N.B. All surfaces except the track were tarred surfaces containing stones of various size and sharpness as noted below. There were no loose stones.
I should like to emphasise that I think all this variation of cal constant is due to variation in surface roughness, and I do not think there is any slipping of the tyre over the surface - except possibly just at the edges of the contact patch. According to my idea of how my bicycle tyre works, when the tyre meets the road at the front edge of the contact patch, it sticks to that bit of road without sliding until, it leaves the road at the other end of the contact patch. The tyre is pressed onto the road with 80psi. This stops it sliding. Although the road imparts friction forces on the rubber parallel to the road surface, I think these do not exceed the limiting force which would be given by the coefficient of friction. As long as this is the case they tyre contact patch does not slide and it does not matter how grippy the surface is as long as it is grippy enough to stop slipping.
I think this behaviour is quite different to what happens with a typical motor car tyre where the tyre casing is so very much thicker and therefore stiffer in relation to the car weight than is the case for the bike tyre. From the papers I have looked at following my recent forum discussion with Mark Neal, it is very clear that car tyres do have significant areas where the contact patch slips. So the grippiness, or coefficient of friction, would make a difference to the calibration constant if we were measuring with car tyres.