Cortland Hurl: power comparison Powertap, Vector, Strava

In Dec 2011 I compared Strava power estimates to Powertap measurements for the Cortland Hurl, a previously popular climb for SF2G. Unfortunately the Hurl has gone out of favor with the Google riders, but recently I did a ride with a leisurely spin up the climb, which has been an occasional fitness test for me.

New to the ride was my use of Garmin Vector, mounted on my heavy (10.5 kg) Winter Allaban. You'd expect a relatively high watts at a given VAM with the big bike with fat tires at relatively low pressure. So I was curious to see how the watts I reported on the climb compared to my previous trend.

Here's the result:

watts vs VAM

The data are sparse, and it would have been a lot better had my Vector ride been at a higher VAM. But it is strongly compelling that the Vector is reading low. The Strava estimates are certainly low in particular due to an underestimation of mass. The Strava method of asking for "bike weight" and "rider weight" omits the weight of equipment, which in my case typically includes a loaded backpack.

Here's the same data, but converted to an effective mass, which is calculated assuming all power goes into moving mass against gravity. Since I'm typically 57 to a historic maximum of 60 kg, obviously a substantial fraction of power is going into wind resistance and rolling resistance and, perhaps, some acceleration.

kg vs VAM

Fortunately, I finally got replacement batteries for my Powertap, so I'll need to put the Powertap wheel on the Allaban and see how the two compare, directly. The small complication is the need to use multiple Garmin head units, and pair each Powertap with a separate Garmin.

Comments

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