Monday, February 25, 2013

Monstercross 2013 Report

Impression and Analysis

Fun race, and totally worth doing. 
I started in the 2nd wave, about 1.5 minutes behind an unknown group that I suppose comprised the real podium contenders. Probably should have bulled my way up there. It took about 5 minutes to get my heart-rate up to TT pace, or actually a little above, but I had to pick through a lot of riders, and it was reasonably dangerous to pass on the paved sections. Even during the first lap I was worried that I had gone out too hard, which might be true. I did spend most of the race sweeping up riders and riding through them; it was only at the very end that I lost a couple places. In the middle of the second long lap I got into a group with two cross bikes, a single-speed mountain bike and a couple mountain bikes. I think that it actually slowed me down, but I couldn't go hard enough to ride away at that point, but when I stopped pulling, the pace would drop back down. In the end, I got dropped on the final steep uphill section away from the last stream crossing, so maybe I was going too hard after all. 
The Garmin trace   

confirmed what I could feel in the race. I started to come off the gas on the second lap, about 110 minutes into the race. My heartrate dropped from low 170s to high 160s. It would have been easier to follow wheels at that point. I was pretty bleary for the last 10km, and went to the back of the group when I realized that I was starting to ride beyond the event horizon.  The plot also shows that my cadence was really pretty exceptional (for me) for an off-road race. 


I drove to the race with my good friend and former training partner, Tom Snyder. I probably went to 100 races with him from '92 until he quit racing in  2001. I hope this means that he's making a comeback, at least for ultracross races.


Tom says that on the first lap, a deer took out the guy directly in front of him. Scary. Link to an awesome video of the wreck

Conditions, Course, and Equipment

The course is two long laps and one short lap of fire roads and bridle paths totaling just under 50 miles  in Pocahantas State Park near Richmond, Virginia. No uphill lasts more than a couple minutes, and there are really no climbs of any significance. 
The course was definitely muddier than in 2012, though the weather was pretty similar: partly sunny, 40F in the morning, and low-50s by the end of the race. I imagined that the 2013 course was shorter, but Strava says that it was the same course both years. 
In 2012  I rode Panaracer T/serv 32s, which have an incised tread, and are basically high-end urban tires. They were treacherous on the twisty sandy sections, but they rolled well on the wide fire roads and the short paved sections. For 2013 I rode Continental Cyclocross Speed 35s at 60psi, based completely on this review. I felt confident in the twisty sections, and I don't think there was any penalty on the road sections.
 The Continental Cyclocross Speed (2013)
The Panaracer T/serv 32 (2012) 
Both years I rode the Fuji Cross Pro, instead of the Litespeed I used for Iron Cross. 


  • 2013: 3:05:23  71/382 overall and  9/46 in 50+
  • 2012: 3:17:33  58/336 overall and 12/50 in 40-49
So I was 12 minutes faster, but basically went backwards in the overall standings. Interestingly, my time would have put me 12th in the 20-39 and 9th in the 40-49 cross divisions. Guys apparently don't slow down as they age.

Links to results

GPS trace of the race

Sunday, February 3, 2013

Calibrating my Powertap and SRM


I pretend to be a metrologist at work, so when I bought my SRM from Will Wong,  I knew  I had a problem. I had two devices (a Powertap and an SRM) that measured the same (or nearly the same) value, and I could run them at the same time. It seemed unavoidable that I should compare them, and then ensure that they were as accurate as possible. I sheepishly admit that I ran the Powertap for an entire season without checking the calibration. 
The internet has many good sources for how to do the calibration--see the list at the end.


I borrowed several calibrated masses from the lab, and a hanger I had lying around for calibrating load cells. Here's the bike in the workstand, with the rear brake locked with a jar of chamois cream.
Note--even though I work at the National Institute of Standards and Technology, the nation's metrology laboratory, that mass hanging off the pedal is W=20 lb. It's probably from the 1950s, though, when we didn't always follow the SI. ;-)


  • Powertap Pro 2012 hub, Garmin 500 reported calibration 11011
  • SRM wired, Dura Ace 7400(?) crank 53x39, L=172.5 mm, Powercontrol V unit S/N 16441 current calibration 34.3Nm/hz Age=?

Procedure: Powertap

  1. Engage 39x25 to maximize torque sensitivity and level the crankarm by eye.
  2. Wake up the powermeter.
  3. Get to the calibration menu on the Garmin 500 (long enter press->Settings->Bike Settings->Tarmac->ANT+Power->Calibrate
  4. Calibrate, wait for successful calibration, 
  5. Sequentially hang masses from hanger; record the torque values. 

Procedure: SRM

  1. Get to the auto-zero menu (Mode+Set press)
  2. With no force applied, zero SRM by pressing "Set"
  3. Sequentially hang masses from hanger, record frequency values

Results and Analysis

For the Powertap, the torque applied to the rear wheel, Ta, is 
Ta = M g L /R
where M is the mass applied, g is the local acceleration of gravity, L  is the crankarm length, and R is the gear ratio. For the SRM, the torque can be read directly without the gear ratio multiplier. 
Not unsurprisingly, both devices are quite linear, so there's no point in showing plots of torque measured, Tm, vs torque applied, Ta. For the Powertap, I used R to compute a linear regression of the measured torque on the applied torque for the Ta>5 Nm. The regression was forced to include the origin:
The results of that regression were interesting: the slope of the line, m  is 
m = (0.977+/-0.002) Nm
That is, the Powertap is reading about 2.2 % low. That's outside its stated limit of accuracy: +/-1.5 %.
For the SRM, the analysis is the same, except there is no gear multiplier and one plots the reported frequency, F, against the applied torque. A linear regression, not forced through zero, returned a slope, m
m = (34.30 +/-0.06) Nm/hz
which is the current calibration value stored in the Powercontrol V head unit. Here's the residual plot:

Note that the residuals are on the order of 1 reading unit. The slight swaying of the masses as I added and removed them caused variations at least this large. 

Conclusions and Limitations

  • The SRM calibration is dead on--fine German engineering.
  • The Powertap reads 2.2 % low, so the factory calibration should be adjusted.
I'm a bit concerned about the assertion about the Powertap calculation, since the residuals show some curvature, and the effective slope decreases at the largest torques. That could indicate more accurate readings at high torques. I have no idea what would produce curvature like that in what is essentially the output from a strain gage. The masses applied generate relatively small torques compared to those in use. Assume that you pedal at C=90 rpm, then the power, P, at the maximum calibration torque, T is 
P = T w = 30 Nm * 2*pi radians/revolution * (1 minute/90 revolutions) * 60 s/min = 125 W
which is at the low end of the use range. 
Next: getting the right calibration for the Powertap and fun things to do with with two powermeters.

Notes and Sources

Gravity by location: 9.80108 m/s^2 from the National Geodetic Survey: