My late run '86 GT (basically an '87) bottoms out when I hit small sharp bumps, drop-offs or pot-holes, etc. Here are my specs: The '87 up-grade 15"x 7" simulated spoke wheels, with Falken 215/60R15 on the front and 224/60R15 on the rear. Eibach 1.5" lowering springs all around, and KYB struts and adjustable shocks, and polly bushings throughout. The ride is nice with the typical sport car stiffness, but the Falken tires send all vibrations on bumps into the car. Is there a solution to stop what feels like bottoming out bang??? Could the hard tough Falken tires be the culprit?
Id say the unmodified bump stops and the poly bushings are the reasons. 60 series tires have an adequate sidewall height to absorb a lot. 70s are even better. The rule goes with the lower the profile tire, the harsher the ride. I think you have a good compromise tire size. Thats whats on my Sebring and my Ferrari kit, and they both ride pretty nice as well as handle good.
Just as a foot note: I just got through making a 1300+ miles trip to Fallbrook, CA from up by the Canadian border in Washington state. The ride was fine except for the hard tough Falken tired that transferred all the little bumps, cracks and rough road surface up into the car. Not the best ride and lots of road noise. I pulled into a 76 station to gas up right before the last 75-mile leg and when I went to start it, all I got was solenoid click. A couple of Mexican kids admiring my car gave me a push by hand, and it fired right up then quit, in a hail of sparks under the car, and everything was dead. The battery ground wire had melted the lead right out of the battery contact, and the bolt was welded to the ground wire. I had to have it hauled to my daughters to jack it up and find out what happened. To make a long story shorter, the tire vibration had loosened the 3/8" lock nut that holds the solenoid wire to the stud and fallen off, shorting out on the block. Not only that, but the top front damper bolt had also vibrated out, and I later found that the screws that hold my driver's side safety belt had vibrated lose and one had fallen out. I ordered new parts and fixed everything, but kind of worried about what will vibrate lose or fall off on the 1300+ miles home trip...lol! Falken all-weather highway tires are not a good choice for a little sports car. BFG next time for sure!
Tires do make a difference. Falken is a cheaper brand at least around here. They are harder rubber to get you better wear, and Ive seen a lot of them out of round. When you get home, have them checked for runout. You can just jack up the rear and let them spin to see if theyre true. To check the fronts yourself, you need to mount them on the back and run them too. I bought a set of them once for a Lincoln and it rode very rough, lots of front end vibrations. They told me that my front end was shot, but took them back. I put BFGs on it and it rode smooth as silk. They did wear faster than tires I was used to because of softer compound.
It's definitely not your tires causing that, unless they are extremely out of balance. As mentioned, it's likely that you've turned your bump stops into your springs. Bump stops are not good at being springs.
While you didn't specify which model of Falken tires you're running, judging by those sizes I'm guessing that they're one of the more standard all-seasons like the ZE-912 or ZE-512 (there aren't many options for 15" wheels these days). They really aren't known for their stiff sidewalls, especially with a 60 series aspect ratio. There are many of us on here running much harsher performance tires with no issues like you described.