lanierb wrote:
RChung wrote:
I recorded the speed data with my power meter but if you have anything like a Garmin (which many people do) you can use that to record the data.If you use the Garmin, be sure to turn "smart recording" (the default) off first. Also, note that the Garmin records data points once per second except that it records an extra data point every time you hit "lap", so the first and last intervals of a lap are actually shorter than one second and are of random length. If you assume that they are one second you will be making a small error in the delta calculations, and the error will be different for each run because of different speeds and because the first and last intervals are of random lengths (so one run they might be very short and another they might be nearly one second).
Right. In this example I did only two runs because I was simply trying to show that it could be done, and done with pretty minimal equipment. I'd do at least one more in a real attempt, then compare the runs against each other. 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.
I don't hit a lap button -- I like to get my position set and start to coast before I enter the test section so I don't have to move my hands or head or anything, even if only to hit a lap button. Instead, I transform speed into distance and from the speed and change in speed and mass I build up a preliminary elevation profile for the course. It's pretty clear where the slope begins, or where it transitions to the flat, and then steepens again; so once that's identified I can count in distance backward and forward to match the test sections. May sound hard but it's not, really, and it frees me up from remembering to mark the sections. Basically, I try to make the data collection as simple and brainless as possible even if it means I have to do an extra calculation when I get home. Many other protocols require that you hit a certain speed at a certain point or hold your speed or equalize mass or something else. I'm usually too stupid to remember all those things when I'm on the bike so it makes sense to account for it later.
I usually use a power meter so my legs are turning but if I were doing coast downs I'd either have to remember to hold my legs in exactly the same position each run or else I'd just soft pedal so I don't have to remember where to put my legs (see the caveat above). Then when you get home try to be better about weighing yourself and checking the air density than I am.
[Edit:] Just to clarify: I prefer to test with a power meter. It's simpler and faster, and because I can "widen" the speed range I can get similar or better precision with fewer runs. However, if I did not have a power meter and I wanted to estimate my drag, I'd do coast downs with speed recording. It's slower and more of a hassle but it will work.