The forces inside the engine trying to get out, are greater than the forces outside trying to get in, or bend the engine.
100 hp engine being contained. How much leverage or pressure is there from the outside? 100 hp outside could do a lot of damage.
I own three flexy flyers and one stable bike. First the stable one: An SV1000S Suzuki with a stressed member system. Mr Squiggle number one: An XS400 Yamaha, stressed member frame. It is the swing arm that flexes, plus a crappy rear shock.
2 other Squiggles. XS11 and an XS1100. Full double loop frame, but the tubing is not quite thick enough. However the frame does the job. The front forks are too skinny and they flex. The fork brace on the XS11 helps.
So flexy flyers can have a variety of causes. It is usually not the stressed member system. It all sounds rather kinky.
Often if a person does not ride a bike like my Suzuki, they will never know what the difference feels like. I actually get used to my squiggle bikes, after riding them for a while.
Last bike I rode that was precise like my Suzuki, was a Featherbed framed 750 Norton. The swing arm on that bike flexed, so we chucked it for a new stiffer swing arm. We also replaced the front forks and a lot of other things.
What is better about the newer bikes, is the front and rear shocks.
Wot Eye said about the bolts holding it together. On the XS400, I just did them up tight, and they have stayed in place.
BTW, the Featherbed frame was designed for about 45 hp, the same as my XS400. We ran an 85 hp motor in it. My Suzuki is 125 hp.
I bolt a sidecar to my XS11, and that needs a full frame.
UK