Doubts about Polar Power readings

[ghostface1]

Hi,



I have been looking at this forum for over a year now, and have been convinced to buy a power meter. Due to lack of money I have invested in a second hand Polar Power unit, because I already owned a Polar s625x. I have also invested in Cycling Peak Software.



The thing is that I think the unit is overestimating my power. I am absolutely positive that I have entered the right values for the chain length and weight, even though I find 230 gram a little low for 107 loops, but I went down to the post office twice and made them weight it on two different scales. I’ve measured it, and used the formula 107 x 12.7 mm and got the result: 1358.9. The chain stay length is 405 mm which I have both measured, and found on the manufacture’s website. So I pretty sure the numbers are right.



It seems to be pretty consistent through the gears and from ride to ride.



My numbers, taken from a random workout, according to CP are:



Peak 20min (310 watts):

Duration: 20:00

Work: 372 kJ

TSS: 36 (intensity factor 1.039)

Norm Power: 322



Peak 60min (234 watts):

Duration: 1:00:00

Work: 842 kJ

TSS: 101.7 (intensity factor 1.008)

Norm Power: 313



When I ride my Tacx Flow in the basement I warm it up and calibrate it so it says +6, which I think gives the best road feel and with a cadence of 90 I can ride 20 minutes in 270 watt in ergo mode.



I know that you can plug in some numbers in analytic cycling, but we don’t have any mountains in Denmark and there are so many factors like wind, bike position, etc. that it won’t make any sense.

I don’t know anyone else who own a power meter, so I can’t make a comparison.



So my question is are those numbers from the Polar/CP correct, or are they way out and what would my FTP be.

I don’t race, I just go for a ride 3-4 times a week, with intervals.



Want made me wonder even more was when I saw T-.mobile rider Aaron on CP have an estimated FTP of 365, which doesn’t seem as so much.



Sorry for my bad grammar and spelling. English is not my first language.



Hope you can help me solve my doubts.

 

[RHR38]

+6 in Tacx Flow for calibration sounds strange to me.. I thought one should do a rollout test for different tyre/air pressure combinations when using tacx?

 

[ghostface1]

That is what I do. I've made "speed up - stop" test and then I tighten the screw which adjust the tension against the tire until it reads out +6 in the calibration. If I calibrate it to +0 the tire tends to slip even though i use tacx own trainer tire, and from what i've read, it also overestimates power at +0. And I think its a more road-like feeling I get at +6.

I also re-check the calibration after i've ended the workout, cause it is known to drift as it gets warmer, but i still get the same reading, +6.

 

[RChung]

from what i've read, [the Tacx Flow] overestimates power at +0. And I think its a more road-like feeling I get at +6.

The Flow's wattage readings are quite sensitive to calibration number. In my case I get the closest agreement to my (static-checked) Power Tap when the Flow is set at +4 -- it very definitely overstated power at +0. I don't know anyone else who has a Flow so I don't know if that's universal or if it varies from trainer to trainer. Two other things: 1) many people have reported problems in using the Polar on a trainer; and 2) "road-like feel" is a red herring.

 

[JustCurious]

...even though I find 230 gram a little low for 107 loops, but I went down to the post office twice and made them weight it on two different scales. I’ve measured it, and used the formula 107 x 12.7 mm and got the result: 1358.9.

230 g does sound low for chain weight. That works out to 0.1692 g / mm. I have a standard ten speed wipperman chain that weighs in at 0.1933 g / mm. Could your chain actually be more than 12% lighter than mine?

 

[Tom Anhalt]

The thing is that I think the unit is overestimating my power. I am absolutely positive that I have entered the right values for the chain length and weight, even though I find 230 gram a little low for 107 loops, but I went down to the post office twice and made them weight it on two different scales. I’ve measured it, and used the formula 107 x 12.7 mm and got the result: 1358.9. The chain stay length is 405 mm which I have both measured, and found on the manufacture’s website. So I pretty sure the numbers are right.

That weight does seem low for the chain length. A couple of things, shouldn't your number of links be even, since you have a set of outer plate links for every set of inner plate links? I'm thinking your chain length is actually 106 or 108 links. Are you using a master link? Don't forget that if you are.

If we assume that the 230 grams is for a 108 link chain, that would be the equivalent of ~250g for a 116 link chain. A Shimano DA 10sp chain is typically 275g for 116 links and a Campy 10sp chain is 260g for 114 links. So....something seems fishy with the chain weight.

Here's a thought, how long has that chain been on the bike? Is it time to replace (or near the time)? If so, just start with a nice new chain and weigh it at it's full length before shortening it.

 

[ghostface1]

That weight does seem low for the chain length. A couple of things, shouldn't your number of links be even, since you have a set of outer plate links for every set of inner plate links? I'm thinking your chain length is actually 106 or 108 links. Are you using a master link? Don't forget that if you are.

Yes, I am using a a master link and weighted and measured it as well.

If we assume that the 230 grams is for a 108 link chain, that would be the equivalent of ~250g for a 116 link chain. A Shimano DA 10sp chain is typically 275g for 116 links and a Campy 10sp chain is 260g for 114 links. So....something seems fishy with the chain weight.

Yes, that is what I thought as well and that is why I took it of again and re-weighted it at the post office at another scale. And when I asked them, they reasured me that the scale was a correctly calibrated. It is indeed very strange .

Here's a thought, how long has that chain been on the bike? Is it time to replace (or near the time)? If so, just start with a nice new chain and weigh it at it's full length before shortening it.

I think I have ridden about 2000-2500km with that chain on the bike. When are you supposed to change?

 

[Tom Anhalt]

Yes, I am using a a master link and weighted and measured it as well.

Then your chain length is actually 108 links: 108 x 12.7mm = 1372mm

Yes, that is what I thought as well and that is why I took it of again and re-weighted it at the post office at another scale. And when I asked them, they reasured me that the scale was a correctly calibrated. It is indeed very strange .

What brand and model chain is it? Compare that to what the manufacturer claims.

I think I have ridden about 2000-2500km with that chain on the bike. When are you supposed to change?

Well...typically, the recommendation is to replace it once it's elongated by 1%. This is easy to do with 12" rule. When the distance between pins has elongated by .125" over a 12" length, it's time to replace. On the tensioned side of the chain run, just hold the edge of the 12" rule up to one edge of one of the chain pins. Since the chain pin diameter is typically 1/8", you just look to see if the other edge of the 12" rule matches up to the corresponding pin edge 12" away. Once the chain has lengthened enough that the entire pin is beyond the edge of the rule, that's a 1% growth.

Personally, I like to do it at half that amount (1/2 of pin exposed), just to be on the safe side and reduce the likelihood of having to replace cassettes. In fact, I don't even measure mine anymore, I just replace them every 6 months. Chains are cheap compared to cassettes.

 

[ghostface1]

The Flow's wattage readings are quite sensitive to calibration number. In my case I get the closest agreement to my (static-checked) Power Tap when the Flow is set at +4 -- it very definitely overstated power at +0. I don't know anyone else who has a Flow so I don't know if that's universal or if it varies from trainer to trainer.

I think I'll try the +4 calibration in the weekend. They have promised heavy rain

1) many people have reported problems in using the Polar on a trainer

Yes, I know polar have problems when used on a trainer. I guess I was very unclear on the matter. I was using it as an example of how many watt i can generate on my Tacx Flow when set on +6 in comparison to what I can generate on the road according to my Polar.
Not sure I've made my self more clear now .

2) "road-like feel" is a red herring.

I'm not sure what the frase "red herring" refers to, but i think i got a clue