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Think of the bottom bracket axle as a bar with a lever attached (crank arm) that your leg moves side to side while pedaling. If you beef up the Bottom bracket area of the frame, the crank arm axle housing if you will, then that bar shouldn't sway back and forth as much. This involves the chainstays also since that whole contraption works together, being that the rear wheel with the drive train is attached to them via the chain. So you make this lowest part of the bike really strong to prevent forces from pedaling input.

Next, if you look at many carbon fiber bikes nowadays, the seat stays are crazy thin. Since your butt supports most of your weight on the bike, you have to adjust the frame to allow shock absorption for your rear, for comfort. So the thinner seat stays are supposed to provide this, along with carbon fiber manipulation along the seat tube and the rest of the frame.

All in all however, we're talking really small improvements here. Pros will notice and deserve the improvements since they are in peak physical condition, but they also get the stuff for free. If you are not a pro however, in my opinion, any bike that fits well and makes you happy is fast enough. Improving the engine always trumps bike tech. Otherwise it's like throwing a carbon fiber spoiler on a stock 4 door Honda Civic. May look cool, but it does nothing for the performance compared to a better engine.
 

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peterk said:
So, now that the frame issue has been resolved, let's talk cranks. I'm sure I have a low end square taper crankset. Does square taper have more flex in it than the external bottom brackets that most bikes come with these days?
Meh, again, we're talking minor improvements here. If you went from your current bike setup to a new CF bike with BB30 and all the new improvements, you'd feel a difference. If you slap an external BB and crank on your bike, your body will respond more to the change in q-factor (which is the distance from the crank to the frame center). If the newer cranks are "narrower" it will feel stiffer to your legs compared to a wider stance on the pedals.
 
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