Originally Posted by BarelyAWake
I could be wrong and in the end it doesn't matter anyway = broken is broken...
But I'm thinking it's not engine vibration that's contributing to frame failure so much as sustained "high speed" (comparative term ofc lol) impacts from uneven road surfaces on the unsuspended rear portion of the frame. These reported failures are all in high load areas, exactly where the most shock would be as it travels up from the wheel to the dropouts, rear stays to seat post - localized where the most weight is, the rider.
I would think if the engine vibration was at fault - you'd see signs of fatigue closer to it - near the motor mounts and/or where the Dtube connects to the crank/head...
Some of each probably but I've seen an uncanny number of frames cracked directly right at the mounts....almost like a sawzall was attached there rather than a motor. Noted, these were all using china girl engines with stock kit mounts.