Sunday, May 11, 2014

Garmin Edge 500 and Mt Diablo: then and now

The Mount Diablo "Time Trial" was today. I felt I rode well, but recorded only my second best time on the relevant Strava segment. North Gate to Summit, which is erroneously named since it actually begins after a bump which follows the North Gate. But since the race today staged on the hot side of the gate, segments which begin at the gate itself included staging time. My previous best was a training ride in September 2011.

I wanted to know why I was slower. Was it due to the tactical portion of the race, with reduced VAMs isolated to a particular portion of the course, or was it due to being generally overall slower? Of course I'm 2.5 years older now, and I've not been training on the bike more than a few weeks, and I'm recovering from a cold which isn't quite gone yet, and the weather was substantially cooler than it was that September day, and I'm presently 1 kg over my "race weight". Lots of possible issues. But I wanted to look at the numbers.

The Garmin I used in September 2011 was different than the one I used today. The older one had been returned when the plastic tabs broke off. This has happened multiple times to me, the most recent in April 2013. It may be a side-effect of taking my bikes on Caltrain, where the Edge is subject to lateral impact.

Here's profile data from the two climbs. The September 2011 profile is a single line, while today's profile is two curves, with the boundary near the junction between North Gate Road and Summit Road.

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This is rather curious because the Garmin 500 is supposed to use the GPS signal to control barometic drift, yet there appears to be a systematic change in the altitude profile from the two rides.

Looking closer at some of the data, the profile on yesterday's ride demonstrates considerable irregularity. I'd noticed this before, where I had to apply extra smoothing to the data than I had expected:

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Here's derivatives of the altitude with respect to distance traveled, taken from the same datasets. No smoothing is applied. The more recent dataset jumps between zero and a large value, while the 2011 dataset varies, but has none of the zero-slope behavior.

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I wondered if perhaps the pressure ports had been plugged.

1 comment:

Mr Nofish said...

I've seen the same on my Edge 500 - this behavior has been called stepping on the Garmin forums.

Off the top of my memory, you might have to revert to 2.8 or even 2.6 to get back the smooth profiles.

I was wondering how/where you found out the Edge 500 is keeping in check the drift with GPS. I'm aware of the Elevation Points features that should somehow help, but it's entirely manual so...

Other than that, my experience is mainly with flat terrain, and I see drifting on a daily basis; more so when weather is changing, of course.