Y'all probably know that it's possible to estimate CdA and Crr with field tests using a power meter. It's also possible to do it without a power meter -- it's more hassle but it's still possible.
There are several ways to do this but if you happen to have a way to record speed accurately and precisely then it's a bit less of a hassle than it used to be. And, many riders now have GPS units on their handlebars that can record speed. That's the point of this post: even if you don't have a power meter but have a way to record speed you can do this.
You'll still need a place to test that's protected from the wind and traffic. And, you'll get better results if you have one of those dedicated wheel sensors for speed rather than relying on the GPS signal -- that's especially true if your test venue relies on tall trees to help protect it from the wind. Don't use "smart" recording -- set your GPS to record second-by-second.
Here are speed data from two coast downs I did a couple of days ago. The speeds are in km/h at one second intervals. As I said, these were coast downs so power, of course, was zero. I forgot to weigh myself and the bike when I got home so I'm guessing the all-everything mass to be around 86 kg. I didn't measure air density but I'm thinking it must be around 1.17 kg/m^3. And the total drop from entry to exit of the test section was about 5 meters. Besides not measuring my mass or the air density, there was a tiny amount of wind in my face during the test runs -- but let's ignore all those problems and for this example let's simply assume wind was zero and everything was exactly as noted above.
15.2 15.8 16.3 16.9 16.9 17.4 17.7 18.1 18.5 18.9 18.6 17.9 17.9 18.6 19.7 20.0 20.9 21.6 22.5 22.5 23.4 23.7 23.5 24.3 25.1 25.7 26.0 25.6 25.3 24.6 24.2 23.9 23.5 23.1 22.9 22.6 22.3 22.3 22.0 21.9 21.7 21.8 21.4 20.7 20.9
26.6 26.6 26.6 26.5 26.6 26.8 26.1 25.4 25.6 26.3 26.8 27.3 27.9 28.4 29.1 28.7 29.6 29.8 30.6 30.3 29.7 29.2 28.7 28.3 27.4 27.2 26.7 26.1 25.8 25.5 25.3 25.0 24.5 24.2 23.9
Importantly, notice that the initial speeds are different for the two coast downs.
Here's the challenge: based on these data, estimate my CdA and Crr. Show your work, and state any additional assumptions you may need.
This was on my commuter bike with heavy duty tires and I was wearing street clothes so don't give me crap about my drag numbers.
Bonus question: it's easy to figure out the average slope over the test section. What was the maximum slope (nearest 0.1% is okay)?
Bonus question #2: why is it important to start the coast downs at different speeds?