Testing the effect of changing temperatures on calibration constant

Interesting study, Mark. I've always wondered how different mile measured splits might end up being when, for instance, I measure a marathon like Baltimore, which requires many hours, where temperatures rise and fall over the course of the measurement, causing mile splits to increase midday and then decrease a little towards evening - as they would be revealed to be if measured via steel tape. I have yet to hear any complaints.

Your research project would seem to me to imply that the differences in constant attributable to the tire types you tested aren't large enough to cause concern. Do you agree?

I wonder, like you, if a solid tire would produce radically different results. I also wonder if the coming "Smart Tire" or one of the other airless tires in development would produce significantly different results. What are your thoughts?

We should all be concerned with the sensitivity of cal constant to temperature change. Most measurements are done in the morning, so the temperature when the pre-cal is done is usually cooler than the temperature during the course measurement. This results in courses that are slightly longer than they need to be.

So for the most accurate measurements we would like to have the cal constant sensitivity to temperature changes to be as small as possible. There are many who suggest using narrow tires with high pressure will minimize cal constant sensitivity to temperature change. But the ideal gas law says the pressure change in your tire due to an increase in temperature will be the same regardless of the initial pressure and the initial volume of your tire. Using a narrow tire with high pressure isn't going to make your cal constant less sensitive to temperature change.

My experiments provide additional evidence that this is the case. The sensitivity of your cal constant to temp change is going to be pretty much the same no matter what tire width you use (assuming your wide tire still has a thin tread) and no matter what initial tire pressure you have. There really isn't a need to pump up your tire to 100psi every time you measure.

Tire "type" does matter. The thicker treaded tire in my experiments did have higher sensitivity to temp change than the thinner treaded tire.

I didn't test solid tires. And since the ideal gas law doesn't apply to them I have no idea what their cal constant sensitivity to temperature change would be.

Sorry if I missed raw data somewhere. BUT what were actual counts and temperatures that day? Oscar

The raw data is in the spreadsheet file, Calibration_tire_testing_MarkNeal, which is at the link below.

https://docs.google.com/spread...WAo/edit?usp=sharing

That file references another file, Calibration_tire_pressure_testing_MarkNeal, which is at the link below.

https://docs.google.com/spread...S0Y/edit?usp=sharing

"There are many who suggest using narrow tires with high pressure will minimize cal constant sensitivity"

Interesting. This has always been my assumption as well. Anecdotally, when I've calibrated with people on large tires and me on small tires, those on larger tires have had higher relative differences in their post-measure cal relative to the pre-measure cal.

ChatGPT seems to be under this same idea as well:

I should have been more clear about my statement that you quoted.

Road tires' calibration constants are probably less sensitive to temperature change than bigger tires, like mountain bike tires. My experiments show this, as the Gatorskin tires were less sensitive to temperature than both the Primo and Bontrager tires. But it's not because they are narrower or because of the higher initial pressure. It's because of other reasons.

The comparison between the 23mm and 32mm Gatorskin tires shows that tire width on its own doesn't matter. Those two tires had nearly the same cal constant sensitivity to temperature change.

The tests for all the tires with two different initial pressures shows that pumping your tire up to its maximum pressure isn't going to make it less sensitive to temperature change.

So yes, you should use a road bike tire to measure courses. But don't worry if it's 23mm, or 28mm, or even 32mm. And don't worry about pumping it up to its maximum pressure every time before you measure.

Has anyone tried to quantify thermal expansion of spokes and rims?

The thermal expansion coefficient for aluminum is 0.0000128 / degree F. A change in temperature of 1 degree F would cause the circumference of the rim to increase by a factor 1.0000128. (From what I've read, spoke tension has a negligible effect on rim circumference.) That, in turn, would change the calibration constant by the same factor. Converting that to the same units as I show in my chart would give -0.0128 m/km/F. That's a small, but not negligible, portion of the -0.05 to -0.11 m/km/F I saw in my tests.

In my tests with an airless tire I was getting about +0.033 m/km. Note that this is a positive number. The effective circumference of the tire decreased with increasing temperature because the solid rubber tire softened with increasing temperature. This leads to an increase of the calibration constant. So the increasing rim size (which would cause a smaller cal constant) would counteract the softening of the solid tire (which would cause a larger cal constant). It may be the case that this partial cancelling out of the effects is most of the reason that airless tires show smaller temperature sensitivity than pneumatic tires.

Thanks for bringing up the changing size of the rim Oscar. That is an interesting component of the airless tires that I hadn't thought of.

By the way, I will post a link to the results of my testing of an airless tire on a separate thread. I posted that to the Facebook group a couple months ago, but neglected to post it here.