I'm putting my slide rule down and backing away from this discussion - I'm clearly out-classed! <img src="/forums/images/graemlins/pfft.gif" alt="" /> My seat-of-my-pants 2D planar estimates are no match for the real 3D thang! While reflecting on this, I realized that I was thinking more of discrete planes (well, we're adding plate, right?) and not the assembled box member.

Thanks for posting up a math example of the stretngth ratios! <img src="/forums/images/graemlins/kewl.gif" alt="" /> I'll try to measure the motor mount area tonight when I get a chance, for real numbers.

FWIW, since plating the top of the rail is gonna be tougher than the inside and/or bottom, how do the calc's work if you only plate one side of the box? How about two adjacent sides (definitely better to fight twisting)? There's also racking to be considered...

Randii (who admittedly fabs without the calcs)