Polar/Computrainer Discrepency

[Matt_K]

Hi folks, I'm new here...

I have a question. I purchased a Computrainer for the upcoming winter season. I am very happy with it, however, I have a problem.







I have a Polar Power sensor on my bike as well, and it is giving power readings consistently ~10% higher than the CT. I am inclined to trust the CT more, since there are less variables involved in how it measures the power. I have re-checked:

Chain weightChain length


Distance from BB to axle centers
Distance from power sensor to chain





​

Am still getting discrepencies. Any ideas??? I'm trying to get both meters as close as possible to the same readings so that I can use them both in CyclingPeaks. Thanks in advance! Matt

 

[Tom Anhalt]

Hi folks, I'm new here...

I have a question. I purchased a Computrainer for the upcoming winter season. I am very happy with it, however, I have a problem.







I have a Polar Power sensor on my bike as well, and it is giving power readings consistently ~10% higher than the CT. I am inclined to trust the CT more, since there are less variables involved in how it measures the power. I have re-checked:

Chain weightChain length


Distance from BB to axle centers
Distance from power sensor to chain





​

Am still getting discrepencies. Any ideas??? I'm trying to get both meters as close as possible to the same readings so that I can use them both in CyclingPeaks. Thanks in advance! Matt

Two things:

First, you should expect some losses between where the polar measures the power (at the chain) and where the computrainer measures the power (at the tire) in that there will be losses in the drivetrain. Three to seven percent losses there alone are not unreasonable.

Second, the biggest variable in the computrainer measurement is the tire press-on force. IIRC, you want this force to be REALLY high if you want the power reading to be accurate. Go to the computrainer forum and search on this subject. If the force isn't high enough (higher than what is typically recommended from Computrainer) then there'll be additional "slip" losses at the tire interface.

Voila! There's your 10%!

BTW, it's also possible to "tweak" the Computrainer reading so it matches another PM. A small bit of searching should get you the info in how to do that...I believe it involve simple tweaking of a small pot under the sticker on the unit.

Hope this helps!

 

[Matt_K]

Thanks! I'll look around for threads on higher press-on forces as well as "tweaking" the CT. I appreciate your response.

Matt