#1072187 - 10/06/21 09:15 PM
Re: Shift rear end right or left ?
[Re: Keithss]
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You do it with just the uppers, if you shorten one and lengthen the other the same # of turns, the rear end will shift but your pinion angle won't change.
Lance 1985 Monte Carlo SS Street Car
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#1072188 - 10/06/21 09:16 PM
Re: Shift rear end right or left ?
[Re: Keithss]
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Left to right shouldn't cause a vibration, but the pinion angle not correct certainly will...
Lance 1985 Monte Carlo SS Street Car
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#1072189 - 10/06/21 09:17 PM
Re: Shift rear end right or left ?
[Re: Keithss]
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You adjust the length of the lowers to set the wheelbase, or to make sure the car is tracking straight.
Lance 1985 Monte Carlo SS Street Car
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#1072193 - 10/07/21 06:19 AM
Re: Shift rear end right or left ?
[Re: Keithss]
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mmc427ss
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Measuring as you did, frame to center of the shaft, should read 3/4" to 1". The pinion centerline is offset in the rear. The best way to check for placement of the rear side to side is measuring the distance between the frame and the tire at the back side corner of the tire. Compare the left and right measurements. To confirm those numbers a plumb bob hanging off the 1/4 panel, on the centerline of the axle, a tape measure from that plane to the center of the axle, will tell offset. Actually a very easy and quick method. Two pieces of string, two 1/2" nuts, two pieces of masking tape, and a tape measure. Just need to have the car sitting on a level area. Tape the string to the 1/4 panel on each side after finding the centerlines, now you can easily jump back and forth with accurate numbers.
If you do both methods don't be surprised if they are not identical. Deviations of body panels, frame, LCA bracket position, and mileage or abuse all play a factor in the numbers.
The real goal is to center the tires in the wells and not rub the frame or the 1/4 lips. Very few G's are perfect from the factory. My 86 the rear is offset 1/8" to the left, so a 1/16" correction would need to be made.
If having adjustable LCA and UCA there is a lot of setup that can be done. First I'd assure the LCAs are exactly the same length. Next i would assure the UCA are exactly the same length. Then I would adjust the uppers to set pinion angle and maintain exactly the same length each side. Then I would have the car for a 4 wheel alignment rack, now the LCAs are adjusted to set the thrust angle as close to zero as possible. Now that the thrust, pinion angle, and UCA are the same length the centering of the rear can happen. First confirm how much offset via measuring. Mark the UCA with a paint marker first will making adjusting easier, you know where your starting point is now. Shorten and lengthen each arm equally. It's easy to lose track of how much, this reduces the effect on pinion angle. Some simple math thinks it will take 2 turn each side to move the rear 1/16" . As a final step double check and adj pinion angle, equal direction each side.
The rear suspension design is a converging 4 link with equal length pairs of arms. The LCAs may now have different length when the thrust angle was set, probably minimal, but this is ok, thrust angle is important. Now if the UCAs get to the point of one being much longer after adjusting for offset than the other this effects thrust angle at launch. The result could be the car driving to the left of right under hard acceleration. For this reason I feel centering the rear using the UCA may have ill effects.
One other consideration with that Moser, does it have multiple positions on the rear for LCA adjustment there? If so before alignment the hole to use will need to be determine. That adj can work for you or against you for instant center location. And one other thing about a Moser the UCA location on the rear is higher that a stock rear, this effects IC. Again dependent on the purpose for the car, ride height, both IC and anti-squat effect the way a car drives.
HTH Bob
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#1072226 - 10/11/21 01:33 PM
Re: Shift rear end right or left ?
[Re: Keithss]
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So personally, I would raise the rear pinion angle to up 2.5* so that it is parallel with the Trans output. This way the 2 u-joints operate at the exact same angle and speed. 100% this is where your vibration is coming from.
Lance 1985 Monte Carlo SS Street Car
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#1072227 - 10/12/21 03:11 AM
Re: Shift rear end right or left ?
[Re: Keithss]
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mmc427ss
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Ditto, agree. I run 3 1/2 engine down, 2 1/2 degrees pinion up. Depending on what type of control arm bushings are in the arms will vary this setting, rubber, poly or solid because of deflection. Pinion angle changes under acceleration, rubber the most, solid very little. A stock G comes with pinion up, engine down. Generally engine angle will be in the neighborhood of 3 1/2 degrees on a streetable G
One thing to remember when a 12 bolt is used. From what i understand the ears on a Moser 12 bolt are higher than a 7 1/2 or 8 1/2" stock rear. When an out of the box adjustable G-body UCA is bought they are, well supposed to be, the correct length for the G body. A 12 bolt would require a different length. Could be why you are seeing that pinion down measurement. But I think the uppers have already been adjusted several times already, Are they still the same out of the box length?
Also if the ears on the Moser rear are higher this effect instant center. In some instances a good thing acceleration, some instances not so good, braking.
I wouldn't be concerned about the rear offset of 1/8" side to side using the 1/4 panel method. Use the UCA and LCA adjustments just to set thrust and pinion angle keeping the pairs of arms as close to equal length as possibly.
Your tire/wheel combo will dictate if and where the tire will touch the frame or touch the 1/4 panel lips. If you have a moderately sized tire in the rear that touches on one side then maybe tweaking the centering of the rear to reduce the touch could be done. Bob
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#1072237 - 10/12/21 10:06 PM
Re: Shift rear end right or left ?
[Re: Keithss]
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Yes, I'm betting it will run smoother at highway speeds for sure...
Lance 1985 Monte Carlo SS Street Car
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#1072254 - 10/15/21 03:51 AM
Re: Shift rear end right or left ?
[Re: Keithss]
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Do you still run a stock diameter driveshaft?
I couldn't eliminate my highway speed vibration completely until I got a 3.5" diameter driveshaft.
Lance 1985 Monte Carlo SS Street Car
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#1072257 - 10/15/21 02:11 PM
Re: Shift rear end right or left ?
[Re: Keithss]
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A 3" diameter one did not stop my vibration, bit the 3.5" diameter one did.
Lance 1985 Monte Carlo SS Street Car
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