It just affects your center of gravity on the bike. It may make you more or less aero but it's impossible to know without testing (I'd bet "no effect" on average). Personally, I prefer a lower BB for TT purposes.

OK, thx!

Since I don't do crits and don't pedal hard through turns and coast through and pedal out of them, I love lower BB drop bikes. The lower the better because it really makes taking technical corners or switchbacks so much more awesome. All things being equal for the same stack and reach and aeroness and maintainability (wrench my bike myself), I'll take the lowest BB bike I can get.

On whether it makes you more or less aero, assuming the same position on top, its' really having more bike+ rider higher above the wheels vs lower relative to the wheels. I think TomA did some research and whether you have more rider around the wheels or less, the overall drag is literally the intergral of drag contribution from each individually (rider on its own + bike frame on its own + wheels on their own). To me that did not sound intuitively right because the presence of one (rider) should affect airflow around wheels or frame vs the wheels and frame on their own, but maybe I am misquoting. One thing though. A lower BB frame (let's say your BB is 2 cm slower), should sheild the rear wheel more than a higher BB frame or looking at it another way, you are filling the gap between front and rear wheel with more frame. That seems like it should be worth something.

That's right. IMO, I think one of the reasons a disc is always faster is because it gives the airflow behind the rider something to attach to. I'm working on a custom TT bike right now with Plane Frameworks and one of the things I'm doing is going with 8.5cm of BB drop as I run 165mm cranks... among other things =) muhahahha

I'd love to see pictures when it's finished. I've had a similar idea. Designing a frame around shorter cranks to bring the rider lower.

Are you looking at road frames or TT frames?

BB drop is a compromise between handling/stability (more drop = lower cg) vs. ground and/or pedal clearance. Aero differences appear to be tangential at most. Bikes that need obstacle clearance (cyclocross, gravel-grinders) and/or are likely to be pedaled while cornering sharply (track or crit bike) are likely to have less BB drop than a Tri bike or a hard-surface oriented Touring or Endurance bike.

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"They're made of latex, not nitroglycerin"

As others have already mentioned, lower BB drop will give you a lower center of gravity to help with cornering, but gives you less pedal clearance to still pedal while the bike is leaning.

So that said, BB drop differences in bikes generally only vary a few millimeters, and the crank arm length of your choice is likely to vary more than that. I'm not super good at math and geometry, but I don't think it's a 1 for 1 comparison in that when the bike leans, a crank that is 5 mm longer isn't necessarily 5 mm closer to the ground due to the angle, but I can't quite wrap my head around if the same math applies to BB drop.

Bikes are still designed to handle corners with 170-175 mm cranks, so I don't think there is a need to sweat over a frame that has 2 mm more BB drop if you plan to run 162.5 mm cranks. However you probably may want to take it into consideration if you were planning to run cranks that are 180 mm.