The B-body Chevy Caprices I used to run when I first started into derbies (because they were cheap and plentiful) have notoriously weak frames behind the rear wheels, especially in snow belt regions where the rust gets to them. Usually one or two good hits is all you get before the whole trunk is crushed into the backseat and causing interference problems with the tires. In between heats I'll get the front-end loader to pick up the car by the sheetmetal and violently shake it to bend it back out of the way.