In Reply To:
edit: so Tom, do you assume a constant Crr, or does the Chung method determine it for you?
The method requires you to assume a Crr and solve for CdA, or vice versa.
That said, if you KNOW the actual elevation differences AND you don't have any appreciable wind effects, it's possible to narrow in pretty quickly on the correct combination of Crr and CdA when using an "out and back" type course. Although there's virtually an infinite number of combinations that will "level" the calculated elevation plot, only one combination of Crr and CdA will "level" the plot, have the proper elevation change, AND be symmetric about the turnaround point. That's why I said above I was lucky to be able to nail down a Crr for my tire combo at a previous TT. The outbound leg and a portion of the inbound leg of the TT was under near zero wind conditions and I was travelling between 25 and 35mph, along with the leg having a known elevation change of ~50 meters.
However, if all you're interested in is comparing relative aerodynamic drag effects in a "head to head" type comparison like the one above, any
reasonable Crr assumption will still give you the
relative aero differences. You just may not be able to compare it to
absolutely to testing on different days, or with different tires/wheels/pressures. Make sense?
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