Originally Posted by
ftwelder
This is important but hasn't been mentioned as far as I can tell.
"Lastly the benefit you state in Q-factor of baving bearings closer together is really a weakness in terms of BB stability and strength. Opposite bearing forces are higher due to greater moment aka Force X distance applied due to opposite crank arm"
I would wager if you rigidly mounted any type of BB shell in a fixture, fully assembled the different crank types and measured deflection at the pedal you would find PF-30 to have the most flex and a quality external bearing BB to have the least.
Yup...the basis for my assertion. From a percentage standpoint...bearings outboard a 68mm shell versus inboard...it ain't small change.
The torque moment percentage wise is likely on the order of 10-20% more torque on inside of shell BB30/PF30 bearings for same pedal force because spacing is quite a bit less. Although unsubstantiated, this may contribute to reduced bearing life as well...which is often reported versus outboard bearings which in my experience have good durability life.