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.


January 11, 2011 Posted by | Uncategorized | 2 Comments

2010 in review, Looking forward to 2011

So my goals for 2010 were pretty simple: ride lots, race long and build a base for 2011.

My 2010 Rocky Mountain Endurance series was a success – 1st place overall in the Men’s 30-39 category. I may not be able to win any given race but I can show up and put in an few decent results when it counts – Call it “victory by attendance”!

The Breckenridge races (Breck 100 and Firecracker 50) were my ‘A’ races and were both utter failures – while my legs were up for the challenge, mechanicals and navigation mishaps ruined the day. The lesson here is to always be prepared, a mantra I’m sure to repeat to myself every Friday during race season.

I also took on a few unexpected challenges:

  • Getting back into trail running. The Fort Collins Trail Running group is a fantastic community and my wife is just nutty for trail running and that alone is enough reason for me to run. My first two trail races (8k and half-marathon) got me 1st place in my age group so that has me psyched up for more; we’ll see in February when I run the Moab 33k.
  • An XTERRA triathalon. I’ve got the biking and running covered but dear god I am a horrible swimmer. Plus I recently learned that my brief swim training this summer really screwed up my shoulders (combined with some crashes and old baseball injuries). No more tris for me!
  • Honeymoon hike; our original plan was to play euro tourist. Instead we backpacked for 2 weeks across colorado at 10k+ ft for 160 miles.
  • Cyclocross. With all that hiking and running I felt the distinct urge to pedal until I puked and needed a competitive outlet to do so before winter set in… CX in a nutshell. The intensity is .. intense. Initially I sucked at it and learned some humility by getting my ass handed to me nearly every weekend. My legs started to come around and I finished the season off strong with a bunch of podiums. I’m currently sitting in 2nd place in the Colorado Cup SS category. To be completely honest, CX is a ridiculous sport which is part of the reason why I love it.

So what does 2011 bring? Definitely more ultra-endurance racing/riding. This year I hope to:

  • Get out on the snow this winter; I’ll probably stick to snowshoeing and touring but may delve into some skating and backcountry/tele expeditions if I can wrangle up the cash to expand my ski collection.
  • Race in the Singlespeed class a bunch more. The mechanical simplicity and style of riding is just .. fun. Riding SS reminds me why I love to bike.
  • Compete in the 24 hour National Championship race. I need to learn how to push the limits of my endurance and riding into the night.
  • Complete the Colorado Trail Race. I am probably insane for considering a 4+ day mtb race but it really gets to the heart of why I love mountain biking and backpacking; Getting out into the mountains, pushing the limits of body and bike and self-sufficient exploration.I’ll probably do a few of multi-day bikepacking rides and races (like the Dixie 200) to prepare.
  • Ultra Trail running: Definitely the Moab 33k in Feb, maybe the Sage Burner 50k in May (Gunnision), probably run as a pacer for the wife in a few races
  • Defend my Rocky Mountain Endurance title. This year’s 6 races look great and are all within reasonable driving distance from the Front Range.
  • Fill in the schedule with some epic long races: Dawn ’til Dusk, Vapor Trail, the Growler, etc.

I really don’t see any short road races, CX, XC or Short-Track racing in store for 2011. I mean, don’t get me wrong, I like to ride all out for 60 minutes, go cross-eyed and puke my guts out as much as the next guy. But getting out into the mountains for some adventure, pushing the limits of endurance, that’s where my heart is. Why compromise? Life is too short to race in a city park. See you out on the trails in 2011!

January 3, 2011 Posted by | Uncategorized | 2 Comments