Loads on engine plates are
* weight load
* torque load (from the drivetrain)
Weight load distributes equally into the plates when bike is vertical. The load acts as shear load and the maximum is at the frame mounts due to the least cross section, fore and aft.
There is also some in-plane bending/tensile load, more so aft than fore.
Drive train torque load acts as an in-plane force couple of tensile loads top/bottom at each rear engine plate. On the P11 there are 2 bolts upper/rear and two small bolts at the frame rails cross tubes counteracting these loads.
On the G15, torque loads are counteracted by 2 bolts upper/rear and the large centre stand bolt which crosses the lower rails.
Due to the vertical distance between these bolts, and torque loads split on 2 plates, the resulting tensile load is small.
Obviously the engine plates need tensile and shear stiffness primarily. This is assured by plate thickness x necessary height at the frame joints. Even the alloy plates can cope with these requirements, as they were given increased thickness over the G15 plates,
compensationg for loss of tensile and shear moduli.
Weakness of plates is primarily seen as creep (material flow) at the engine bolt holes due to high contact pressure caused by small diameter bolts, and also by twisting/tilting/vibration of the engine itself.
Strangely, my G15 rear engine plates also shows ovalization of the gearbox adjuster hole. Obviously the gearbox tries to twist a great deal. Reason enough to take corrective action minimizing the gearbox bolt/plate clearance.
The Commando has a similar arrangement, but torque reaction loads are cushioned, so there is less likelihood of material creep at the engine and adjuster bolt holes.
