BE1 wrote:Not all "rough" surfaces are "bumpy".
Many roads in Ireland are constructed with a top dressing of loose clippings rolled into the Tarmac. This is not terribly bumpy but is very rough and on occasion I have lost speed on shallow downhills. Under these conditions my ill informed guess is that lower pressure would considerably increase resistance.
I'm not implying that lower pressure will roll better, but that the optimal pressure will be lower than for a smooth surface. e.g. on track 'boards' (or rollers as it the rolling road drag tests above) you want the tyres as high as they will possibly go and that as the surface becomes rougher the best pressure falls - at some point on every surface there will be a crossover where the extra tyre flexing causes more drag than the tiny ups-and-downs and jiggling of the harder tyre - I'd guess that the rougher the surface the lower this optimal pressure is.
But it's only a guess...