A few more points about the '50s/'60s British-twin frames.........
The rear of the Triumph frame was actually bolted to the back of the center spine, which probably explains why such vehicles would wag their rear end when pushed into high speed corners!
The BSA frame was a solidly welded unit, and handled better than the Triumph, but still had its limitations due to its narrow center spine.
The Norton Featherbed (whether wideline or slimline) was an amazingly simple but excellent design and basically had no foibles for the time.
And........ both the Triumph and BSA had "solid" phosphor-bronze bushings for the swing arm pivot, and the Norton Featherbed had "flexible" rubber bushings for the swing arm pivot !