Tuesday, July 10, 2007

TT Frontal Area

In my methodical attempt to improve my time trialing, I plan to do coast-down aero testing to try changes in body position, etc. But before I start doing coast-downs over and over I need to nail down as many known variables as possible to make sure I'm not wasting my time and getting junk numbers (although the testing is a great way to get in good hill repeat sessions).

One of the variables is frontal area. Most folks just combine frontal area (A) with drag coefficient (Cd), call it CdA and are done with it. But if I have a good measurement of A, then I can isolate Cd pretty accurately. Then when I make positional changes on the bike, I can get a preliminary idea of how much it will help just by measureing A again.

To measure A, I took a digital photograph, imported it into AutoCAD, properly scaled it using the tape measure in the picture, traced my frontal area, and measured it directly. I found it to be 4.185 square feet, or 0.3888 square meters. I was pleased to know it was fairly close to the 0.35 square meter estimate I had been using. Based on the limited coast-down testing I did the other day, I think my current CdA is about 0.297. That's good news, becasue that's not a great number. For my size, I should realistically be able to lower it to something between 0.230 and 0.265 depending on whose research you read. Conservatively, that would be at least 89% of my current drag.

Power required to overcome drag is directly related to CdA, so a 89% improvement in CdA would mean I could go the same speed at 89% of the power output it used to take. That type of improvement would be well worth the effort. It might not show benefits at first, though, because a more aggressive position might limit my power output to 89% of my old position until I can train enough in a new position to get the power back up. Six months of once a week time trial training should do the trick, I hope.

Here's the photo with my frontal area hatched in AutoCAD:

Andrew Coggan believes the optimal position on a TT bike for aerodynamics and power output is having your shoulder 5cm above your hip. I don't know if I measured properly, but as you can see in this photo, my shoulder is at least 18cm above my hip in my current position: