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Discussion Starter · #1 ·
Ever sence i installed my new 9" i'm getting a shaking that starts around 50-55 mph, I've installed new rear tires & checked "runout" on the axles/drums, U joints are new, I was told it might be a driveshaft problem, but why after installing a rearend, Any ideas would be greatly appreciated.. Its a 78 cutlass G body.
 

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LJM97Z,

I had this happen one time. I installed a new U-joint on a Dodge van and it cam back with a vibration like yours. It turned out that I had not centered the caps on the U-Joint in the rear end yoke. I slid it over and the vibration was gone. It is a place to start.
Scholman
 

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Discussion Starter · #3 ·
Thanks guys, it is a new rearend (all new), just the houseing was fabbed up from a reputable shop in iowa, I'm still trying to determine where my pinion angle is at, The 3rd. member is -4, the driveshaft slopes upward 2*, I was told 2 different things on this, One shop said its at -6*, the other said -2*, I'm trying to cover all the basics, I have a good 1500.00 in this rearend, All the parts are from Quick Performance out of Iowa. The cars a 4 link control arms.

The one thing i have noticed is there is a 1/2" difference between the DS & PS front wheel well to the tires.
 

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Who made the driveshaft? and are the trunnions timed properly? Driveshaft angle sounds good. Check wheek balance again, all 4. Does it shake in the seat (Rear) or the steering wheel (front)?

I had a Mustang once checked everything, trunnions were turning in the driveshaft yokes. Had a new shaft made, perfect!
 

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Discussion Starter · #6 ·
The driveshaft is factory, so this may be the problem, its very possible the problem was there with the old rearend but just wasn't as noticable. I'm going to have the driveshaft checked 1st.
 

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Discussion Starter · #9 ·
Well i changed out the front tires/wheels with another set i had, replaced the inner/outer tie rods/drag link & steering box, front end is much tighter, also changed the rear tires/wheels, The shacking is still there, it is no better/worse, it happens around 60-65, I'm really puzzled on this, I do feel it in the seat though, The tranny mount is good, Is it at all possible that the transmission could be causing it? Like a bent spline, The only other thing is it has to be something in the rearend, i just don't know what.

I've also currected the pinion angle & alligned the rear tracking.

What else should i look for, could it be axle/pinion/carrier bearing issues, all this stuff is brand new.

One thing it does do, when i'm on the throttle hard its not that bad, but gets worse off the throttle when its at those speeds.

Just to clarify, Its not a "vibration", more of a shacking/wobbling.
 

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pinion angle

When getting on the throttle the rear usually squats, changing the pinion angle , Check pinion angle vs trans output shaft angle. adjust the pinion to get it correct.
A wrong length drive shaft from mixing parts ?, Go to inland empire drive shafts and check out their how to for drive shafts.
 

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Like timothale, said pinion angles change under acceleration and decel, are you using the factory 4 link or aftermarket links, ( you can buy adjustiable top links , to replace the factory ones . to change the pinion angle) what rear springs are you using , get the car sitting at the right ride height,
 

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Differential/Driveline
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Normally when you have a angle problem you will feel the shake during accel or decel. Have the D/L checked for straightness and balance and verify the rear joint is correct to the yoke.
 

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Since it's a GM G-Body, you might also check and be sure the 4 link bars are OK. This happened to my son's MCSS one time and it was a broken lower link that allowed the rear axle to move around...

Russ
 

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Discussion Starter · #14 ·
Thanks, I had the driveshaft checked, it was out, so they balanced it, I also re-adjusted the pinion angle for -2, the trans mount is good also, none of this changed the way the car reacts from before, I also installed non-adjustable upper arms from UMI, I then installed lower "adjustables" to currect the pinion agle & side too side tracking, still does the same thing, It really feels like a slightly bent driveshaft to me, but they showed me it on the machine as it was spinning with the dial indicator, The rear joint should be fine as it is new, its a combination joint, its sitting in the saddle good & snug from the looks of it.

I do know that the 9" sites off to one side on the pinion then the factory rear, could this cause a problem at a side angle? It still looks like the driveshaft is going fairly straight back in the tunnel.
 

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Discussion Starter · #16 ·
I fought a buzzing vibration like that on a Chevelle with narrowed 9". Turned out the big tires and wheels needed load balanced on a Hunter machine.
I run 2 sets of tires/rims with the same problem, so i'm thinking i need to rule that out. But who knows. Is this type of balancing done on like dyno machines??

275/60/15 Coopers & 26x11.5 MT ET Street bias ply.
 

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Thanks, I had the driveshaft checked, it was out, so they balanced it, I also re-adjusted the pinion angle for -2, the trans mount is good also, none of this changed the way the car reacts from before, I also installed non-adjustable upper arms from UMI, I then installed lower "adjustables" to currect the pinion agle & side too side tracking, still does the same thing, It really feels like a slightly bent driveshaft to me, but they showed me it on the machine as it was spinning with the dial indicator, The rear joint should be fine as it is new, its a combination joint, its sitting in the saddle good & snug from the looks of it.

I do know that the 9" sites off to one side on the pinion then the factory rear, could this cause a problem at a side angle? It still looks like the driveshaft is going fairly straight back in the tunnel.
Where on the driveline did they show you it was straight? On the ends? or? Do they have a balancer to fine tune it?
 

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side angle

Spicer, U joints have a web site that has charts on how to compute the actual working angle, using the horizontal and verticle off sets. up and down and side to side. Normally the trans output-crank shaft is down and the pinion angle should be UP, both angles paralled at load position. If your suspension components have a large difference in length the pinion angle will change a corresponding larger arc swing. The 2003 Ford GT, modern version of the old GT40, has the rear half shafts running at an angle. It looks strange when I was used to looking at a Jag or Corvette rear suspension. We had links a few months ago when someone else was having drive shaft problems
 

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I run 2 sets of tires/rims with the same problem, so i'm thinking i need to rule that out. But who knows. Is this type of balancing done on like dyno machines??

275/60/15 Coopers & 26x11.5 MT ET Street bias ply.
I could not get it to vibrate off the ground. Finally stumbles into a tire store that does a lot of big drag tire stuff and he tole me I needed an experienced operator to force balance them. Here is a link for locating the nearest shop.
Hunter GSP9700 Wheel vibration Control System solves wheel vibration and tire pull problems that balancers and aligners can't fix
 
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