Using N2 in bicycle tires, unlike in racing car tires, is somewhat controversial because this practice has benefits that may be small in relation to the extra cost for non-elite bicyclists. It seems that you are suggesting that for course measurement purposes, Mark, using N2 may have no effect, or an undesirable effect because the tiny leakage of oxygen through the tire is potentially desirable when the tire expands during a measurement.
We have seen that some rubber compounds do not expand when the road temperature increases. My latest (high quality Michelin) hybrid bike tires seem to remain constant or actually give slightly higher counter readings during times of increasing temperature. Maybe their rubber is of the type that actually contracts slightly or remains stable under increasing temperature. I have not made direct comparisons of nitrogen vs. air-filled hybrid tires.
In Mike Sandford's (sorry I referenced Pete in error, Mike) study, high pressure inflation seems to have little or no correlation with temperature coefficient. The study demonstrates that skinny tires don't change size as much as fat ones. But it seems to me we still have some anomalies in our effort to determine what kinds of bike tires are best for certification measurements:
- Some tires seem to shrink or remain relatively stable in diameter during periods of increasing road surface/ambient temperature increase.
- Solid rubber tires may actually demonstrate slightly higher cal readings when the temperature increases between pre-and post-cal.
- Hybrid tires with N2 may also show possible shrinkage during temperature increases.
- Tires with air could, in theory, experience a loss of oxygen through the tire during measurement that could offset the expected increase in tire size. How would we be able to determine the truth of this, and whether the offsets tend to balance?
- Water vapor in air-filled tires, not present in tires properly inflated with N2, could exacerbate temperature change, but we do not yet have a way to calibrate this effect on our measurements.
I conclude at this point in our discussion that solid tires
- Seem to provide a more stable constant during periods of changing temperatures, as compared to pneumatic tires.
- Have the potential to provide better measuring accuracy by reducing the impact of temperature changes on the empirical distance covered in a single count.
- Are no more fatiguing during long measurements than comparable pneumatic tires.
- Eliminate the possibility of a flat front tire and the resulting disposal of a measurement that is interrupted by a front flat.
Whenever we get back to holding an annual in-person meeting, an interesting activity could be a mass test of different types of tires over a short road course and a cal course.