It looks like it's bouncing on both the slick sidewall & the rear suspension, probably the sidewalls a lot more. In my opinion big slick sidewalls shouldn't show a ton of wrinkles like your car does as it stages. And 5 lbs of air pressure in the slicks sounds way too low to me, especially for Hoosiers. The sidewalls look to be repeating a wind-up/un-wind bounce cycle, so the tire never really gets "up on the tire" under load during the first 60-100 ft. I'm surprised the car doesn't exhibit some amount of tire-shake at this air pressure. You might keep adding air pressure until the slicks almost flat-out spin (zing) then back the air back down a little, how much air it really needs will kinda depend on the rubber compound (soft, medium, or hard) & the sidewall stiffness you have. You can also sometimes tell if you have the right amount of air pressure by how full/wide the burnout tracks look.
Part of the bounce might also be from the wheelie bars, I/C length/height, & both the front and rear the shock valving, but I would adjust the easy stuff first (air pressure, wheelie bars, & shock valving) before breaking out the wrenches & sockets and adjusting the harder stuff.
Your rear shock "rebound" (extension) valving setting sounds really too tight? I could see where you needed it that tight if you wanted to slow/calm the suspension's initial "hit" because you were using either a decent amount of nitrous, or a short I/C setting (or both). And a soft bump setting could also be adding to the bounce cycle. True, a softer "bump" adjustment is sometimes used for controlling how well the car stays hooked down course on a rough track. And also for calming down the effect the wheelie bars might have on unloading the slicks during a wheel stand, but your bump setting might be too soft . Are you sure your QA1 double adjustables (with both knobs on the bottom of the shock) have 30 clicks of usable adjustment each knob? I thought that they were only around 20-24 clicks each, but I could be wrong.
As a starting point, if your slicks are a medium to hard compound & fresh, you might try around 6.5 to 7.5 lbs of air in the slicks (shoot for little/no wrinkles while staging). If your I/C was around the 45 -50" long range and around the 3.0 - 6.0" high range you might try the rebound set around the 60-80% tight range, & the bump set around the 40-70% tight range. And make sure the wheelie bars are high enough that they won't slap the ground or mask a problem, but still act as a safety device just in case.
Good luck, nice looking car. And as I will say again, the '69-'70 Mustang was the best body style Mustang ever made......EVER! :twisted:
460 street '66 Ranchero.......................finished someday.
460 race '70 Maverick..........................finished someday.
All 'glass Top Sportsman '69 Mustang......ummm, check back when I win big playing the Texas Lotto, or online poker.
page. Some ancient & newer local race video.
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