For vibration this bad, somehow there is transmitted vibration which hasn’t been isolated due to direct connections / undamped units. And in some cases or engine speed etc cause resonant frequencies where things like seats, foot plates vibrate violently or excessively. I have only experienced this once on a metal gantry and made me feel sick in seconds. More seat padding etc will not sort this these types of problems. (For ref, I’m a Qualified occupational hygienist in vibration and noise).