Those vertical tubes between the upper and lower tubes are a no no.
I would cut them out. Also the channel section gussets are way
too strong. Both things will lead to bending induced stress
concentrations in the upper and lower tubes right where those parts
are welded in. They add noting to the strength of the motor mount.
On the contrary actually.

Paul Lamar

OK, I'd love lo learn.  I can believe that the vertical tubes are a no
no. (add stress to the supports without adding any support or strength.
But those gussets just seem so natural.  I am so confused.

David Leonard
The Rotary Roster:
http://members.aol.com/_ht_a/rotaryroster/index.html

A properly designed space frame is made up of ONLY  pure triangles. If you make a 
trapezoid or rectangle out of a triangle the loads are no longer pure tension 
and compression in the tubes. The tubes then resist the loads in bending 
near the joints. This concentrates the stress on opposite sides in the 
welded joints. In this case top and bottom of each of four joints.
When a tube is in pure tension or compression (pure triangle) the stress is evenly
distributed in the tube and in the welded joints. When you weld in a 
large beefy gusset or another tube between tubes like that you convert 
the triangle to a trapezoid.

Make an accurate balsa wood 1/4 scale model and things will become clear.

Paul Lamar
 
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