head position while on the drops

I got on the trainer and tried the following two positons, both representing a "chase to close a gap" mode (I couldn't necessarily hold these positions for a half hour, for example).



In the top photo, I've got my head up to focus on where I'm going. I used to ride this way all the time. In the bottom, I'm shrugging my shoulders and relaxing my head, looking up to look forward. I can see fine, it's just I'm looking up instead of straight. In the top photo it actually seems my arms are more bent (sloppy of me). Yet despite this advantage, you can clearly see my head is higher above my back: the difference to the bottom position is a full 5.6 cm.

With a head width of 17 cm (which I measured), Cd = 0.8, air density = 1.1 kg/m³, I can calculate the effect of a 5.6 cm drop in head position on power, assuming power depends on frontal cross-section and the head down allows more of my torso to draft. The result: 6.7 watts @ 45 kph.

Chasing to close gaps is always a dicey proposition. Small differences get amplified, since it's speed difference rather than speed which determines how long the chase will be. 6.7 watts can easily make the difference.

None of this is new, of course. Here's a photo of Jacques Anquetil setting the Hour Record:

Comments

Nicholas said…
I see you've done the analysis on a bare headed case, surely the gap closing situations you are considering would be generally in racing, so you'd be forced to wear a helmet?
January 11, 2010 at 2:04 AM
djconnel said…
A helmet is an additional source of variability. I assume the effect of the helmet is similar in both cases, since my road helmet does not have a prominent tail to it.

There are guys who ride the Noon Ride helmetless, and I don't have any criticism of them for that. But for essentially all organized outdoor cycling events, helmets are required.
January 11, 2010 at 5:43 AM
Nicholas said…
I guess you are riding something WW like a Limar 104.

I don't think I can say just by looking which of these will be faster, head position wise:
http://photos-b.ak.fbcdn.net/hphotos-ak-snc1/hs240.snc1/8721_162911311891_689721891_3293613_1115329_n.jpg

or

http://photos-c.ak.fbcdn.net/hphotos-ak-snc1/hs220.snc1/8721_162911316891_689721891_3293614_1989100_n.jpg

And something like this muddies it even more:
http://photos-e.ak.fbcdn.net/hphotos-ak-snc1/hs220.snc1/8721_162911321891_689721891_3293615_3350871_n.jpg
January 11, 2010 at 9:09 AM
djconnel said…
Okay -- this is a good point. It's on my "to-do" list to redo the test with a helmet. In any case, there's a few things I want to try:
∙ photo from the front, rather than side
∙ put does on my shoulders, to more precisely quantify torso/arm position.
January 11, 2010 at 9:41 AM
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