Expected CdA Precision
Posted: Thu Sep 18, 2025 9:04 pm
I'm curious about the kind of precision other users are seeing in their CdA testing. I've read of +-0.002 in a couple of places in this forum, but haven't seen the data that goes with it - I'm more interested in understanding all the different parameters that went with such precision.
The best I'm seeing (when my test route had 0 cars/bikes interference but had some non-gusty tail/head-winds, with a 4.5 minute out-back loop) is around +-0.005 for the 95% confidence interval. Which, roughly speaking, makes it so that I can't really tell apart configurations that are at 10-15 W of difference.
Today I tested the difference between raising my stack height by 6cm. The analyzed CdA values I got for baseline were 0.185, 0.181, 0.171, 0.172, 0.178, 0.178 and that for the higher stack height were 0.178, 0.180, 0.171, 0.175.
The best I'm seeing (when my test route had 0 cars/bikes interference but had some non-gusty tail/head-winds, with a 4.5 minute out-back loop) is around +-0.005 for the 95% confidence interval. Which, roughly speaking, makes it so that I can't really tell apart configurations that are at 10-15 W of difference.
Today I tested the difference between raising my stack height by 6cm. The analyzed CdA values I got for baseline were 0.185, 0.181, 0.171, 0.172, 0.178, 0.178 and that for the higher stack height were 0.178, 0.180, 0.171, 0.175.