Originally Posted by Bob Ross
Sure, at least I will try to. First, drivetrain responsiveness. What I am talking about is the perceived difference in the way that two, otherwise identical bikes will accelerate when the only difference between the two bikes is the BB drop difference of, say 6mm. The lower BB bike will feel as though it is "more willing" to accelerate when the rider is standing. This applies to situations where a rider is jumping out a corner, leaving an intersection or climbing out of the saddle. The perception pretty much goes away once the rider is back in the saddle. What is going on here are a couple of things.
First, the obvious one; when a bottom bracket is lower, the entire bike is lower, everything. Not just the BB and seat tube, but top tube, saddle, bars ... all of the weight of the bike is lower. Since a bike "rocks" around the fulcrum point of the contact patch of the tires, the closer the structure of the bike is to that point, the less force is required to rock the bike, it feels as though there is less bike fighting the rider.
Second, and this one is hard for me to communicate, since I really don't understand fully how it works from a bio-mechanical perspective. When a rider stands up to accelerate, usually, there is some degree of rocking motion that the bike goes through. The lower the BB, the less lateral movement the BB and cranks make during a rocking cycle. Bikes with very high bottom brackets keep the riders pedals farther away from the ground and the rocking fulcrum, therefore making for more lateral movement of the BB during rocking. At the same time, downward pedaling forces during rocking have a tenancy to add to the rocking motion. But the higher the BB, the more those forces will add to that motion. Lower bottom brackets have less effect on the natural motion since those BBs are closer to the rocking fulcrum and don't "rock" as far. With the higher bottom brackets, the rider needs to compensate more for the pedaling forces and the bike seems to take more upper body effort to ride naturally, or neutrally.
Yikes! That wasn't very clear. OK, a few related thoughts. There are some interesting instances where frame builders really need to pay attention to this stuff. Picture a full-on track sprint bike. This frame will need a maximum BB drop of about 57mm if that much, for two reasons both related to pedal clearance. First, on shallow tracks, pedals can hit on the inside during sprint speeds. Second, on steeper tracks, pedals can hit on the outside at slow speeds. So the issue here is just how much drop to use. The trick is to determine a drop which will keep the rider safe, but only just safe. For those of you who have not seen international level sprinting up close, I will also point out that very few sprinters do much rocking as all. The main reason is that at the power levels that those folks have, a rocking (road) type of action would be hugely inefficient and slow. When they are using ALL of their muscle groups, full on, the bike hardly moves at all since they are not dancing on the pedals at all.
Another area where builders need to find a happy medium is with any type of off-road bike. This will include Cross, Mountain Bikes, BMX, trials, etc. The considerations are obvious; ground (log, barrier and rock) clearance versus center of gravity. So for most off road bikes, the compromise point is really determined by the rider. Riding style, components (crank length, pedals) and terrain will determine the drop.
Now back to road bikes. The drop considerations are really the same here as well. Pedals clearance versus drivetrain response. I try to keep our BBs as low as possible while at the same time, safe. OK? Hope I didn't get too far off course. I can be pretty long winded. Thanks for the follow-up question.