Crank lengths - 172.5 to 165 adaptation?

Sheila Leard

My new bike has 165 crank lengths. I've been on 172.5 forever. I no longer have a Quarq PM. I have the Stages.  Today I rode and was at a loss of how I could go from a long ride averaging 2.3 w/kg to 1.3 w/kg.! The last two weeks I've had  broken training but to lose that much power is horrible. 

Is there an adaptation phase to going to shorter cranks?

 I'm going to experiment with Zwift and see what's up. Maybe I had a bad day. I read in the Wiki @John Withrow s stuff on cassettes and crank set. I have a 11-28 and 52-36.

Thanks for the feedback.

John Withrow

@Sheila Leard is this the first day you rode with your stages?   I would be shocked if your power was materially lower with 165 cranks vs 172.5 cranks.

I would not be shocked if your actual power output (feet to the pedals) was pretty much similar, but the reading you are seeing from the Stages is materially different to what you were previously seeing on your Quarq.  "If" this is the case, the number doesn't matter all that much as long as it is consistent and repeatable on whatever bike/PM combo you will be racing on (or training on for a block).

There are several ways to test if it is a PM calibration issue. You could borrow a powertap wheel (or Powermeter pedals) and measure power from the PT wheel and the Stages at the same time...   And then the PT wheel and the Quarq at the same time...   Or go ride a known loop and see what your NP is and what your time is...    Then do the same thing with the other powermeter and try to hit the same time/average speed and see if the Power is similar...

Or you can hop on a trainer with your old Quarq installed and warm-up, then do a long'ish ~15-20 min interval at like 90% of FTP and careful note of what your HR profile looks like...   Then swap your put the Stages back on, and cover your power reading, but not your HR...   And try to repeat the same "effort" and HR profile for the same ~15-20 min interval...   Then compare what your HR looks like for the two efforts (hopefully it's very similar) and see if the Power readings match what you would expect (hint:  if they're not very similar then the 2 powermeters read differently)....

Sheila Leard

Thanks @John Withrow for some really great insights. I will experiment and dial this in.

Brian Hagan

@Sheila Leard - I have a similar issue with my Powertap P1S (left side only) pedals and my Quarq. 

I am much stronger on my right side, mainly because of arthritis in my left ankle preventing range of motion.

The Quarq measures power from the right foot (chain ring) and estimates it for the left.  Since it uses my strong side, I believe it is overestimating for the left.

The P1S pedal measures power from the left weaker foot (similar to the Stages) and therefore underestimates for the right.  This is why I believe I get a lower power read from the P1S than the Quarq.

My Kickr reads somewhere in between the two, and I am guessing is the most accurate.

I agree with @John Withrow    " the number doesn't matter all that much as long as it is consistent and repeatable on whatever bike/PM combo you will be racing on (or training on for a block)"

Except.....  I am a little competitive, and it does matter in Zwift, since speed is calculated mostly from W/KG.  Because of this, I've started to use the Kickr as the power source in Zwift.

Tim Sullivan

You definitely didn’t lose that much power. It is most likely a calibration issue. Your last power meter was over reporting or this one is under reporting. I switched to shorter cranks and did not notice any power drop off. I just noticed a smoother pedal stroke and my knees is a more comfortable position. 

Gary Lewis

@Sheila Leard If you have a smart trainer, link it with Zwift and view the power output while at the same time running any of the apps that measure power on your phone and connect it to the Stages.  I use IPSensorMan on my Android.  This will give you a comparison of those two, if you don't want to switch your crankset back.  I found that my trainer is about 5-7 watts higher than my Pioneer PM.

Shaughn Simmons

@Sheila Leard
Going with shorter cranks seems to be a recent trend with the pros lately.  Nice move.

Give yourself about two weeks to adjust then look at power, calibration etc.., and compare.

Nice work!

Sheila Leard

@John Withrow @Brian Hagan @timsullivan @Gary Lewis @Shaughn Simmons

Thanks so much Team. All is good! Maybe there was a break in period ... I don't know. But  - I went on Zwift and compared watts .. very close.

I'm not having any knee issues with the shorter cranks. I was always getting off the bike with pain on the top of my kneecaps. Excited to see how this goes.