Resistance vs drag
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Iwas interested in the comment in the article Van Aerodynamics (CM 1-7 Jan): "Ecotek's own trials are carried out on a rolling road". This will only show the difference, as the article says, in the rolling resistance, ie the tyre and mechanical friction losses, not the aerodynamic losses.
The complete "drag equation" can be obtained by a coast-down test, measuring the rate of deceleration from maximum speed. This should ideally be carried out on a straight and level road and in both directions (not easy in today's traffic), measuring the time for every 5mph reduction and plotting a graph of speed against time.
This would make interesting comparisons between vehicles showing tyre rolling resistance (a constant figure) and aerodynamic drag (proportional to the square of the speed). Low tyre rolling resistance is useful for urban work while aerodynamic drag is important for trunking.
Experiments have proved that even a 100mm radius on bodywork edges dramatically improves aerodynamic drag compared to a square edge, although increasing the radius further did not give similar improvements. Gorden Spooner, Woodford Green, Essex.