Thursday, April 23, 2009

Torque Offset

My riding so far this year has been really crappy. Hardly a bright spot to be seen: never once have I been climbing thinking "I feel really good". However, when I looked at my maximal power curve, there was one shining exception: the Menlo Park Grand Prix. I had been able to sustain higher power for longer there than I have so far on any climb. But did that make any sense? I'd done the same race in 2008, riding in the pack last year as I did this year, and power was lower. I told myself it was the new course, but that didn't add up. The reality is I wanted to believe I was capable of the numbers my computer told me I was generating on that 8th of March.

Then this Tuesday, "reconfirmation". Despite feeling especially poor in the extraordinary heat, I was able to put up some confidence-boosting numbers during a 5-minute interval on San Bruno Mountain. Nice. Until I was descending and noticed my power hovering around 100 watts, despite the fact I wasn't pedaling. Whoops.

Cycling is a balance between power delivered to the rear hub, the resistance from the rolling tires, resistance from moving through the air, resistance from climbing against gravity, resistance from bearing friction, and acceleration. Sure, when descending air resistance may be considerable. But when descending at a steady speed (no acceleration), wind resistance it's balanced with a negative resistance from gravity. Potential energy is being spent to transfer kinetic energy to the air (eventually heating it). Energy is conserved. There's no way the powertap should read positive power when I'm coasting.

torque histogram
Comparison of torque distributions in last year's versus this year's 35+ 3-4 Menlo Park Grand Prix. Last year I flatted out late in the race, so total samples were fewer. I applied a 2.1 Nm shift in values to this year's data (see shifted curve) to accommodate an improper torque offset on my powertap this year.

PowerTapPowertap reports power, but it really measures hub torque, and separately the rate at which the hub is spinning. Torque times spin rate equals power. The spin rate measurement is rock-solid, but the torque measurement is trickier. Basically, you have an electrical element whose resistance depends on torque. But one day to the next, and one element to the next, the resistance at zero torque may differ. To correct for this, Powertaps have a variable "torque offset" which defines the resistance associated with a zero torque value. This can be set when coasting, using a somewhat complicated set of button pushes, which can only be done when coasting:
  1. left button until top row is selected,
  2. long right push to get power flashing, displaying torque in newton-meters,
  3. long right push to zero this,
  4. short right push to bring back watts.

Fortunately, all of this is usually unnecessary, as unless the option is disabled (doing so involves a far more complex still set of button pushes, or the PowerAgent configuration menu), if it detects coasting of a sufficient duration, it zeros the torque on its own.

The problem with torque auto-zero'ing is the PT must recognize that the rider is coasting. Coasting = zero cadence, and cadence is determined by using patterns in the measured torque. But if the torque measurement is sufficiently wacky, due to a really bad torque offset, it can't accurately determine cadence. This can happen if you swap wheels (the head unit must then be taught to "learn" the ID of the second wheel; yes: another complicated set of button pushes).

At Menlo GP, Cara and I were sharing a wheel, so a head-unit "learn" occurred that day, presumably on mine. Then this Tuesday, I'd left my head unit in my office, so borrowed Cara's, "teaching" it to use my wheel: another wheel swap.

The moral of the story: when you swap wheels with a PowerTap, you must zero out the torque. RTFM. It's not that hard. But relying on auto-zero in this case isn't reliable.

Anyway, my extracted maximal power curve has now been whittled down to a size more consistent with how I've been feeling on the climbs this year. Uninspiring. Road racing has to wait.


Mr. C said...

I learned this the hardway as well, while swapping the head unit between bikes. Interestingly it only happens when going from my powertap pro wheel to my powertap SL wheel, and not in reverse

djconnel said...

Thanks for the comment! The issue is that auto-zero'ing requires a zero cadence, but it can't determine cadence well if zero torque is sufficiently off. A cadence sensor would help. My Perl script searches my power files for cases where the histogram of torque has a large peak following relatively low values. But, for example, my girlfriend's roller workouts trip the script: she's like a machine on the rollers, holding her torque in a tight +/- 10% band.

If i compare cadence from 2008 to 2009 Menlo Grand Prixs, I see plenty of zeros in 2008, but 2009 has virtually none. I rode each similarly, showing the Powertap "virtual" cadence number requires a decently accurate torque signal.