[Nathan Streeter]

Adding onto Joshua Miller's comments, here are some more considerations:

1) Gearbox efficiency goes down as shafts become misaligned... if you're concerned enough with adding acceleration to add another ~3 pound CIM to each side, it seems logical to increase the thickness of your plates and add some bends. Two .090" plates bolted together with standoffs will seem fairly rigid if you try to twist/bend it by hand; however, with 3.67-CIMS of torque, I'm guessing you'll have significant flexing. This flexing will be no good for your efficiency, shaft strength, or gear wear...

2) Stiffness of a profile due to bending is (b*h^3)/12. The base and height are both in the "cross-section" of a profile, with the base being the side parallel to the axis of bending and the height being the side perpendicular to the axis of bending. For the cross-section of your gearbox, b=5" or so and h=.090" or so. If you compare this to a "standard" .250" plate, you have an h of only 36% of .250". If you raise 36% to the third, you have only 4.67%... meaning your flat .090" plate is only 4.67% as stiff as a .250" plate. If you go up to .125" plate, you'll have 268% of the stiffness of your .090 plate, which is 12.5% of the standard .250". I'd definitely still recommend adding flanges to your .125" plate, though... Using just 1/2" flanges with a 5" wide .125" plate, you'll get 480% of the stiffness of your traditional .25" plate. For still only being 40-50% of the weight, that sounds like a good design...