Originally Posted by
gcottay
I have not. Have you? If so, how did you do it?
Originally Posted by
The Smokester
Well, I AM interested in measuring CdA and the other parameters that go into a model describing(understanding) bicycle dynamics with a view to understanding the differences between DF and 'Bents in real-world use. This is just an amusement for me, but I am taking the equations of motion for the various bikes I ride and fitting GPS data to time, position and elevation data for runs over the same courses. To determine CdA I fit many unpowered downhill runs to a model using these gps data and a derived estimate of the varying slope (velocity vs slope). This allows me to combine pleasant rides (i.e I don't need to obsess over the measurements during the ride) with the accumulation of interesting (to me) data. I preselect data to maintain uniformity in weather conditions, bicycle and rider. It is too early to tell whether this approach will be meaningful since I haven't had the 'bent very long.
Thanks. There are several ways to measure drag (both aero and rolling) from field tests and the choice among them depends on how much precision you want or need. Coast downs of various protocols normally give something like one-and-a-half significant digits of precision (i.e., you can tell big changes in position or equipment or position but not small ones). I do something vaguely similar in spirit to Smokester's GPS runs but with a power meter under windless conditions. The precision is quite good but it requires some special hardware. I think I've developed another way to do it with coast downs that takes doesn't have onerous hardware requirements (but takes much more time -- can't get something for nuthin'). I'm trying to find out what other people have been doing so I can figure out whether my approach has already been done.
I would be interested in knowing what other technical references on such empirical modeling there are on the web.
There are a few, though I've mostly been focusing on the ones that use power meters. Candau et al. did coast downs in hallways using electric timing lights. Would you be interested in that?
As a side note, I notice from watching various Tours de ... that breakaway groups are allowed to get quite close to the finish line before the main peleton is unleashed to catch them and the peleton almost always wins. I have wondered whether these sophisticated teams have real-time models of the course and bike performance that allow them an edge in judging just when to start the chase. Anybody know?
Yeah, they make guesses and occasionally they guess wrong. They know that in a flat run-in to the finish, the peleton can pull back a solo escapee at close to 10 sec/km and a small group at 8 sec/km. On a stage with a steep hill finish, the GC contenders can pull back close to a minute per km depending on who is up the road. Everyone knows what the finish of each stage looks like, and with radios they know who is up ahead and how much of a danger they represent.