You have a classic problem with off center load on a post: fence gate
posts, fence corner posts, and side loaded power poles have the same
issues. Your problem will be ground creep if the load gets too high.

You post will want to pivot around the center of what is underground.
The lowest part goes one way, the top goes the other. Underground
resistance is more substantial than the top but still can move.

A bare post will really move quickly, just not enough surface area.
Encasing it in concrete helps by increasing the surface area exposed to
the dirt. Tamping gravel in around the post helps by increasing the load
bearing capacity of the dirt around the post.

I'd suggest 2 plates about 8"x16" minimum, one at the top toward the
road, one at the bottom away from the road. Treated lumber, concrete
block, steel plate are all fine. Bigger is better, it needs to be wide
enough so the dirt does not flow around the plate. Think bigger if you
have sand or black loam.

Ignoramus5876 wrote:
Our old mailbox was hit by cars, snowplows etc, too many times. It is
falling apart. (no foul play involved, just a lot of idiot drivers and
snowplow operators)

I made a swinging arm for a mailbox, so that when the mailbox is hit,
it swings away and then back.

So, now is the time for installing a new mailbox post. I bought a 30"
post support that is made to be beaten into the ground with a
sledgehammer. It is like an arrow with four fins.

I am now having second thoughts and am not sure if this is a good long
term solution. One of the reasons is that there is going to be quite a
bit of tipping moment due to a little longer swinging arm. (my guess
about 40-60 extra foot pounds of moment of force).

I want this mailbox to stay vertical and not "tip".

I live in Northern Illinois, so we have frequent freeze/unfreeze
cycles of soil.

So... What's a good way of mounting a mailbox post? Maybe I should set
that mailbox post support at least partially into concrete? (ie, digh
a shallow hole, beat it into the hole level with ground, and fill the
hole with concrete?

i

"Ignoramus5876" wrote in message
. ..
snip-------
My own plan for now is as follows. I have this giant 40" or so stake
for posts that I bought at Menards. I dug a hole for 2 bags of
concrete. About 16 by 16 wide by 10 inches deep. I will pound the
stake into the ground in the middle of the hole, so that after
pounding, it should be level with ground on the same level as if I
never dug a hole. Then I will fill the hole with concrete. This way,
the concrete will support the stake in a vertical position.

That, I think, will be good enough -- about 170 lbs of concrete and a
stake going even deeper.

The nice thing about this is that later, I could remove the actual
post.

I will also add "eyes" so that later, I could lift the concrete out of
the ground with a "shop crane".

i

Seems to me that you'd want to go deep----below the frost line, so the
ground can't heave the installation as it goes through the freeze/thaw
cycles.
..
Mind you, I'm not an engineer, so maybe I don't understand it as I should.

Harold