Rob84 wrote:
... I was initially worried about relying on high- and low- speed runs for precision work because CRR and Cd vary with speed... ...Ultimately it’s actually not that hard to validate a delta CRR method. You can do runs with the same tire at different pressures, where you have a small but roughly known CRR change (say, based on BRR’s data at 60-80-100 psi). If the deltas you measure lines up roughly with what was determined expect from 3rd party CRR tests at different pressures, then I’ll consider it validated.
Crr increases quite a lot with speed on the roads where I tested... with the caveat that it could have been a mysterious "something else". Temperature effects Crr too.
BRR's data for Crr vs pressure would only be valid for the apparatus he was using. This would certainly not be valid on a road where vibration losses are a factor.
What you really want to do is compare the overall resistance of one tire vs another, at race speed and expected race conditions. If you are testing without airspeed measurement, then very low wind is your best bet, although you will miss aero differences. I suspect on most wheels the VFR will be aerodynamically better at yaw, because it is smaller.
Question is: What is the precision of your testing relative to the small difference in Crr you expect? Doing a bunch of test runs where you are trying to keep *all* variables the same (or compensate for the changes), will show you if you have a reasonable chance of picking the winning tire with any certainty. No tire swapping is necessary to do that.
Also, note that you need to nail the weighted rollout for each tire.