In article <[email hidden]>, Mike
Jacoubowsky said:What I don't understand is why it's OK to have a stem with a very steep
upward angle, positioning the bar quite a bit higher than normal... but it's
not OK to use a taller fork column with a lower stem (to achieve the same
position).
My instinctual reaction is that it has to do with the bending moment of
the length of steerer tube exposed above the top of the head tube. The
length of steerer tube within the head tube is restrained at both ends,
therefore there are no internal bending forces to resist. From the head
tube up, the steerer tube is acting as a vertical cantilever. Force is
applied to the stem from the weight of the rider on the handlebars
which creates a bending moment in the steerer tube.
Your former instance has a shorter cantilever to the point of applied
force, therefore less bending moment. If the length of steerer tube
exposed in the latter instance became too great, I can even see the
internal portion of the steerer tube bowing because the connection at
the top of the head tube really isn't rigid, and at some point would
begin to act as a fulcrum.
But the difference in lengths being discussed in your example is so
minimal compared to the diameter of the tube that this is probably just
a theoretical pile of [censored].
FWIW,
Dan