Delete the rear offset? Getting a 9 inch

smed7880

Greasemonkey
Jul 27, 2010
132
171
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Why did GM offset the rear axle housing in the g body? What purpose?

If doing a new rear 9 inch housing should the offset be deleted?

If offset deleted will the upper control arm mounting be off? Im getting adjustable ones, but want to make sure im not going to create an unresolvable issue.

Ls2 4l60 holley blackheart mounts etc

TIA
 
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Built6spdMCSS

Geezer
Jun 15, 2012
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I have a 9", installed square to the frame, still offset to one side. Double Adjustable Uppers too. Car tracks fine, maybe offset for driver weight. If you look close the engine is offset to the passenger side.
 
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81cutlass

Comic Book Super Hero
Feb 16, 2009
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My understanding is the only reason the rear end on G bodies is offset relative to the frame is poor factory tolerances (it didn't matter so GM cut costs and let it float back and forth).

Adjustable upper control arms allows you to center it.

On a 9" the driveshaft will not sit center in the tunnel however because the pinion doesn't sit in the middle of the housing. Are you maybe getting confused with that? I know some people will put a slightly longer/shorter axle in one side to put the driveshaft pinion in the center of the tunnel, you just end up putting the brackets offset relative to the pumpkin.
 
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smed7880

Greasemonkey
Jul 27, 2010
132
171
43
Will be a 9inch g body housing.

Thanks!
 

Built6spdMCSS

Geezer
Jun 15, 2012
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I know some people will put a slightly longer/shorter axle in one side to put the driveshaft pinion in the center of the tunnel, you just end up putting the brackets offset relative to the pumpkin.
9" already has one Axle longer than the other...
 
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Rt Jam

G-Body Guru
Mar 30, 2020
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Ontario Canada
Adjustable upper control arms are not for centering a rear housing. Use only for adjusting pinion angle. I would only do this on a panhard set up a tiny bit.
The 2 pairs of arms in a triangular 4 link should have 2 equal length uppers and 2 equal length lower.

I F you chose to have a driveshaft parallel to the centerline of the car. The offset should be built into the housing. Place the yoke where it needs to be and the housing will be equal in every way on both sides.
 
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Built6spdMCSS

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Having the driveshaft in the center or off to right will absolutely not affect anything. Money spent solely just to do that would be a waste. Literally would be negligible...

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Built6spdMCSS

Geezer
Jun 15, 2012
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I F you chose to have a driveshaft parallel to the centerline of the car. The offset should be built into the housing. Place the yoke where it needs to be and the housing will be equal in every way on both sides.
I'd like to see how you figure that with how a 9" is made...
 

81cutlass

Comic Book Super Hero
Feb 16, 2009
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Yeah, a 9" often does and can have different length axles side to side but depending if you want the pinion in the center of the tunnel or the pumpkin in the center of the tunnel (depending on if it's a custom or off the shelf) determines the length of the axles. If the pinion is (kinda) centered, with an offset pumpkin, as is the case of most aftermarket 9" housings, quick, TRZ, speedway, etc., the brackets are biased to one side of the housing more than the other. Just buy whatever housing you like, and wherever the pinion lines up is fine.

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But like I said I assume you are talking wheel mount flange to frame relative to each side. You want the same gap between the wheel and the frame on the driers and passenger side. If the wheel mount flange is not centered relative to the frame, the tire will hit the frame on one side and not the other.

As far as using the upper control arms to center the housing, yeah, adjustable links are primarily to adjust pinion angle. If you make the bars slightly different lengths it can help put the housing where you want it left to right. Or lower arms too if they are adjustable.

If you are hard core drag racing and yanking the front wheels off the ground for 200ft, and worried about putting rear steer in the car so it launches straight yes, the upper bars should be the same length. Slight lengths in upper bars can make the car push right or left. Hard core chassis guys will have really long calipers to dial adjustable arm length in PERFECT because it can matter that much. That is for 9 second or faster stuff.

Personal experience, I have my upper control arms set up to be different length and an running bottom 11's with a stick shift car and don't give 2 craps about rear steer. I don't yank front wheels of the ground. I put my housing in the center of the chassis, and run 15x10's with 275 drag radials. It doesn't cause any problems. The arms might be 1/4" different in length.

No need to over think it, buy an off the shelf housing, buy your wheels. If the wheels clear the frame on one side and don't clear the other side, get some adjustable upper arms and set pinion angle and adjust the uppers relative to each other so both wheels clear the frame.


I can't find the thread but I remember there being a topic on the forum here where GM had purposely made different length hole-to-hole upper control arms for G bodies from the factory. I'd give someone a virtual high five if they can find that thread with the parts book. Even GM played with different length arms to get the housing centered if it was really bad.
 
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Built6spdMCSS

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Jun 15, 2012
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Well the above answers my post here..
I'd like to see how you figure that with how a 9" is made...
I just bought the normal Gbody one, slapped it in and been driving. Street car stuff I guess.
 
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