Driveline vibration out of now where! Help!
1972 dart swinger, 440/727, 3.23 geared clutch type 8 3/4 rear. I just got finished rebuilding the front end and doing the K-H disk swap. Finally got that all done and now noticed i have this pulsating vibration in the driveline somewhere.. Almost like swerving over onto the rumble strips back and forth..not quite that bad but... It had been fine and sits most of the time in the garage so i dont know where this came from all the sudden. I jacked it up and ran it through the gears on the jackstands and it still did it. I crawled under the car and grabbed the drive shaft and shook it and there doesnt seem to be any play in the u-joints..Im stumped. Like i said, its been fine and seemed to just develop out of now where.. any ideas?
Does it come and go at a particular speed
or just get worse and worse the faster you go?
Is it tied to a particular rpm
or throttle opening?
What happens if you cruise the car into the vibration zone, and then put it into neutral?
Did you change the ride height?
If it does it on jackstands with the wheels removed, and has a sweetspot speed where it is worse than at other speeds, then you have eliminated several usual suspects; namely all of wheels and the entire front end.
I assume the jackstands were placed under the axle tubes,out near the bearings. And I assume the rear springs and more specifically the bushings in them,were not touched. Those springs are pretty good amplifiers, which is why they have rubber isolation bushings.
With 3.23s, the rear axles turn at 1/3.23=31% of driveshaft speed, and so, at a cruising speed of 60mph, or about 2400rpm, the axles are only turning at 740 rpm. And so it's hard for me to think your large amount of "rumble-strip" vibration could originate in the axles; they would have to be bent fairly badly to cause that degree of vibration.
And I know of no single source of vibration that can come and go.
But I do know that the A-body has a natural frequency of about 1 cycle per second that can be excited by another vibration, and the two together can combine to make a come-and-go, who
ooooo-whoooooo-whoooooo, which can be both felt and heard. But it's not a sudden onset, and is far more subtle than "rumble-stripping". I also know that the tires, at 740 rpm, are far more likely to aggravate this ~60 cycles per minute body vibration, because they are related like 740/60= ~12 to 1 ; so the whoooooo-whoooooo-wh increases and decreases both in volume and intensity, at various speeds often being worst at 55/60 mph.
But, by taking the wheels off, you have eliminated this.
But the driveshaft can also excite this body frequency, when the U-joints cycle thru their angles of operation. I don't know if you are aware of how a U-joint works so A quick review is this; Any time the driveshaft and the thing it is driving or being driven by, are not in a direct line from eachother, the crosspins speed up and slow down with every revolution. When the angle difference is small the vibration is small and vice versa.
The front angle is usually 1 to 3 degrees depending on the ride height and rear tire size. The rear angle changes with the rise and fall of the rear ride height AND the amount of power input as the springs wind up. For a streeter the rear angle can vary from zero to maybe 7 degrees.
The ideal would be if the rear angle is equal AND opposite to the front angle, as then the vibrations cancel out and you get a smooth ride. But this situation,in a non-stock vehicle, if it happens at all, only happens at one ride height and power input. And worse is if the rear end winds up on the springs with the nose going up and the angle going negative; then the angles add up and instead of cancelling the vibration, now they amplify it. That's bad news.
But again, on the jackstands, with the wheels off, and with no load applied,This is not likely to be a factor.
So IMO that kindof leaves just two options; driveshaft run-out and crosspin wear. And as mentioned, a worn out trans mount can allow the vibration into the body at the front, and worn-out eyebushings can allow it to enter the body at the rear.
Crosspin wear is usually caused by little to no pinion angle at all in the rear, which causes the needle rollers to hammer grooves into the pins. So the recommended minimum angle is one to two degrees to keep the bearings rolling and working the grease around. Crosspin endplay does not change much over the life of the joint, cuz that's where all the grease piles up. So depending on how you push/pull the driveshaft,a worn-out joint may not be felt. But as said, as soon as it's out of the saddles, you'll know.
Driveshaft run-out is easily measured, but the joints have to be known to be good first.
None of this,however, addresses how it could have been fine before the work began, and now, it's not.; except perhaps a rideheight change, or a spring swap....... I'm guessing.
