[471A]

My 3k pound blown 29 A Sedan begins to shudder at 60 mph then drives out of it at around 70 mph on 120 mph low to mid 11 second quarter mile passes. Front and rear wheel/tires are static balanced. Thoughts on why there is a substantial "shudder" 60-70 mph ?

[ericnova72]

Driveshaft would be my first thought, either shaft not straight or true enough(runout), bad u-joint, out of balance shaft, or the slip or rear axle yoke is not machined true enough. I checked 6 factory 70's 8.5" rear yokes on a Coordinate Measuring Machine(CMM) before I found one with less then .010" runout between the u-joint mount ears and the center splines, The original made a brand new race driveshaft vibrate and the shaft was guaranteed to have less than .005" run out. The original 1972 rear yoke was out .047" from GM, fine with 2.73 gears but a real problem with 4.56 gears. Had the same symptom you describe, but at 50-55mph, then again at 100-110 mph.. You could feel it come and then drive through it, made the inside rear view mirror "buzz".