rruff wrote:
I'm measuring temperature and pressure, so air density shouldn't be an issue. Regardless, it's a fact that on a sunny day the tire temperature will be a lot hotter than the air. Just checked near my house today after I got back from a ride. Air was 70F, road was 103F, tire was 86F... while going ~15mph. So if you are adjusting Crr for temperature, just using air temp doesn't cut it.
Go out and measure CRR against ambient temp. Plot it and you will get an equation good enough to adjust CRR. Tom A had done something similar I think it's somewhere on ST.
While I didn't do the exercise for temp or road temp, I did measure them and see a correlation between ambient temp. I suspect if I did the exact same thing I could get equations for those. I didn't because it is not practical for me to use those during testing.
Now if I had a ladida tire pressure/temp sensor, maybe I would.
But right now you and I have much bigger sources of error to worry about.
My comment on air density is that I also get much better consistency in my results when I don't use air density. This may be partially due to the fact I measure over very long loops (15km) where barometric pressure, temperature and relative humidity vary more. I am pretty sure I can find two points on the course that are more than 2% difference in air density at a given time. If your CRR is varying, your PM is varying (due to temp) and air density is varying, that isa lot of varying. My point, poorly made was the fewer variables, the better.
The reason I test over 15km loops comes to comments made in the future of testing thread, I don't have time to do 15 measurements every 3km loop when I am out riding. I want to train and get aero data at the same time. If a device allows me to get better or marginally better data with less futsing, that's a win. Very much a personal opinion.