I started writing this as a technical document but let’s face it. I should be doing other things and so should you so I will write this more as an abstract.
Over the course of the last 4 years I have been to many test facilities including A2, CMEFE, and done multiple velodrome tests regarding dome top bottles. I tested multiple bottles between the arms (BTA), down tube, seat tube, and with multiple behind the saddle single & double mounts.
When the standard bottle is BTA it is in turbulent air and essentially a free nutritional storage place without an increase in drag. When adding a dome top bottle facing forward there is negligible difference when compared to a standard bottle, but when the bottle is facing rearward (dome toward the saddle) the drag was slightly lower than a forward facing dome shape bottle. Now the differences talked about above were consistent but very very small, we’re talking fractions of a Watt yet this explains why there is a business for BTA systems.