Say we will use for the Locost book chassis another type of square tube: sides of 30mm and 1.0 or 1.25mm wall tickness.
Is this type of tube similar in strenght to 25mmx1.6mm?

Amadeu

Without doing the calculations, i would say yes it will be about the same strength in bending because of its greater second moment of area. Not sure about in lengthwise crushing/buckling.

David

The other point I would make is that 1mm wall tube will be more difficult to weld compared to the thicker stuff.