QUOTE=lightspree;19039747]No, the cylindrical shape is much less efficient at utilizing the space. Think about it. It's basic geometry.

What is the volume of a four-inch diameter cylinder that is eight inches long?

What is the volume of a four-inch wide, eight-inch long container that is square in cross section?

Rectangular shapes can utilize the space even better.

And the bottom of the main triangle is not utilized with conventional water bottles.

C'mon, guys. Rub your remaining two brain cells together, and make a spark.[/QUOTE] Aside from your geometry errors, on Touring bikes the "bottom of the main triangle" AKA downtube, often has a third braze on for a water bottle.