I have gone back and forth about whether to get behind the seat bottle cages. I have read they are less aerodynamic. I have read they are more aerodynamic. Ack! Who to believe. The difference seems to be minor. Currently I have a bag under seat. I use it to store a couple tubes, Microflate, CO2 cartridges, and a set of Park Tools.
But I have a friend who is an Ironman, and he swears by his rear mounted bottle cages. I could also store my tubes, tools, CO2 cartridges, and inflator on it. Right now my under seat bag hangs down. I am sure it is not very aerodynamic. So I think the behind the seat cages might be an improvement, or at least not too much of a loss. And if I go to do an Ironman, I will want to make sure I drink a lot of water/Gatorade.
I started looking around at different models of behind the seat bottle cages. There is a variety of designs. Some with the bottles high, some with the bottles low, and a couple modifications such as with the bottles pointed forward under the seat. I read horror stories about the rear mounted bottle cages being missile launchers, and launching the bottles when the bike hit a bump in the road. The local bike shop had a simple one made by Minoura that is just an aluminum bracket. It was only $15. But I wasn’t sure how I would store my tubes, tools, etc.
Profile Design makes one, but I read reviews about them breaking. My Ironman friend said he had a Carbon Bontrager model. But that he wanted to get a Xlab rear mounted bottle cage holder. He specifically mentioned the Flatwing which is one of the models I was looking at. I will do more research.