Crank length vs power

Like many, I have an older road bike on the trainer with a well worn drive train. Over the past couple of weeks, the front derailleur broke (no problem, found one on the 'bay for 20 bucks) and on the very next session, the rear shifter gave up the ghost. I’m not too keen on dumping good money after bad on this as the chainrings have also seen better days. So, ultimately I will replace the groupset on the bike… sentimental attachment to the frame.

Anyway, to my point. When swapping over the assioma pedals I noticed that the crank length was 170. I had always assumed they had been 172.5 because that is just what I had been running on other bikes and I was quite the novice when I had purchased this one way back when. My question is then, since I had set the crank length 172.5 when I set up the pedals and now have them on true 172.5 cranks, how would this effect the readings. Previously would the readings have skewed higher or lower then they would now? Or would there be no real difference.

  • YES, there will be a very real difference. Crank length is a key factor in power calculations for pedal PM’s.
  • I haven’t had enough coffee to answer this with confidence, so I’m relying on others more alert :stuck_out_tongue:

Power readings should now be lower with the crank length and settings properly aligned, I believe. If you’d been running 175s, the opposite would be true.

Power = Torque / time
Torque = Force (what the power meter measures) * distance (crank length)
Power = Pedal force * crank length / time

So since you lied to the power meter and told it to use a longer crank length than actual in the power equation, it was inflating your power numbers (reading higher than reality).

One important caveat: Assiomas use the crank length set in the recording device (Garmin, Trainer Road, etc) as the crank length. Whatever you set in the Assioma app will be ignored on other devices / apps. So you need to check what you had set in the devices and apps you use to record with to see what crank length you were actually using. Maybe you’ve been using whatever default was set there?

25/1700=1.5% difference

I just did this on accident when I took the pedals off one bike and then put them on another bike that had cranks that were 10 mm shorter. My power readings were significantly higher until I realized that I needed to change the setting on my head unit. Once I made the change it was reading back to normal.