motoguy128 wrote:
Neither. Aero bottle on down tube, 1 BTA, nothing behind. Learn to live off the course.
I've never found a way to mount the bottle where I could reach in aero position. I have to be sitting up. So for me, it's useless and I suspect so far behind me that it's aerodynamically not much better than downtube, definitely worse than aero bottle I'd suspect.
While your idea has merit to it and is better than what a lot of people do, you're assuming that you'll be regularly drinking from the rear bottle, and that doesn't have to be the case. I do all my drinking from the front bottle, the rear is just for storage, not for drinking. When the front bottle is empty I just trade bottles. I do have to sit up to trade, and try to do this on a false flat where the aero penalty is reduced, but anyway it only takes me a few seconds. It's a very simple & flexible system. On halfs (don't currently do fulls) I carry a stainless steel down tube cage just in case I need to temporarily stow an extra bottle, but have yet to use it. (I use a stainless cage as it has the lowest frontal area of any cage I've seen.) I'm pretty confident that my behind the bottle setup is very aero, haven't tested it in isolation but it was one of a number of changes that have dropped my CdA substantially. I did test a downtube conventional bottle and it added significant drag. I'm also aware of testing that indicates that while aero bottles are much better than regular bottles, they're generally still slower than no frame bottle at all.
Now if you have a high concentrate bottle then you can't swap, then a down tube aero bottle might make sense. But usually you don't drink concentrated solutions that often so the penalty of reaching back isn't as great, especially if you do it on hills.