Tuesday, November 13, 2012

Low-Key Soda Springs Road: VAM analysis

Last post I described my perception of the Low-Key Hillclimb up Soda Springs Road. I did the climb without obvious metrology: I relied only upon perceived exertion and the timer on my Garmin. It's interesting to see how data match perception.

My PowerTap wheel it far too heavy to haul up the hill, but a good surrogate for post-ride power analysis is VAM (vertical ascent rate). This works especially well with a hill like Soda Springs road which sustains a relatively constant steep grade the entire way.

I took my Garmin Edge 500 reported altitude as a function of time and differentiated it. That creates a highly noisy signal, so I convolved that with a Gaussian of sigma 30 seconds. I considered points only from the point I started riding, so points at the beginning and end wouldn't get smoothed with points from before the start or after the finish, which would cause sag in my VAM rate early and late in the ride. I then did a similar thing with road grade, which I extracted by differentiating the altitude with respect to position before smoothing it with respect to to time in the same manner.

Here's a plot of the smoothed VAM along with the smoothed road grade during the climb:


For the first few minutes the climbing rate was extremely high: close to 1600 meters/hour, which are Tour de France type VAM numbers. But then between 3 and 5 minutes my pace settled out to around 1275 meters/hour. Old La Honda gains 393 vertical meters, and we hit that altitude gained 17:27 into the effort. Old La Honda is less steep, so there's more wind and rolling resistance, but still it shows the pace wasn't exactly slackidasical to this point.

At around 21:30 in my VAM dropped for a bit before recovering to around 1270 meters/hour. Approaching the finish I was able to boost it up back over 1400 meters/hour.

The droop is interesting. What caused that? Looking at the grade data makes it clear: in this section the road leveled out a bit, and rather than immediately upshifting and keeping up the effort, I got lazy and failed to proportionately increase my speed, letting my climb rate fall. Eventually I got my act back together and my climb rate returned.

The finish surge was good. It's natural to have a boost in power at the end when you no longer need to worry about the long-term metabolic costs of anaerobic efforts. But the surge wasn't too big, either: had I had too much in the tank at this point it would have indicated I'd been too conservative in my pacing.

Overall I found this plot very encouraging. When I go out too hard, as I clearly did here in the enthusiasm of the start, it's common for my climb rate to decay steadily the rest of the way. Here I was able to hold a relatively steady climb rate before the surge for the finish. Part of the reason for this may have been, as I discussed last time, that much of the pain I was feeling was discomfort from running-specific muscles which were not becoming increasingly fatigued by the cycling effort, but rather whined and complained at a fairly constant rate the whole way.

The one potential source of improvement here would be to get on top of changes in grade quicker: shift up immediately, increase the speed, and not succumb to the temptation to recover. If acceleration is a big deal due to rapid speed changes, or if the grade gets small enough that wind and rolling resistance come into serious play, then optimization can be more sophisticated, but in this case I think it's clear I could have done slightly better at this point of the course.

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