Grant Erwin wrote:

I have a friend who is about 6'5" tall and weighs maybe 240 pounds, a big
solid blue-collar type, very strong.
... He called me
last night with a problem. His bicycle seat post - raised way up due to his
height - is bending on him. He bought a replacement and it too is bending. He
asked me to consider designing and fabricating him a suitable replacement.

I would suggest getting one of his damaged posts and measuring the wall
thickness. Hit it with a grinder and a file to at least estimate what
sort of steel and hardness you are dealing with. (The strength of
steels is very closely related to hardness.)

If it's fairly thin wall mild steel, you might just try using tube with
double the wall thickness. This will almost double the strength. I
don't think solid aluminum is likely to be very good since once you are
past a certain wall thickness you get into a region of diminishing
returns.

Don't use too hard material! This is the sort of part where failure in
bending is *much* less serious than failure by breaking.

Ted