy and reproducibility on a 1.2 mile stretch of road with a steady incline of 0.4%. I repeated the test on 8 of the nine rear cogs, i.e. the 12...23. I am using an FSA compact carbon crank with a 50/34 chainring in combination with a 11..23 cog set. I was monitoring boith speed and cadence throughout the trial. Here are the results of the readings from the large chain ring:
Gear... watt @ 19mph...% from average
50_23... 226... -3.48
50_21... 230... -1.77
50_19... 237... 1.22
50_17... 234... 0.04
50_15... 232... -0.92
50_14... 235... 0.27
50_13... 230... -1.77
50_12... 241... 2.93
Average Power...234...
The errors are pretty tight and resemble a combination of riding variations and reading errors. Regardless, the error was within 3.5%. In fact, ignoring the 50_23 combination which shouldn't be used anyway and the 50_12, the error is less than 2%. The same bike gave me a 25-30% variability on the trainer.
So I am quite satisfied with the performance and reproducibility from the road test and can second the opinion posted by others before that this device works well on the road and poorly on a trainer. I should also note that I calculated the power that would be expected given my weight, bike weight, incline and so forth at 237 Watt for the above trial. My average came in at just 1% below that. Again, that would suggest that the unit also reports absolute values correctly, unlike one of the websites that suggested that the unit over-reports by 20%. I have repeated this test twice with nearly identical results. The second test was done at 20mph with a slight steady headwind requiring a ~285 Watt power output. The differences between the cogs was within 3.5%.
Hope this is useful for someone.
Harry