I searched the threads for an answer to this but no luck, so I'm starting this one.
A brief history. I bought this truck (1949 GMC FC101) as a stalled project out of NM about 3 years ago. It was a frame off and completely disassembled when I got it. They had swapped out the rear axle with a early 80's Blazer and that's as far as they got. The truck then sat for 20 years (I found a parts receipt and some parts wrapped in newspaper from 1998 when I was inventorying the parts that came with the truck).
It is now on the road with a SBC 350, a 700r4 and the original 3.73 gearing. I did replace the straight axle front suspension with a IFS unit and coil overs. And took out every other leaf spring in the rear suspension. The rear shocks are rather light duty.
Now the problem...I get a fairly serious vibration at 60mph that feels like it originates from the rear. I feel it in the seat, not much in the steering. It starts at approx 55mph and then starts to go away at 65mph and pretty much stops at 70mph. I have had the wheel balance checked and the driveshaft balance checked. No improvement.
I'm starting to think that it starts a harmonic vibration in the rear suspension, possible caused by the undersized shocks.
Looking for suggestion in how to isolate the issue and correct it. Any help would be greatly appreciated.
Tommy Boy
PS if your interested in seeing the truck you can check out my Facebook page, I have a few pictures there and will be updating it with a more soon.
