Wednesday, October 29, 2014

An Oldie, but a Goodie...Field Testing Frame Differences with a Power Meter


I originally posted this back in 2008 on the Slowtwitch.com forum and is an example of the types of equipment differences that can be determined with careful field testing.  Here's the link (click on the title below) to the full thread on Slowtwitch...it's long but informative, and there's some pretty good "back and forth" with a few skeptics:

Something Borrowed...Something Fast!

 
So take this guy and his position:


...and put him on this guy's bike and adjust it so that they fit identical.


Then, let the first guy test both of these "back to back" using the same 404 wheelset with PT SL hub and cover....Any guesses on what the aerodynamic drag differences (if any) one would see?

Using RChung's most excellent methodology described here: Method to the madness

Here's the results for the P2K with the 404 wheels (the wheels on the P3C above):



And here's the results for the P3C:



So...what's the bottom line?

Well...taking the same rider, the same wheels, the same basebars and brake levers, and with the seat and extensions adjusted to deliver the same positions...at basically zero yaw conditions I apparently measured a drag difference of ~.023 m^2 of CxA (or Cda, whichever you prefer - .228 m^2 for the P2K and .205 for the P3C). Using Doc C's "rule of thumb", that basically equates to ~2.5s per km of time savings.

With only an extremely small amount of crosswind however (I could just barely detect it on my skin, not enough to even move leaves on trees), the drag on both setups drops significantly, with the P3C setup dropping more at .190 m^2 vs. the P2K's .220 m^2 for a growing difference of ~.03 m^2. That translates to a difference of ~3 seconds per km...or a full 2 minutes over 40K.

Can you imagine comparing the difference between a P3C (or even the P2K) against a frame that actually increases in CxA with increasing yaw, which is actually fairly common?