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.

image

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:

image
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.

image

I wondered if perhaps the pressure ports had been plugged.

Comments

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.
July 20, 2014 at 4:27 PM
Post a Comment

Popular posts from this blog

Marin Avenue (Berkeley)

Image
On Saturday, I got an email from Tim telling letting me know that Murphy's Spring Classics was going to start up Marin Ave. I'd missed that. Murphy announces his courses the day before, and indeed it was already well into the night before when the course announcement went out. I'd skipped over the details, jumping straight to middle and end games. Marin? I laughed at the irony, because not long before when he'd told me he was choosing to repeat the Nifty Twn Fifty which we'd done together last year versus selecting Murphy Mack's Stage Mullett two-day, I responded I didn't think I had it in me to ride Marin Ave two years in succession. Well, so much for that. I was committed, so I went downstairs to swap my 11-26 cassette for my Recon 12-27. The name is easily overlooked: "Marin Ave" fails to strike the same impression as, for example, "Redwood Gulch" or "China Grade". But among those in the know, Marin Ave is infamous i
Read more

hummingbird feeder physics

Image
I couldn't figure out how the hummingbird feeder worked. Why didn't it overflow? Forces need to balance, of course. Neglecting surface tension, there liquid level is higher in the inner reservoir than in the feeding chamber, so there must be a corresponding pressure difference. Suppose the pressure in the inner chamber were zero. Then the column height difference would need to be atmospheric pressure / (density of liquid × gravity). But this is over 9 meters! Obviously the height difference is only approximately 1% of this. So the pressure difference inside versus outside is only approximately 1%. The inside is only slightly below atmosphere. So air is getting in. How? Does it diffuse through the liquid? If this were the dominant mechanism, it wouldn't take long for the pressure inside to go from 99% to 99.3%, for example, which should be plenty to push the column of liquid down in the inside chamber and thus push liquid out through the holes. It would over
Read more

Gallium Pro geometry

Image
On Caltrain yesterday, I was surprised to see among the usual set of commuting bikes a SRAM Red-equipped Argon Gallium Pro. The bike immediately stole your attention. The design was one which has been trending: clear-coated carbon with plenty of text detail so the viewer is sure to realize he's gazing upon the result of advanced, proprietary engineering and not at yet another paint job on the same old OEM frames being pumped out of the same old Taiwanese factories. But it worked: the bottom bracket area was huge, the downtube a large-diameter "inverted Kamm tail" design seemingly designed to maximize wind resistance, the top tube a broad, eccentric shape which screamed "vertically compliant yet torsionally stiff". All it lacked were the pencil-think seat stays Cervelo popularized, but this was a machine designed for stiffness over anything else. It seemed dramatic overkill, since the bike was small (the Argon "XS" I suspect). There wasn't muc
Read more