Since I have had vibration problems off and on for nearly 6 years...I thought I would post some general information that helped me. The first bout with vibration was easy to fix. The whole car was in need of a complete restoration. Brakes and front end components were changed out and it drove pretty smooth for a while. Then, when the rear axle got upgraded, another vibration came at about 50+ (getting onto the freeway), I did some research and had the driveline balanced and it went away. This time, I had replaced the old sway bar for an Addco 1 1/8" unit, disc brake upgrade, new 8.5 10 bolt w/posi and had the driveline shortened to accomodate the new rear axle. The first test drive felt fine. That's because I didn't take the car onto the freeway yet... Above 55+mph, it would rattle like crazy!! The dashboard would rattle, I could feel the vibration in the seat too. I took the front tires off, checked and double checked all of the components and especially the sway bar (since it was new) I couldn't find anything obvious and was getting frustrated. I did more and more research here on the site and decided to check the tires. The rears are BFGoodrich 255 60 15's and had 95% tread when I bought them from a friend. The fronts are 235 60 15's bought from the same friend. All are mounted on Corvette rally's...I got to the drivers side front tire and happened to notice that it was leaning as it stood in the middle of the garage floor (unlike the other 3), so I rolled the tire in the garage and noticed that there was a flat spot next to a rather large "lumpy" area. I remembered that my friend had his 69 project car sitting on these tires for a long time and didn't think much about it, but looking back, he had a jackstand under the passenger front and the majority of the weight of the car leaning on the drivers front tire!! This had apparently caused some sort of cord separation or flat spot in the one tire. I never thought that these tires could be "bad" because the tread looked so great on them and had very few miles on them. (And the fact that I had rebuilt about 1/2 of the underside of the car) It is a real good idea to look for the simple things first! Here's a list of how I went about finding the other vibrations in the past:

-Check the driveline for an out of round condition. Check U-joints
-Check components of the steering and front suspension. Worn components like ball joints, Pitman arms, idler arms, etc, can amplify a mild vibration at highway speeds. Check the rubber boots for cracking and keep everything well lubed!
-Check the pinion angle. This is a measurement from the output of the transmission and the input of the rear yoke.
-Body mounts, bumper supports, and frame positioning can be culprits of body vibrations!
**The tires/rims are the first thing I would check from now on! Then work your way from the driveline to the front of the car. A bad vibration/droning noise while on a cruise isn't fun to anyone, and can be a real hassle to correct! Patience and making one change at a time until something improves is a good way to go about fixing a vibration problem. I hope this can help someone in the future, 'cause I sure had a hard time finding it this time...
I would like to see others chime in and post some solutions to vibration problems so that others can get some enjoyment out of cruising their first gen's instead of rattling fillings out of their mouth ;)
Dano:thumbsup:

You just about cover everything..
All I can add is:
1/ flat spots on tyres from a hard emergency braking, or abuse wth the E brake. A common occurance with 2nd hand tyres.
2/Sloted rims: Use tapered nuts to line these up on the studs, then one by one replace with the correct nuts
3/Paint/debris buildup on contact surface between hub and rim.
4/A rim that just touches the balancing weight on a drum.

I had one at about 3200 rpm.....centerforce clutch. The weights open at about that rpm and that was causing a driveline vibration. I installed a Hays clutch and all good....took me 2 years to find it....

The most common cause of a vibration is tire/wheel balance. Always check this first. A tire that has been run out of balance for any length of time will develop a single flat spot.

A driveline imbalance will be at a higher frequency. It will be faster than a wheel balance vibration in proportion to your axle ratio. Multiply wheel rpm by your axle ratio to get driveline rpm.

Something else to try is to drive the car at the speed at or slightly above where the vibration occurs and clutch it or pop it into neutral and coast. If the vibration stops, it is crankshaft speed related. Things to check are flywheel/clutch/torque converter, accessory drives, balancer, and/or fan.

The most common cause of a vibration is tire/wheel balance. Always check this first
No check the stuff that requires no spanners or money 1st lol like
vibration occurs and clutch it or pop it into neutral and coast. If the vibration stops, it is crankshaft speed related

Perfect timing for this thread, I'll be watching this one.

I have a speed related driveline vibe at about 40mph+, can I assume that my tranny is OK? due to the fact that it does not go away with clutch in or with tranny in neutral. I am checking driveline angles next.

Bought 2 BFGoodrich 235 60 15's and now she runs straight and smooth on the freeway!!
Dano:thumbsup: