"Chris" wrote in message ...
How do aircraft manufactures determine the maximum gross weight for a
given plane?

They pick a number and then verify that it is OK through a number of tests.
There are a lot of requirements for aircraft to do "this or that" at various weights
up to gross.

For example, the Navion can be approved for various gross weights, some of
it is climb horsepower related. If you have the stronger gear and a bigger engine
you get a 100 lb increase, etc... Some of it is structural in cruise. Some of it
involves sturctures for landing. etc...

I know I've come across articles that mention on a cool day
you can go over the max. gross weight by a percentage, I think 20% was
thrown out there but not sure

Burn whatever said that. If it were purely a performance issue, that would be
true. But that's not necessarily the case. Note that in the above example,
the two limiting factors on the Navion are performance AND the landing gear.
A cold day might give you a bit more performance but it isn't going to make
the landing gear any stronger.