Bad Vibration
Thread Starter
Launching!
Joined: Mar 2012
Posts: 299
Likes: 21
From: Phoenix, AZ
Bad Vibration Im getting a really bad vibration when Im accelerating in the 65+ mph range and it seems to go away when I let off the gas, or when Im just cruising. But say Im on the freeway doing normal freeway speeds and I punch it to pass, the vibration gets so bad that I have to let off.
The vibration doesnt feel like a typical drive shaft buzzzzz, but more of a womp womp womp that shakes the body of the car. The vibration seems slower than a driveshaft and it reverberates through the body.
Ive done some work to try and narrow it down:
Replaced the tailhousing of the 4l80e.
Balanced the driveshaft.
Pinion angles are not great, but I dont think they would cause this sort of vibration.
Pinion angle engine is 4 degrees down measured from the crank pulley, driveshaft is 2 degrees heading down towards the pinion, and the pinion is up 4 degrees.
New wheels and tires (needed them anyway)
I jacked up the rear today and ran it to see if I noticed the axles wobbling or anything strange coming from the rear end. Everything looks normal. HOWEVER, I did feel that same sort of vibration (but to a lesser extent) with the rear tires in the air. So Im guessing its safe to assume that the problem is somewhere forward of the rear end.
I did notice there is a slight wobble coming from the flexplate and torque converter. But its hard to say if its normal or not?
Im at a loss as to what could be causing this. I have a hunch it has something to do with the torque converter/flexplate, or even transmission.
Any suggestions?
The vibration doesnt feel like a typical drive shaft buzzzzz, but more of a womp womp womp that shakes the body of the car. The vibration seems slower than a driveshaft and it reverberates through the body.
Ive done some work to try and narrow it down:
Replaced the tailhousing of the 4l80e.
Balanced the driveshaft.
Pinion angles are not great, but I dont think they would cause this sort of vibration.
Pinion angle engine is 4 degrees down measured from the crank pulley, driveshaft is 2 degrees heading down towards the pinion, and the pinion is up 4 degrees.
New wheels and tires (needed them anyway)
I jacked up the rear today and ran it to see if I noticed the axles wobbling or anything strange coming from the rear end. Everything looks normal. HOWEVER, I did feel that same sort of vibration (but to a lesser extent) with the rear tires in the air. So Im guessing its safe to assume that the problem is somewhere forward of the rear end.
I did notice there is a slight wobble coming from the flexplate and torque converter. But its hard to say if its normal or not?
Im at a loss as to what could be causing this. I have a hunch it has something to do with the torque converter/flexplate, or even transmission.
Any suggestions?
Without knowing rear tires and axle ratio, I'd assume you're probably 2000 rpm at 65. Your driveshaft is spinning at 1500 rpm (.75:1 OD); the axles, however, are at ~400 rpm, which is 6 Hz (revs/second). That sounds close to what you described. I would suspect something within the rear end itself, especially since it manifests itself with the rear tires off the ground.
Keep in mind, though, that the pinion rotates up under acceleration. Depending on suspension type and condition, this could be by several degrees, resulting in mismatched angle and high-speed vibration. And your measurements could be off depending on what you used to make them (I like Harbor Freight's digital angle gauge).
Where are you located in the valley? I've troubleshot bad vibrations and might be able to give you a second opinion. Shoot me a PM.
Keep in mind, though, that the pinion rotates up under acceleration. Depending on suspension type and condition, this could be by several degrees, resulting in mismatched angle and high-speed vibration. And your measurements could be off depending on what you used to make them (I like Harbor Freight's digital angle gauge).
Where are you located in the valley? I've troubleshot bad vibrations and might be able to give you a second opinion. Shoot me a PM.
TECH Senior Member
Joined: Jun 2016
Posts: 22,293
Likes: 3,616
From: Central Cal.
Without knowing rear tires and axle ratio, I'd assume you're probably 2000 rpm at 65. Your driveshaft is spinning at 1500 rpm (.75:1 OD); the axles, however, are at ~400 rpm, which is 6 Hz (revs/second). That sounds close to what you described. I would suspect something within the rear end itself, especially since it manifests itself with the rear tires off the ground.
If the engine is at 1500 RPM, the driveshaft is spinning at around 2000 RPM. The engine spins at .75 of driveshaft RPM.
