Saturday, January 9, 2010

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:


specialist 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?

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.

specialist 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:


And something like this muddies it even more:

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.