My last set of tests turned made me realise just how much the road surface can affect the rolling resistance of a pair of tyres. The only differences between the two tests that I carried out on Stapleton Lane (rough with pot holes) and Desford Lane (very smooth except for two metres of gravel) were obviously the surface and the fact that the tests weren't done at exactly the same time. Whilst the results from both lanes indicated that one tri-bar position was better than the other, those carried out on the poor road surface resulted in a wider spread of calculated CdA figures. However, when I looked at the differences between medians for both sets of tests they were similar:
Desford Lane, 0.259 and 0.265m^2 or 0.006m^2 difference
Stapleton Lane, 0.309 and 0.312m^2 or 0.008m^2 difference.
I had assumed a coefficient of rolling resistance (Crr) of 0.005 for both sets of tests, but in reality the Stapleton Lane tests must have yielded a higher Crr. However, if everything else REALLY was the same, then CdA values for both sets of tests should have been identical for each respective tri-bar position.
I took the CdA values that I had obtained from Desford Lane of 0.259 and 0.265, plugged them into the Stapleton Lane elevation calculations and solved for Crr. I got values of 0.0084 and 0.0085 which are pretty close in my book and were as they should be: greater than for Desford Lane. Once again the methodology delivers perfectly logical results (I'm sure the Robert will say "and why shouldn't it?") and leaves me feeling even more confident about what I'm trying to do.
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