A guy that worked for me years ago was big into demo derbys. I remember him saying that Imperials were banned because their frames were stiffer than regular New Yorkers.
He and his brother would swap the frames out hoping nobody would catch it. If they did get caught, they had a backup car to run with the regular frame. No idea to the truth, but it may explain why they are still around.
My dad ran 8-10 derbys a year here in Michigan from the mid-70's until the mid-90's (mostly USA Demolition Derby out of Belleville). By the early 80's they had banned Imperials because, properly set up, they were darn-near unstoppable.
