Friday, March 27, 2009

Cadence

Today's workout was a recovery ride with two high cadence drills, each consisting of the following:
  1. 5 min @ 100 rpm
  2. 4 min @ 105 rpm
  3. 3 min @ 110 rpm
  4. 2 min @ 115 rpm
  5. 1 min @ 120 rpm

Obviously this suggests the need for a cadence monitor. As I mentioned in my last post, the Powertap does make an attempt at cadence by monitoring the periodicity of the torque measured at the rear hub, expecting to see torque fluctuate as first one foot then the other foot moves between the downstroke. However this doesn't work as nicely as a good old-fashioned magnetic reed switch mounted to a chainstay to pick up the field from a magnet taped to a crank arm, and Saris will even sell you one for $50. For moderate cadence, the Powertap's scheme works okay, but for spinning drills, it tends to get confused and underestimate. But even at moderate cadence, the number fluctuates, such that trying to maintain cadence by watching the number isn't very productive. Smoothing helps reduce the fluctuations, but it also delays the feedback received from changes in cadence. So the best thing here is to mostly go by feel: for each interval, sustain a cadence I feel I can sustain for that interval.

I started my ride meeting the Noon Rider crew, providing a convenient deadline. Then I "warmed up" by riding with them over Arastradero before letting them go as they charged up Alpine. Okay, I admit I didn't need to warm up for a recovery ride, but the Noon Ride is addictive, and even a small fix helps. Then I began the business at hand.

My first set took be to the Portola Road - Sand Hill Road intersection. After a break, I started my second set on Tripp Road, finishing it where Whisky Hill T's into Sand Hill Road. I couldn't sustain continuous high cadence: turns or cars at times got in the way. But when I was pedaling, I kept the rpm's high.

Here's some analysis of the data. After pruning out the points for cadence below 45 (coasting, or contaminated by diluted by coasting during the sampling period), I average my measured cadence for each period, along with the total pedaling time for each period. I do that for each of the two sets.

RPM vs interval time
RPM for each interval vs. pedaling time

Since I am confident my last (and shortest) intervals were the highest cadence, and well above the listed values, the Powertap appears to be having problems with cadences over 120. This is consistent with my prior experience, and in fact with the Saris FAQ, which claims an upper limit of 130 (since cadence naturally fluctuates, this would show up as a reduction in average cadence for reported average cadences modestly below 130).

I was glad when the second set was done. High-cadence work is hard. On that final one-minute interval, I try to keep it on the verge of bounding on the saddle. On group rides, Coach Dan likes to show his spin, typical of those who've spent productive time at Hellyer. The ability to sustain power at cadence is non-trivial. Not only does it help in quick accelerations, but for descents and/or tailwinds it allows one to generate power to higher speeds. I was dropped at the Patterson Pass Road race a few years ago when I found myself gapped off the back on a gentle descent with a raging tailwind. I was giving my 53/12 everything I had, but I just couldn't close that gap. Better neuromuscular coordination at high rpm's could well have made the difference (so could have not getting gapped in the first place! But that's another matter.)

Rather than simply spin back to work, I finished my ride with one-legged pedaling. "Coach Troy" Jacobson's voice rang through my brain: "Pull up on the top of your shoe! Pull up on the top of your shoe!" One-legged pedaling is a prominent component of the Spinervals 23.0 technique workout.

So another virtuous day on the bike, but a hard day on the bike, one which had me sometimes tempted to return to the seductively simple suffering of the Noon Ride.

P.S. You'd think the Powertap would report half cadence for one-legged pedaling. But clearly it wasn't doing so yesterday: reported numbers during my single-legged drills were in the 60 range, right where I'd expect. It would be interesting to see the algorithm it uses.

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