Quote:
Originally Posted by geeteeohguy
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.
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I didn't really have to do extensive re engineering, the 2.75" driveshaft tube dia was undersized for what was required of it, this was my fault as I forgot to take into account the increased driveshaft speed with the overdrive trans in the rush to get the car back on the road. I went with 3.5" dia tube for 2 reasons, (1) the factory sized 3.25 " dia tube was not readily available and the cost of the yokes were more. the 3.5 dia tube seems to be the go to size. (2) 3" dia was border line in that at a cruising speed of between 70-80+ it was near the true half critical speed of that dia and length of tubing required to fit in my car. Above or below the critical speed is fine to a point but to operate at this speed continually will result in failure this is true for any reciprocating engine or shaft.
The only way to increase the critical speed of a shaft is to make it shorter or increase the dia of the shaft/tube and in the case of our cars we need to increase dia as the length is fixed. as I said above the shaft was spinning right near the true half critical speed for the dia and size I had installed