Shift rear end right or left ?

Posted By: Keithss

Shift rear end right or left ? - 10/06/21 08:32 PM

I see that my rear end is off center by about 1 inch, need to shift it towards the drivers side 1/2 inch.
I measured from the frame alignment holes, to the center of the drive shaft.

I have a Moser 12 bolt with adjustable upper and lower control arms.
Can it be moved left with the control arms?
If so how? I am thinking maybe to loosen (shorten) both the upper and lower on one side, and tighten (lengthen) equal amount on the opposite side.
The reason I want to adjust it is that I think it is causing a vibration at the drive shaft, or drive train.
Any ideas.
Posted By: SSLance

Re: Shift rear end right or left ? - 10/06/21 09:15 PM

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.
Posted By: SSLance

Re: Shift rear end right or left ? - 10/06/21 09:16 PM

Left to right shouldn't cause a vibration, but the pinion angle not correct certainly will...
Posted By: SSLance

Re: Shift rear end right or left ? - 10/06/21 09:17 PM

You adjust the length of the lowers to set the wheelbase, or to make sure the car is tracking straight.
Posted By: mmc427ss

Re: Shift rear end right or left ? - 10/07/21 06:19 AM

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.

Posted By: Keithss

Re: Shift rear end right or left ? - 10/08/21 04:18 PM

Well this is how it ended up
Had the drive shaft balance checked Good
Axle centered within the wheel wells, based on the quarter panels within less than 1/8 inch. Center of pinion about 1 inch to the passengers side.

Added 1 inch under the trans mount. G-Force mount ( has a lot of flex) That is as high as I can go without modifying the tunnel.

Angle off the front of the harmonic balancer down 2.7 deg.
Angle off the front u joint down 2.5 deg.
angle at the front of drive shaft down 1.6 deg
angle at the rear of the drive shaft down 1.7 deg
angle at the rear pinion down 1.7 deg.

all of these measurements could be off by as much as .2 deg.

Have not road tested to see if the vibration has gone away or reduces any.
Live on a dirt road and been raining for a few days, must wait until is quits and dyes out .

Posted By: SSLance

Re: Shift rear end right or left ? - 10/11/21 01:33 PM

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.
Posted By: mmc427ss

Re: Shift rear end right or left ? - 10/12/21 03:11 AM

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.
Posted By: Keithss

Re: Shift rear end right or left ? - 10/12/21 08:11 PM

Did a little more adjusting today. I think I have it a lot more better.
I did put a 3/4 inch spacer under the trans mount.

Engine down 2.9 deg measured off the harmonic balancer.

Trans down 2.8 deg measured off the u-joint seat .

Drive shaft at the trans and pinion 2.8 to 2.9 deg. down

Pinon 2.7 deg up measured off the u-joint seat.

I do have adjustable UCA and LCA. The upper are screwed all the way in to the shortest possible.

Still not road tested yet.

Does this look more better to you guys?
Posted By: SSLance

Re: Shift rear end right or left ? - 10/12/21 10:06 PM

Yes, I'm betting it will run smoother at highway speeds for sure...
Posted By: mmc427ss

Re: Shift rear end right or left ? - 10/12/21 10:33 PM

Here's what Dana (Spicer ) has to say about driveline angles. Page 5 is the important info. I consider this the bible on driveline info. The old pdf version of this seems to have disappeared, but his newer version seems to saythe same thing.

Drag racers, roundy-round racers don't drive on the streets and have a different idea what works for them.

Try the setup in the car now and see how it works for you. If still not happy I'd remove the shim under the trans mount to bring the engine trans back to close to 3-3-1/2 degrees and leave the rear at 2 1/2. Drive it an see how that works.

Under hard acceleration the pinion will move upward, in the case with the pinion now, UP at 2.7 degrees, it would exceed the 2.8 of the engine/trans angle. Pinion angle will change anywhere from maybe 1/2 degrees to as much as 2 or so with rubber bushings.
Posted By: Keithss

Re: Shift rear end right or left ? - 10/14/21 10:59 PM

Well today I got the car out for a test drive.
BIG improvement. There is still a vibration when I get up to around 65 to 70.
Next I will have the tires balance an make sure this is not where the vibration is coming from.

On another topic, to remove the timing cover, how far do I need to drop the oil pan? Do I need to lift the motor?
got an oil leak.

Posted By: SSLance

Re: Shift rear end right or left ? - 10/15/21 03:51 AM

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.
Posted By: Keithss

Re: Shift rear end right or left ? - 10/15/21 09:56 AM

I had a new drive shaft made when I put in the Moser 12 bolt, and switched to 1350 u joints.
Right now I do not remember what the diameter is, but I know it is bigger than the stock one.
Posted By: SSLance

Re: Shift rear end right or left ? - 10/15/21 02:11 PM

A 3" diameter one did not stop my vibration, bit the 3.5" diameter one did.
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