Most riders, coaches and even researchers treat CdA as a fixed number. Our first paper shows why it isn't. Using Reynolds number theory, published wind tunnel data and our own field tests with riders including Anna Kiesenhofer, Mattie Dodd and Casper von Folsach, it shows that a rider's CdA can change by more than 10% across their speed range. That's a bigger effect than most of the error sources usually blamed in aero testing.

It also traces how the "constant CdA" assumption took hold through a chain of papers from the 1970s to the 1990s, and why ignoring it can show up as implausible rolling resistance values in field testing.