*****Question for Polar Power users *****

[tmctguer]

for those of you who are using the Polar Power unit, what values did you enter for your chain length and weight -- the ones recommended by Polar (for the common chains listed in the Power handbook) ? or did you take time to measure and weigh your own chain?

the recommendations for a Campagnolo Record chain are 1,448 mm x 279 grams. however, prior to replacing my chain (with another Record chain), i measured it and the correct chain length was 1,353. i went ahead and changed the length value for the chain.

then i took my chain to a post office and weighed it. i was surprised to find that it weighed 1.569 lbs, or 712 grams. this is more that twice the MFG's estimated weight.

i haven't changed the chain weight yet, but will probably do so and do a few test rides on a predetermined course -- 1 with the old values (1,448 x 279), 1 with the new length and old weight (1,353 x 279), then a final one with all new values (1,353 x 712). i can then compare the results to see if there is any difference in the power output value.

anyone else who has worked through this? i'm wondering whether the change in values are material enough to alter the complex power calculation used by the Polar. i found the formula on a website, and there were tons of square roots going on..........

 

[lanierb]

You would be best off using the length and weight recommended by Polar. The reason is that the actual length and weight of your chain don't matter as long as the the weight PER UNIT LENGTH (what you get when you divide weight by length) is correct. The Polar recommended length and weight should give you the correct value quite accurately, even if as in your case you seem to have shortened the chain by removing some links.

On the other hand, your post office scale is obviously way off because the weight weenies site lists the actual (verified) weight of the Campy Record chain as being very close to 279g. So, if you use your own (inaccurate) measurements you're going to be way off.

Lanier

 

[tmctguer]

i was going to find a 2nd scale to confirm my chain weight. i thought about the length/weight ratio, and was going to reduce my chain weight proportionately, but have not gone there just yet.

i did 4 test rides this AM up the same hill with various combinations of weight/length. here are the results:

test course: .426 mile hill, 8% grade
gear: 39 x 13
ride stats: 75 RPM, 8.8 mph, 143 BPM, 2 min 56 sec

279 grams (polar #)x 1448 mm (polar #) = 310 watt avg
279 grams (polar #) x 1353 mm (actual measured chain length) = 335 watts
399 grams (most i could enter) x 1353 (actual measured chain lenght) = 473 watts
399 grams (most i could enter) x 1448 ( polar #) = 445 watts

based on the results above, you are correct in your comment about weight/length ratio.

i am going to proportionately reduce the 279 gram weight by the amount i shortened the chain (95 mm, or 6.561%) to 261 grams, then do the test ride again. i will also try to reweigh my chain using another source.

just to compare, i went to analyticalcycling.com and entered all the info on my ride. it calculated estimated power at 277 watts. this lower wattage amount ties to a lesser chain weight following the weight/length ratio.

i will post when i go to round 2 of testing.

 

[Weisse Luft]

Sounds like you are on the right track. To reduce some of the "magic" to science, the tension measurements are made from the length and unit mass of the chain. The sensor functions just like an electric guitar pickup, measuring the vibration frequency of the chain. Higher frequency, greater tension. Greater weight than programed, lower frequency and lower tension. Greater chainstay length than programmed, lower frequency and lower measured tension.

Keep this in mind, a dirty chain will show a slight decrease in measured power. Changing the lubricant (oil to wax) will only show a change if the mass of the chain is altered.

There is no magic, only science here. Use the guitar analogy if confused.

 

[tmctguer]

i just re-weighed my old chain on a new scale. it weighed 260 grams.

looks like my estimated reduction was almost right on target! i have made the change, and will post new test ride results tomorrow.

a lesson learned for me is to weigh my parcels BEFORE i get to the post office. it was 250% too high !!

 

[tmctguer]

i redid my chain weight to 260 grams, and rode another test ride on the same hill as yesterday. the only differences was the temperature (about 32 degrees cooler). with the new chain weight, i produced 319 watts.

this looks normal for me. i think i have solved the chain mystery. thanks for your replies.