La vie de vélo

Adventures in cycling and life

Powertap calibration

So I’ve been riding the PowerTap Comp (the cheapo wired version) on my road bike since ’09 and it has performed very well. Or so I thought. After a couple frustrating indoor workouts, I started to wonder. My wattage was some 30-40 watts lower than normal. I had been off the bike for a few weeks (mostly focusing on running/hiking/skiing) so I figured the low readings were due to a decline in fitness. I’m sure that was part of it but decided to do a “stomp test” to check the strain gauges in the powertap just in case. Here’s how I did it (more for my own reference than for anyone else):

  1. Zero the powertap manually.
  2. Enter “test” mode: hold mode+select, click select until ‘t’ is flashing, click mode, click mode a few more times until you get to screen 5. The middle number is the torque offset, bottom is the current reading, top is the difference- ie the current torque in in-lb.
  3. Get the cranks parallell, shift to 39×17, grab the rear brake and hang a 30 lb weight from the pedal (i used my barbells). Note the current torque – in this case it was 83.
  4. Calculate the expected torque value:

    torque = (weight in lbs) × (crank length in mm) × (1 in/25.4 mm) × (cog teeth/chainring teeth)
    torque = 30 x 175 x 1/25.4 x 17/29 = 90.09

  5. Calculate the percent difference between the two, (83 – 90)/90 = -0.077

I repeated the test with different gears and different weights and the result was always 7 to 9% lower than expected. That combined with the torque offset of 437 (~14% too low compared to the desired 512) indicate that something is wrong with the strain gauge. Ugh. I’m contacting Saris right now to see if they can help me out.

On the plus side, it means that my pitiful wattage numbers as of late are not entirely due to the off season fitness drop 🙂 It’s possible that I’ve been doing my intervals way harder than I thought! For how long – I don’t know.

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January 11, 2011 - Posted by | Uncategorized

2 Comments »

  1. How did you get on? Did you have to get it fixed? My torque offset is 330, so a 150w workout is quite hard especially when I used to sit over 200w for 4 hours. I suspect mine needs recalibrating (or something has broken)

    Comment by Tony | May 6, 2011 | Reply

    • Tony – I ended up getting a replacement from Saris. Actually it was more like an upgrade… sent them the old wired hub and few hundred bucks and they sent me a new Pro+ hub built with DT rim. The Pro+ hub doesn’t have those stupid carbon windows which leak water which is nice.

      330 torque offset seems WAY off. Once it goes out of the 500-524
      range (that seems to be all they care about and the sole determinant
      of their “is it dead” decision) call Saris and ask about upgrade paths. They’ll
      try to sell you on rebuilding your hub but you can eventually get them to
      offer the replacement which is a smoking deal.

      Comment by perrygeo | May 6, 2011 | Reply


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