vibration
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Junior User
Joined: Sep 2009
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From: ca
vibration
hello,my truck ran really smooth ,before i lowered it 4'' with djm shackle and hangers now it vibrates only at the speed of 25-30 mph other than that it runs good ,any ideas,oh before i forget my truck had the flex o matic suspension 31'' instead of 28'' leaf springs,
Fleet Mechanic
Joined: Dec 2006
Posts: 1,338
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From: UPSTATE NY
its wierd how things only vibrate at a certain speed. but thats harmonics for ya. i had a similar problem at 50 mph and it ended up being my passenger side wheel bearings. they are extremely picky about how tight you have that center nut.
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Junior User
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From: ca
i didn't do anything else to the back,do you think it could be the leaf springs being too long,because the standard lenght is 28'',i might be maxxed out on the travel,the shackle is facing toward the rear more
Fleet Mechanic
Joined: Dec 2006
Posts: 1,338
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From: UPSTATE NY
oh i didnt realize you were talking about the rear. i saw djm and ibeams immediately popped into my brain. sounds like that new ride height for the rear would cause a different driveshaft angle and if those ujoints have seen better days or never seen grease you'd definitely get vibration. only real way to inspect them is remove the driveshaft and then make sure it has full smooth range of motion. takes like 5 minutes. if you have a long wheelbase then chances are you have a 2 piece driveshaft with a center support bearing. this center support or "carrier bearing" could also be causing you problems. another possiblility is poor pinion angle.
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Junior User
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From: ca
why would it start vibrating right after i lowered the rear,do you think its the ujoints,its a short bed so it only has 1 drive shaft,so you dont think it could be the leaf springs
Last edited by cervantes; Oct 24, 2009 at 09:05 PM. Reason: correction
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From: ca
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From: Kernersville, NC
Driveline angles are critical.
U-joints are designed to run in such a way that they have to compensate for angular rotation. Too much or too little angle can cause minor-to-severe vibration. Too little angle causes them to run nearly straight and too much will surpass the tolerances in which they were designed to run.
By lowering your truck, you have undoubtedly put the pinion yoke in more of a straight line between it and the output yoke on the tranny.
Get some shims to put between your springs and spring perches, tilt the front your rear axle down a couple of degrees so that the pinion angle is greater, and I'd almost bet lunch that your problem will be solved.
U-joints are designed to run in such a way that they have to compensate for angular rotation. Too much or too little angle can cause minor-to-severe vibration. Too little angle causes them to run nearly straight and too much will surpass the tolerances in which they were designed to run.
By lowering your truck, you have undoubtedly put the pinion yoke in more of a straight line between it and the output yoke on the tranny.
Get some shims to put between your springs and spring perches, tilt the front your rear axle down a couple of degrees so that the pinion angle is greater, and I'd almost bet lunch that your problem will be solved.
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Junior User
Joined: Sep 2009
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From: ca
where do you get shims and what kind (material),another question i lowered the front 4'' and the rear 5'' so its still higher in the back by 1'',thats the way it was before i lowered it
Last edited by cervantes; Oct 25, 2009 at 10:33 AM. Reason: add
Senior User
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From: Kernersville, NC
Most any heavy-duty truck alignment shop will have them. You may even pick them up at NAPA.
They will most likely be aluminum.
The relationship between the front of the truck and the rear has no effect on driveline angle. It's the relationship between the rear axle and the next u-joint (transmission or center bearing) that is important.
You could jack up the front of the truck a foot or more, and it wouldn't affect the driveline angles. Lower or raise the rear as little as 1/2" and you've made a change, although it would be very insignificant.
They will most likely be aluminum.
The relationship between the front of the truck and the rear has no effect on driveline angle. It's the relationship between the rear axle and the next u-joint (transmission or center bearing) that is important.
You could jack up the front of the truck a foot or more, and it wouldn't affect the driveline angles. Lower or raise the rear as little as 1/2" and you've made a change, although it would be very insignificant.
Freshman User
Joined: Jan 2009
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70_RangerXLT is right on the money. Pinion angle is critical. The U joints are probably under a bind. Lowering the suspension reduces length between the tailshaft and the pinion and changes the pinion angle in relation to the driveshaft. Shims will restore both.
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