It definitely got your attention. You gotta have the car wheel trailing just right. If your off it becomes apparent quickly. I don’t know, I’m thinking the age of the frames and maybe some inner rust in the tubing allowed for some flexing after a while of riding. The lugs for mounting were just brazed on 90° to the bike frame so there’s that possible influence. Or too, there could’ve been something harmonic going on between the the vibration of the knuckle motor, two frames and a particular section of road. I don’t know, I never could get it dialed in. Sold the car, still have the bike, but don’t ride it very often. A buddy had one of the Russian Ural bikes that was fun and smooth. I’d like to have one for off road camping.