A Good rule of thumb is that a tube will buckle when the stress reaches 0.25*E*t/r

Where
E = Elastic Modulus (70,000 for Al)
t = thickness
r = radius

So for a 80x2 it will fail at 860MPa and 100x1.6 will fail at 550MPa. The Yield strength of a good Al like 6082-T6 is 250MPa so both values are above this which is good. I personally then like to add a generous safety factor to these which is why I prefer 80x2. I have experience of 76.2x1.6 beam tube buckling on one of my boats.


Gareth