Quasar (6/8)
An illustration of the seating position.
As can be seen, there's not much headroom for big folks. This is made more of a problem nowadays, since modern helmets are substantially larger (ie, thicker padding) than helmets of the 70s, when the Quasar was designed.
Look closely at the mirror pods and you will see the scars of a few low (or no) speed tumbles when learning to ride the Quasar!