wrote:

Chalo wrote:
The frame can carry pretty much whatever you put on it, for a while at
least. (Remember the Viet Cong hauling 500 lbs. or more along unpaved
roads on old French road bikes?) It's the wheels, and the square taper
crank if it has one, that are at the most risk of failure from heavy
loads.

How exactly will a heavy load affect the bottom bracket spindle? I
would think the bottom bracket spindle is affected only by the amount
of force/power the motor/rider can put into it while turning the
cranks. No matter what load is being supported above the ground by the
frame and wheels, it does not cause the motor/rider to produce more
power/strength and stress the bottom bracket more.

The load in this case is the rider. The rider can load the BB spindle
in three primary ways: Asymmetric torque from pedaling forces on the
left side, symmetric torque from standing with pedals level, and
bending loads that are applied whenever either pedal has any force on
it (in any crank orientation) and whenever the chain has any tension on
it.

A heavy rider increases the magnitude of all three of these loads
compared to a lighter rider.

If the load on a bike is in the form of luggage or the like, then of
course the BB spindle will not be affected by that.

Chalo Colina