I found that my bicycle could roll down a 1:50 slope on smooth concrete brick but not on smoothish tarmac. This was at c. 1.5m/s. The bizarre thing is that means the resistive force was c. 2.1 newtons, and I was losing 32 watts. But, if I scale that up to my optimal condition speed of 7 m/s, I get 150 watts, which is ridiculous, because I am pretty sure that I am losing 40-50 watts of rolling resistance and 75-85 watts of air resistance at that speed (16 MPH, fully upright, working clothes). And I know that my effective aerobic capacity is only 125 watts, based on lots of different experiments. Or perhaps rolling resistance is NOT linear in speed, after all! Regards, Nick Maclaren.