That's a question I've been interested in quite awhile...and with my getting access to one recently, I decided to run a few tests/analyses.

Just in case anyone besides me was interested, here's the result of a constant cadence cassette sweep (only 9 of the 10 cogs of a 12-25 cassette, 60 rpms, with a repeat of the 53-14 at 75 rpm for some slightly higher power.) After the sweep, I did some accelerations/decelerations for the purposes of doing some inertial mass estimates (more on that later).

Taking the average of the power and "virtual speed" (i.e. the result of the gearing, cadence, and assumed wheel rollout) over the last 2 minutes of each step (and the last 1 minute of the 75 rpm step), I then plotted P/V vs. V^2 for the following:

Assuming an "all-up" mass of 85kg, and a rho of 1.2 kg/m^3, that y-intercept works out to represent a Crr = .0051 and the slope of the line works out to represent a CdA = .350 m^2. Sounds like a fairly "normal" road bike position (on the hoods) and Crr.

Now...about that estimate of inertial mass. My intent was to plug the file and the calculated Crr and CdA into my VE spreadsheet and then modify the mass entry until the small "hills" formed by the accelerations/decelerations "flatten out"...I tried that, but I think I need to reconsider how varying the mass entry affects the calculated rolling resistance force (I might need to separate out the mass terms used for the rolling resistance calcs and the other places the mass term is used). Although, with an assumed 85kg mass, it's pretty darned flat as it is. (I actually had to slightly increase the CdA to .353 to flatten out the "steps")

I think I might just pull the fan/flywheel cover off and measure up the flywheel and just manually calculated the moment of inertia ;)

http://bikeblather.blogspot.com/
Last edited by: Tom A.: Jan 28, 11 16:04

### Edit Log:

• Post edited by Tom A. (Dawson Saddle) on Jan 28, 11 16:04