AG wrote:
There is really strong growth in the gravel / adventure race scene. 3T even bought out the Explore, an aero gravel bike.
Any test out there on frame bag aerodynamics?
Specifically could filling in the triangle with a frame bag like the one below potentially reduce drag?
It could, but that doesn't mean it will.
I'd expect the main mechanism for reduction of drag due to a bag would be reduced vortex shedding by the down tube and thus less drag by that part of the frame. Whether shedding is reduced or eliminated will depend on a few factors, mostly the shape of the down tube as it dictates non-bag aerodynamics and the relative widths of the tube and bag (i.e. do you get a relatively clean transition from one to the other).
However, regardless your ability to reduce the drag generated by the down tube, it's pointless if you also add another, bigger, source of drag, and if you're also adding unwanted weight and handling issues, then it's just a mistake!
Triathlon bikes are ignoring the UCI constraints these days, and yet no-one has just filled in the front diamond. They could do it easily and cleanly. I suspect the main reason is the handling issues it may cause (or perhaps it's just handling issues that might be perceived). So a well implemented bag on a calm day might have some advantage but I wouldn't be inclined to go that route personally.