FrostyJ wrote:
I was a bit nervous about the repeatability and had the same concerns. I in a way got "lucky". It turned out that my original position was the fastest. Run #1 was my baseline run. Then as I started changing my position, lower, higher etc my drag went up fairly significantly. I was thinking there was something wrong with my baseline so I went back to the original setup and I was back to my original drag number. I think the reason I was able to repeat the run so well was because I have ridden the same position for two years so it feels very natural. I think your point is valid. If my new position would have been very different from the original I think it would have been hard for me to repeat as well as I did. Hopefully Jim or Andy will jump in and address the repeatabillity issue.
Our system has a variability of about 0.6% within a given set of laps. We call this "intra-run variability". We take, say 20 laps, remove the first 3 laps on each end, and measure the std deviation of the laps. Std Deviation/Avg Cda is about 0.6%.
Between runs, our system has 0.8% variability. We call this "inter-run variability". But this is under certain conditions: no air density transients, stable bike fit, and consistent riding. Air density transients, such as what happens if someone were to open a loading dock for prolonged periods of time, are deadly. Outdoor humid, warm air can easily change the air density by more than 2%. That's about 0.005 m^2.
A stable bike fit is one where you're not only comfortable enough not to shift around on the saddle, but that there is enough contact area on your saddle and elbow pads that you don't slip. The contact points must be positively stable -- translation: that it's impossible for you to move around. Jim's pretty good at making as sure as possible that his riders are stable on the bike. That said, occasionally we see examples of slip, shimmy and "one-cheek riding" around the turns. You have to stay vigilant as a test director. But sometimes people move around and you miss it.
AndyF
bike geek