Tuesday, November 17, 2009

Chung TT Bike Results

This plot shows the results of my recent Chung Method aerodynamics testing with my Cervelo P2K. I wore full aero gear (skinsuit, TT helmet, shoe covers, aero front wheel), but used my standard Powertap wheel on the rear.

Because there is very little air resistance on an 8% climb at 150 watts, I was able to accurately discern my Crr using the climb side of the virtual profile. Then with a little iteration, the descent side of the profile, where CdA greatly overwhelms Crr as a force against the bike at 43 mph, allowed me to determine my CdA. I found it to be very close to the CdA result I got about a year ago when I did extensive coast-down testing on a 1/2-mile 3% descent. I found it to be 0.232 then, and this week's testing shows0.230. That's a pretty good number, but I'm not a big guy (5'9" and 146 lbs) and I ride in a very aggressive position, so it's not completely unexpected.

The first set of runs is with the setup I used for most of this season. For the second set of runs, I moved my aerobars from 4" apart to 3-1/4" apart (moved each of them inward 3/8", or 9.5mm). It was definately enough of a change to feel on the bike. I felt narrower. I didn't expect to see a difference in CdA from the Chung testing, but I did. My next task is to do repeat runs on different days to check precision/repeatability.

I trimmed all my data away except for the climb and the descent for each run, and I reset each run's virtual elevation at the known elevation.