[geeteeohguy]

Originally Posted by TB1

Bit of an update after checking and adjusting the pinion angle and using the Tremec app I had the the driveline angles dialed in, took the car for another drive on the hwy at 70+ mph same issue bad vibration so then I started looking into critical speed of shafts (my Engineering back ground got the better of me) and I totally over looked this possibility when getting the car back on the road, I had the drive shaft shortened a couple summers ago when I installed an 8.5" rear and the factory tube had to much runout to be used so they used 2.75" dia DOM tubing with the factory weld in yokes to save me the cost of a totally new shaft. This was perfect with the 4 speed as I never really went much above 60-65mph as the engine was around 3000 rpm and you couldn't carry on a conversation in the car. After some searching I found that the true 1/2 critical speed of a 2.75" .dia shaft at the length in my car was right about 2800 driveshaft rpm and with my car at 70 mph the engine was turning 2000 rpm but the DS was actually spinning at 2000x1.32= 2640 rpm too close felt like it was trying to form a pretzel. I went back to the local driveshaft shop and talked to the owner and he build me a shaft/ 3.5" dia .065"wall thickness with 1330 yokes at the rear I used a 1330 to 3R conversion joint as I didn't change the pinion yoke. I was able to test drive the car today up to about 80 MPH and its as smooth as a 56 year old car gets. over all very happy with the outcome.

I'm a bit confused on the limited speeds/vibrations. I've been driving 4 speed first gen GTO's since the '70's, and have experienced vibrations on some cars that usually had bent or out of phase driveshafts. All of these cars were stock GM, not modified. I could run these cars up to 120+ mph without vibration with the stock stuff....smooth as glass. When I had the 3.55's in my '65, the 'sweet spot' was 90mph at over 4,000 rpm. Not sure of the need to do extensive re-engineering unless the aftermarket stuff is way out of the ballpark.