Vibration when accelerating

rcmaniac791

rcmaniac791

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So I just put new leaf springs on my Fifth Avenue because the old ones were sagging bad. I've driven it a bit and at speeds of 40 mph and above there is a minor vibration coming from the rear end only when accelerating. There is a slight increase in the drive shaft angle. At speeds under 40 mph, the thing drives completely normal. I don't know if it's the u-joint or what. I'm planning on getting under there soon and inspecting it, maybe try to shoot some grease in it and see what happens. Any opinions would be appreciated.
 
There are several possible problems, but it's almost certainly the U-joints. Checking them accurately means either having both rear wheels in the air with the transmission in neutral or removing the U-joint straps at the rear axle. U-joints can seem fine while the car's on its wheels or when the driveshaft is under tension, even when all the needle bearings are gone. Adding grease to a bad U-joint won't fix it, nor will a good joint with no grease cause an issue (at least until it's gone bad--which doesn't take long).

My next guess would be excessive pinion angle. Did you replace the iso-clamp insulators?
 
When I check it i'll put the whole rear axle on jack stands. I did replace the insulators. I actually replaced all of the hardware that wen along with the leaf springs. I did go from a 4 leaf to a 5 leaf. I will know better once I crawl under there tomorrow. Got to love working on cars, it seems that once you fix one thing, something else goes! o_O
 
I second the u-joint theory. One going tight or the needles have worn little divots into the races usually. Loose would normally tend to vibrate at cruise also.
If the back of your car is higher than it's been in a long time, the u-joints are running at a different angle than they're "used to" and now the wear pattern can cause a problem, or magnify a minor problem from before. Seems 90% of the time, it's the rear joint that goes bad.
 
I second the u-joint theory. One going tight or the needles have worn little divots into the races usually. Loose would normally tend to vibrate at cruise also.
If the back of your car is higher than it's been in a long time, the u-joints are running at a different angle than they're "used to" and now the wear pattern can cause a problem, or magnify a minor problem from before. Seems 90% of the time, it's the rear joint that goes bad.
 
rcmaniac791 said:
When I check it i'll put the whole rear axle on jack stands.
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Actually, you'll want the frame on the jackstands and the rear axle hanging free. With the axle on jackstands, you're compressing the drivetrain and tightening the joints.
 
Good point. I'll make sure that I jack it properly.
I drove it today, and it seems that there's only a slight rumble between 40 and 45 mph now. Beyond that speed, It feels completely normal.
 
Ever heard of the calm before the storm? :D

shitstorm.jpg
 
NoCar340 said:
Ever heard of the calm before the storm? :D

View attachment 12841
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Agreed 100%
It could be a few different things but it could be a u joint. Snapping a u joint at anything past a crawl is not good. Things don't often fix themselves so consider this as NoCar implied....the calm before the shitstorm. Get that puppy in the air and have a look. Safety first. You don't want to even think about a front u joint letting go on you. Especially at any kind of speed.
 
Just finished checking both the front and rear u joints. They are both smooth as glass and there's no play in them at all. The one thing that I did find is that when the rear tires were off the ground that I was able to pull the wheels in and out a very small amount, like maybe a 16th of an inch, but it did thud. The rear wheels would move in and out, but the differential did not move. Not too sure what that is, but I'm relieved that my u joints are solid.
 
The end play on the axle shafts is totally normal and nothing to worry about. If it was up and down,,,,,,,,,,,,,then you'd have a problem.
 
While you were under there, did you check the pinion or transmission yoke for up-and-down play? If not, you can do that when you have it back in the air to check for a smoked transmission mount. :D
 
I didn't check it directly, but as far as I could tell, everything seems solid. I have no idea what the vibration is, but it seems that the more that I drive it, the better it gets. The family drove it to dinner tonight, drove like a champ.
 
If it were me, I would just keep an eye on it (so to speak) and see if it does indeed cure its self or gets worse. Maybe its just as mentioned before and is wearing in again. you did say it has and increased pinion angle. I had this problem when I put 3/4 ton springs on a 1/2 ton truck. After about a tank of fuel, it drove fine for 10k miles before I sold it.
 
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