Right, well some of the discussion of tire pressure is very interesting, but somewhat misses the pont. The pont is that it’s not just the tire pressure, but the bike that is a variable that may change during the day. That’s why we do calibration rides before and after.
A will accept that the outside diameter of the wheel will change with the internal pressure of the air in the tire. But it’s not the only variable!
Although you may have a constant amount of air in the tire, as measured by your pressure gauge, while the wheel is static in the back of your van, that does not mean the external diameter of the wheel is constant.
Temperature will change the diameter. Ride a course when it’s dry on a hot day, and then after a tropical thunderstorm do it with cool standing water on the course. Your counts will change a lot although you may have lost no air from the tire, and after you have put it back in your van the gauge may read the same as the start.
Temperature variation is caused by the obvious, sun, shade and puddles. Puddles, in my experance make a big difference. There is also heat generated by work, riding fast over the ground makes tires hot. I ride my bike a mile or two before running it on the calibration course to get it warmed up. Otherwise my first two runs are one or two counts off.
A quick burst of rain can change the temperature of the black top from more than 110 to 65.
The difference that temperature affects the diameter is not just the expansion or contraction due to the temperature of the gas in the tube, but also the expansion or contraction of the rim and spokes. The bimetal thermal compensator made from a rack of rods, like the one that’s built into a good grandfather clock pendulum, is one way to overcome the affects of temperature change, but impractical to implement in bike wheels.
Big nubbly tires on mountain bikes generate more rolling resistance, and therefor more heat in the tube. Mountain bike tires are normally inflated to lower pressures and have more surface aria that expands more readily. So small variations in temperature or barometric pressure may varied relatively big changes in diameter.
We can have a go at calculating the expansion coefficients of the gas, the elastic surfaces and the metal components, the angle of the sun, the depth of the puddles, the rotational speed of the wheel as it sheds the water from the puddle, the .......
Or we can take the unit as a whole, and refer it back to some form of constant reference, like a calibration course.
I agree that monitoring the bike to make sure that you are not losing a significant amount of pressure during the days measurements is a good idea. But even if you don’t, the lost pressure will show up in the post measurements calibration rides.
The last thing we want is people “Adjusting” the pressure between the pre-and post rides. This can only lead to a larger variation in errors, both from pressure and from operator error.
I have a professional foot pump with a plunger handle that is worked with both ands and held steady by the feet. It has a good quality pressure gauge on it. I use it before I start my first calibration ride. By inflating the tire to a known mark, 100 psi, I always start with about the same pressure.
Once I have warmed my bike up by doing a mile or two first the rides on my standard course are often very close to each other. But they are not the same, or in the same range as rides on a previous occasions. On different days the temperature, wether and wind seem to have more of an affect than starting tire pressure.
P.S. Should we have a space on the application form for the riders, before and after, blood pressure and alcohol level?