Power Meters for Cyclocross

I was a bit torn on whether to use my almost new 2012 SRAM Red Quarq power meter for cyclocross this season.  Firstly, the chainrings with the hidden bolt that threads through to the crank aren’t used on the Red PM (because the torque is measured in the spider, and if the rings were bolted to the crank, 100% of the torque would no longer pass through the chainrings/spider), and there currently aren’t chainrings available for cyclocross.

I was able to install chainrings using standard crank bolts by trying a few times until the friction was enough to tighten the bolt sufficiently without accessing the second allen key on the back of the bolt.  I may try chopping one of my allen keys in the future to see if I can manipulate it into place, since there might be just enough clearance.

Wear after 2 months of Cross

More of a concern though was wear.  I know it is silly but cranksets are the centerpiece of a groupo and cyclocross is very hard on the finish.  Mud and grit get rubbed on the crank arms by your feet as you wrestle the bike at strange angles.  This is made worse by low-tension pedals which make foot/ankle interference even more likely.  I may try a treatment to refresh the clear coat by the time road season comes around.  If I do, I’ll let you know how it goes.

More importantly, now that I’ve thrashed my crank for a few races, how useful is the data?

Race Power Data

Below I’ll provide data from seven races.  I’ve raced a couple more this season but I was having problems with my Garmin 500 not recording data properly (since resolved since performing a ‘full reset’ and turning off ‘smart recording’).

Race

Subjective
Rating

CrossResults
Points

Placing

Race
avg/NP

30 min
avg/NP

Boulder
Racing #1

3/10

N/A

DNF

261/307

* 25 min
only

CrossVegas

10/10

209

1

299/326

306/344

Boulder
Racing #2

7/10

211

5

283/317

296/338

UCI USGP #1

3/10

285

67

261/316

270/322

UCI USGP #2

5/10

281

61

262/321

271/335

UCI Co Cx Classic

8/10

225

20

278/307

296/323

UCI Boulder
Cup

7/10

251

28

263/299

270/304

I think the most useful data from the above chart is the 30 minute average / Normalized Power and the CrossResults points (lower is better).  The 30 minute average accounts for sitting up the last lap and shorter races by focusing on the fastest 30 minutes of each race.

Overall there seems to be little correlation between power numbers and race performance.  While it is true that CrossVegas was my best race and had my best power numbers, it was also at 2500 feet of altitude, half that of the other races.  I think the best example races to compare are the USGP Day 2 and the Boulder Cup: Both races have similar power numbers but I did much better at the Boulder Cup by all metrics.

I think there are a few reasons the data doesn’t account for performance.  Most importantly, the running sections.  30 seconds off the bike per six minute lap is would reduce power by 8% (about a 25 watt reduction for the above data) compared to riding constantly for the entire lap.  Even small changes in the amount and difficulty of running sections will affect the data significantly.  Keep in mind that this reduction in NP also reduces TSS scores.

The next reason for variability in the data is technical ability/tire pressure, etc.  The faster a corner can be taken, the more speed is carried, and less power is required to accelerate.  When you’re riding well technically, less power is required to complete the course, and a better placing can be found with lower power output.

Summary

I haven’t found the power meter to be terribly useful for cyclocross racing.  Race day power data doesn’t seem to correlate well with a successful race.

However, that doesn’t mean the power meter is useless on my cyclocross bike.  I train more with intervals during cyclocross season and the power meter is very useful for training.  I also have found that I was generally under-estimating TSS scores for race day.  After accounting for riding out to the race, warming up, course preview, and riding home, I might burn 2000 kJ and accrue 175 TSS points.

About Russell

I have been racing bicycles for a decade. This blog will chronicle my efforts as a Category 1 road racer lining up with the pros.
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2 Responses to Power Meters for Cyclocross

  1. Sean says:

    Great post; I’ve been enjoying your CX related write-ups. I’ve got a power related question for you: what kind of variation do you see in your functional threshold power throughout the season? Or your 5 second output for that matter. Do you have a different power profile at different times throughout your season depending on the kind of racing you are doing/training for?

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