The Commando frame was MUCH stiffer in torsion than either the Featherbed or the P-11 and was designed primarily to keep the front wheel and back wheel in the same plane regardless of side loads. That monster top tube was a major part of the concept.
It was not as stiff in the bending plane, but still quite a bit stiffer than the older technology. When we transferred the design concept to the AJS Stormer, we started to see the limitations of the Commando concept in anything other than street use. The early Stormers (including the factory M-X team bikes) had the top tube welded to the headstock down towards the bottom, with a stiffening gusset wrapped round the top tube and welded to the headstock almost all the way to the top. We had several frame failures where the gusset finished, about 4" back from the steering head.
I did some brittle lacquer testing, putting loads in with a hydraulic actuator to simulate the loads from the front suspension. It was immediately obvious that the transition from the gusset to the round tube was a big stress raiser, and that's where the cracks showed up. Eventually, the design morphed into the one that Bombardier (I think) bought the design rights for. It consisted of two semi-circular sections with a full length triangular insert welded in. The cross-section was circular at the seat loop end and obround at the headstock. It didn't go into production until after I left, and I think the factory M-X program died shortly afterwards.
The reason the Featherbed and the adapted Matchless frame of the P-11 don't break very often is that they're a lot more flexible in response to shock loads. Im' sure the Reynolds enthusiasts will forgive the remark, but one of our techs said "they're welded up from old gas pipe".
I also did some "high-speed" (64 frames/sec on Super-8 film) movies of the Stormer team bikes at one meeting and compared their handling over the sharp bumps with the handling of other, more conventional bikes (MZ and Bultaco in particular). The Stormer could be a real kick in the butt, quite literally. You could see there was almost no flex in the frame when you came off a jump and the shocks couldn't handle the load. There was one series of frames where the back end kicked up so hard, I'm surprised Malcolm Davies wasn't singing soprano for a week. We were testing different shock ideas and one mechanic got quite nasty burns on his hand when removing one from a bike that had been out on the circuit for a few practice laps. Because of the stiff frame, the shocks were getting hot enough to burn the paint off.
If I can find that film, I'll see about posting a piece of it. I'll also have a go at some sketches of the frame details I mentioned.
