Well I gotta jump back in for a minute and share the results that I found with my '50, 3604. I'd done all of this on the first set of rims and tires but never recorded and numbers so this time I charted all of my measurements.
Put her up on jack stands this weekend and ran it up to the magic 45mph mark and did notice some of that throbbing happening although not as bad as when it’s on the road. I next dropped the rear (open) drive shaft down and ran it up to 45 again. NO vibration. I reconnected the drive shaft and put an indicator on it and came up with about .010” to .015” RO.
Next I indicated the machined surface of the hubs just inside of the axle flange, both sides had less than .010” RO.
The rims were the next step, outside of the bead seat gave me .025” on one side and .038” on the other.
The really important RO measurement was the radial RO of the tire. I ran a 1” masking tape around the center tread for a track and got an indication for total RO of .035” on one side and .040” on the other. I orientated the rotation of the wheel to the hub to cancel out the high spots and since the high spots were not exactly opposite each other I only reduced the condition by a couple of thousands. Rotating the tire on the rim might give me a few thousands less RO but one must consider that the Roundness on a steel truck rim and production rubber tire probably isn’t going to be good enough to give you better results so it gets to be kind of a fruitless effort.
The last measurement was for the lateral RO of the tire, the results being .035” and .050”.
Now if I were going to run this truck at 150mph I would no doubt try to get these numbers closer or running at 85 or 90 on the freeway these number would be about at the top of the RO tolerance and one might feel some vibration. But at 45 or 50 mph this is well within tolerance.
So this leaves me with one last rotating part to address, the drive shaft, and I’m going to take it over and have it balanced this week.
When this doesn’t solve the problem then there is only one answer left. There does exist some Radial Run Out on every rotating system. This is going to set up a vibration at some point at the structures natural resonant frequency unless there is some method of dampening it. This is normally absorbed by the automobiles suspension. When I look at the original 7 leaf heavy rear springs on this 3/4 ton truck there isn’t much give at all and with them being 58 years in service there probably isn’t much sliding going on be twix the leaves. So I think what might be happening is that any run out of the tires is being transferred almost directly to the frame and at around 45 mph this frequency begins to resonate with the frames natural frequency of vibration and the two add together and create this amplified throbbing that I’m feeling.
There is one last thing to try, and that is to remove the springs, separate them, clean and inspect them real good and against the factory recommendations, lube or add those Teflon strips. This might help absorb some of the vibration from the slight yet inevitable tire run out and keep it from transferring to the frame.
Denny Graham
Sandwich, IL
