This is why you have to gap and body work these cars when they have the suspension loaded like they are sitting on the ground. If you have any car sitting on jacks at the frame, their gaps are all over the place.
Even hardtops do the same thing. The front rails droop. Depending on how tight the gaps are, the doors will interfere with the fenders. Being a convertible has really nothing to do with it. And yes, convertibles suffer from what I call "cowl shake". You can actually see the dash and windshield shake over bumps. Even my T-Top turbo car did it.
I have experienced it myself, I made the mistake of adjusting a fender gap while the car was on a jack stand (was replacing a cross threaded bolt on the firewall)....yikes. It stands to reason that designing a conventional frame for a high volume car that didn't flex was not a priority in the 60s. The steel was cheap enough but adding extra weight, as well as the tooling and assembly complexity/precision required, were not high on the priority list. Throw some rails under some shiny paint with some whitewalls...call it done. According to GM, most of our cars were supposed to be refrigerator magnets or steel shelves by about 1974...
When I put the "1964 Skylark-Pervertable-GS-Prototype-Wannabe" up on jacks, I put the front pair of stands at the body mount hole behind the front tires at the "bend", and the rear pair of stands at the body mount hole in front of the rear tires. And it has the flex. I am more worried about some millennial texting or a soccer mom turning me into a listin in the obits, than the car failing me.
If you want to see just how much these cars flex, put a `68-`70 GTO on the lift, even a hardtop. The front enduro bumpers on these cars are bolted to the frame like most others but the GTO bumper looks more like it's part of the body. When on the lift the bumper will separate 1/4"-3/8" forward and below the leading edge of the fenders. Another thing to try, while driving the car down a bumpy road, reach out and lay you thumb or finger on the gap between the door and the quarter. You'll feel the movement between them.
I noticed that Honda’s second gen Ridgeline has the cab and bed seperate; the first gen was one piece-I wonder if they had trouble with the first design flexing under load? Also pondered the same thing with El Caminos (never had a problem with my ‘85 but never overloaded it and then twisted it) and Avalanches; with full size trucks I’ve noticed how much the cab moves in relation to the bed when the terrain isn’t flat. Patrick
Years ago before I had bought my two convertibles, some wise old guy told me to always pop the catches on the top before the car goes up on the hoist. The frame is designed to support on 4 wheels, not on 4 jack pads. Apparently, according to this old wise guy, there's enough flex in some convertibles to rip or stretch the top out of shape, particularly if the car had suffered an accident at some time in it's life. So, that's what I've done, before it goes up, pop the catches, and then once it lands seal it up. The top on the Wildcat is 27 years old and is just beginning to show signs of distress.
Do a search on Cowl Shake. I have struggled with this issue and had some success but it is the nature of the beast. With the top down, driving at speeds between 25 and 35, it feels like the steering column wants to do a little dance from left to right. I have added rear frame stiffeners found on the GS models. checked and tightened front wheel bearings and installed Bilstein shocks on all four corners. The wobble is less but still there at low speed. At any other speed she drives like a Buick should. As a side note, with the top up and securely latched you don't feel the shake at all.