TECH Regular
Joined: Feb 2002
Posts: 426
Likes: 45
From: ohio
If this only happens in acceleration, I would suspect driveshaft angles. If it happens during steady speed it could be bent driveshaft and angles as well. If it’s the Flexplate it will change when and if you can downshift the engine, while the driveshaft speed will remain constant.
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I agree. What sort of suspension work have you done, everything upgraded or are there 50+ year old bushing present?
Last edited by Pop N Wood; Jan 14, 2020 at 03:55 PM.
Thread Starter
Launching!
Joined: Mar 2012
Posts: 299
Likes: 21
From: Phoenix, AZ
I messed with the pinion angle a little bit the other night and it seems a little better. I was out of town this weekend and didnt have much time to test on it. I definitely still feel some vibration, but not as bad as before.
Thread Starter
Launching!
Joined: Mar 2012
Posts: 299
Likes: 21
From: Phoenix, AZ
Rear suspension is all new UMI stuff.
Front suspension is stock.
Banned
Joined: Feb 2013
Posts: 3,540
Likes: 207
Since the vibration is noticeable on acceleration only I would try adjusting the front of your pinion downward a couple of degrees and go for a test drive if your UMI rear suspension allows for easy adjustments. GM assembled the A-bodies, and almost all their other cars, with a few degrees of pinion tip-in like this from the factory.
Thread Starter
Launching!
Joined: Mar 2012
Posts: 299
Likes: 21
From: Phoenix, AZ
I took it for about a 300 mile round trip and drag raced it in the Zip Tie Drags and the vibes are still there. I noticed a few things:
I notice vibes at even lower speeds, maybe around 40-50 mph, but its barely noticeable.
The vibes are the worst at part throttle around 65mph.
It gets worse if I slowly increase speed but actually gets better the faster I go. I finally was able to get it out on the open interstate and went 90 mph and while the vibes were still there, they are not as bad as going part throttle at 65mph.
I did a shakedown pass at the strip not going all out because I was worried about the vibes and finished the 1/4 mile at 85mph and I did not notice vibrations AT ALL.
I notice vibes at even lower speeds, maybe around 40-50 mph, but its barely noticeable.
The vibes are the worst at part throttle around 65mph.
It gets worse if I slowly increase speed but actually gets better the faster I go. I finally was able to get it out on the open interstate and went 90 mph and while the vibes were still there, they are not as bad as going part throttle at 65mph.
I did a shakedown pass at the strip not going all out because I was worried about the vibes and finished the 1/4 mile at 85mph and I did not notice vibrations AT ALL.
On The Tree
Joined: Nov 2013
Posts: 106
Likes: 3
From: Omaha, NE
I will agree with what was said earlier. A body cars are difficult with vibrations. My 71 Cutlass had vibration starting at 45mph or so that would increase in intensity with speed. I am lowered 2" all around making the appropriate angles even more difficult to achieve. After adjusting pinion angles a dozen times and having my driveshaft balanced by 2 different shops the solution to my problem was a new driveshaft from The Driveshaft Shop with a front CV joint. Haven't had a reason to take the car over 100 yet but smooth as glass up to that point now. It's a tough $800 pill to swallow but after paying the money I was finally able to enjoy driving the car.
for me it always turns out to be the tires.
i build my own drive shafts and rarely need a balance weight
I set my driveline angles with a HF digital POS angle finder
I also offset my own steelies, cut em down and widen em and stuff.
but its always the tires. I have 100% given up on getting places to balance them right and just bought my own tire machines.
i build my own drive shafts and rarely need a balance weight
I set my driveline angles with a HF digital POS angle finder
I also offset my own steelies, cut em down and widen em and stuff.
but its always the tires. I have 100% given up on getting places to balance them right and just bought my own tire machines.
Thread Starter
Launching!
Joined: Mar 2012
Posts: 299
Likes: 21
From: Phoenix, AZ
for me it always turns out to be the tires.
i build my own drive shafts and rarely need a balance weight
I set my driveline angles with a HF digital POS angle finder
I also offset my own steelies, cut em down and widen em and stuff.
but its always the tires. I have 100% given up on getting places to balance them right and just bought my own tire machines.
i build my own drive shafts and rarely need a balance weight
I set my driveline angles with a HF digital POS angle finder
I also offset my own steelies, cut em down and widen em and stuff.
but its always the tires. I have 100% given up on getting places to balance them right and just bought my own tire machines.