Morning Bill, what your describing is exactly like the problem I’ve had with my 1950, 3604, which is completely stock. At around 45mph I get a vibration at the rear, I can see the tailgate hopping up and down in the mirror and feel the hopping all through the truck. I don’t have any buddies any more, their all dead or can’t drive anymore so I don’t know if it can be seen from the rear but I can see it.
I first thought it was the 15” split rims, which were in almost new condition since the truck had spent most of its life in the fire barn. I broke them down and discarded the old rubber, blasted them and PPG’d them with DBU, mounted brand new Power King bias plys, tubes and flaps. I found a dealer that had the newest dynamic load or Road Force balancer and the tires/wheels runout checked out OK. I indicated them on the truck to see if the spindles or axles were bent, OK there. All that didn’t help. Found a real nice set of 16.5” tubeless rims did all the same as above to them as far as prepping the rims and mounted new Coopers up on them, and had the Road Force balancing done, that didn’t stop it.
I put new double action gas shocks from JC all around, that didn’t help any.
I put a thousand lbs of bagged sand over the rear to see if it would change the mode, that didn’t change a thing.
I had the complete driveline out, stripped to bare metal and inspected for damage or wear, all was like new there. I rebuilt all of that with brand new u-joints all three of them, replaced all bearings, bushings and center support rubber. Dial indicated the open drive shaft for runout, OK there. I put it up on jack stands and ran it up to the speed that the rear end hopping starts at, that be right around 45mph, and it was fine on the stands, no back end throbbing or related vibration other than the normal vibration one would expect from a machine running at 45mph. So that tells me that it I’m back to something to do with the tires/wheels when they are on the ground.
Every bit of the front end steering and suspension is completely rebuilt and aligned, there is no slop in the steering and there is no oscillation in the steering wheel at any speed.
My final conclusion is that it is a resonance node that sets up a vibration at around 45mph, unless someone comes up with a better idea. I’m really looking forward to getting the cab and box off the frame and see if there is a problem somewhere in the frame that I can’t see with everything mounted up.
Good luck and please do keep us posted if you get it figured out.
Denny Graham
Sandwich, IL
