Centering the rear end and setting pinion angle

[jordan]

I am putting in an XV level 1 suspension kit in the rear.  I am modifying things by using a Dr.Diff 1" spring relocator bracket and shackles.  I have cut the stock rear perches off and installed the rear leafs and Dr.Diff kit.  I set the rear assembly on new perches and slightly tightened 1 u-bolt on each side so I am able to move it yet it stays in place. I dropped a couple of plum bobs at the outer quarter sheet metal, and at the outside of the rear frame rail; towards the  wheel well.  When I measure these distances,  they are different.  I know the quarters have been replace at some time because the car was rear ended.  So I am not confident using these as my references to make measurements.  Should I use the leaf springs as my reference, since these wont change?  If not, what is the reference to measure the equal distance from and to the outside axle housing flanges?  Am I suppose to line the pinion inline with the trans output shaft?  The engine and pinion are offset to the right, correct?
     
Does the car have to be level to start making driveline measurements from the trans to the pinion?  I measured the flat face on my tailshaft of my 4l60e (not 727) and it shows 85* with the arrow left of the 90* mark.  Does this mean it is up 5*??  Then I need to measure on the yolk face of my pinion, what degree do I need to weld the axle assembly to the perches?  If the pinion needs to point down, then should the needle of my angle finder point to the left or the right of the 90* mark?  My car is on a lift.  Do I need to lower it and level it to the rocker panels to make measurements?

I know I really need to get this right.  I dont want vibrations, and I want to stuff some 295 series tires under there on 9.5" rims x 18" or 19" diameter.  I dont have the rims yet.  Do I need to get them first?  Do I need tires on them too?  I was hoping to get this set up before I order wheels because I cant afford to screw up on wheel backspacing too.  Im in a bit over my head, but am sure many of you have been there, done that.  I can use some guidance and feel confident I can make this work, but I sure need some help from the CC community.  Thanks 
 

[jordan]

      Bueller?                                 Bueller?                                                                           Bueller?

[Chryco Psycho]

It is somewhat difficult to help you , if the needle si left of 90* we still need to know which side of the car you are looking from , My guess is the trans is 5* down at the rear to keep the driveshaft speed even you need to match the angle with the pinion 5* down also . The suspension needs to be fully loaded as the angle of the spring will change as the diff rises .As far as centering the diff I was very careful to center mine perfectly going from the frame & suspension & I was still close on one side to the body as virtually all are,  you could center off the body panels instead

[jordan]

Thanks Chryco.  I was measuring from the drivers side looking at the passenger side.  It makes sense now after conceptualizing it that the trans is down 5*.  I think I am going to measure from the springs to the axle housing flanges because the springs wont change side to side.  I know I may have some space to the body greater on one side vs the other.  I will get tires that fit the smaller side.  I now know I need to lower the car and rest the weight on the springs before I measure for the pinion angle. 5* down on the pinion is the opposite of the trans, correct?, since the pinion is facing the opposite direction of the trans?  For example, if the trans reading is 85*, then the pinion reading should be 95* if the angle finder is orientated in the same way?  I did some reading in the tire fitment sticky that these cars like the pinion angle down 5* more than the trans so when the car is under torque (acceleration) that the pinion rises to an equal angle of the trans.  So do I set the angle the equal but opposite as the trans, or do I add a little extra down angle so that it rises under acceleration to the correct angle?  Am I over thinking this?   I am heading out to the garage now and am gonna try to get some work done. 

[Chryco Psycho]

Check out the video link just above this post it will illustrate the driveline angles , yes the pinion angle wil climb with load

[jordan]

I watched the video recently, and that is what has me a bit worked up about getting this really right on.  I dont want any crazy vibrations when I get it done.  Its too much work to do it wrong and have to do a "do over" again this winter.  I shouldnt expect to be so lucky.  I know the cars came from the factory without tight tolerances, but that probably what made them sort of crappy drivers.  Here I go, wish me luck.   Thanks

[jordan]

I got it centered to the leaf springs.  Same distance to the end of the axle tube.  Then I leveled the car at the door sills.  I measured the angle of the tailshaft housing at 3* down.  I then rotated the pinion until it read 5* down.  Does this look right?  If so, I will tighten the u bolts a bit and raise the car and tack weld the perches to the axle housing.   If not, what should I rotate the pinion angle to?  Here are a couple of pictures.  The first one is the pinion.  The second is the trans tailshaft.  It seems like I can rotate the pinion up a few more degrees.  Opinions?  Thanks for looking. 

[Chryco Psycho]

Hard to be sure exactly how much the diff will rise or how soft the springs are but if you can drive the car tack it at 5 down or raise it to 4 down & tack it there , it should not vibrate

[jordan]

The springs are supposed to be softer than stock, but have a different arch to keep them from winding up.  I cannot predict how much the pinion will rise.  I put a monster stroker with killer torque in.  I am sure that will have an effect on the rear springs!  I am planning on putting on a pinion snubber to help too.  The car is not drivable yet.  I need to finish the rear rebuild and try to get it in in the next couple of weeks.  Rims and tires come next!  Lots of questions coming up on that soon.  Thanks for the help Neil.  This was my first attempt at a rear end regeometry and your support sure did help.   

[Chryco Psycho]

 

[cuda346pk]

I know this is an old thread but I didn't take pics of my gauge readings so I hope no-one minds me hijacking this one a little bit. I used the Firm Feel lowering hangers on the front of my leaf springs. I have some vibration so checking pinion angles I used same gauge and had my car sitting at its ride angle with jack stands under leaf spring perches and just behind lower ball joints with tires all at 7" off the ground. Measuring at the flat of the yoke I get 6* up. Driveshaft is level or 0* and flat end of u-joint at tailshaft measures 4* down. Does this mean I would need 10* wedges on the backside of the rearend? Since the biggest I am seeing is 4*i am guessing I will have to redo the perches. Dang it, if I had known that was gonna be the case I would have gone ahead and used Dr Diffs relocation shackles and hangers instead of the firm feel heavy duty ones.

[Chryco Psycho]

2* should get you close but I would try to shim up the rear fo the trans as well then use a 4 * shim between the springs & diff

[cuda346pk]

Ok. I thought about it last night and figured it had to be a 2* and I was thinking something wrong. Just for knowledge on my part why would you shim the trans to be 2* down as opposed to 4. I am guessing to reduce the angle the u joints see. I guess an added plus to raising the re ear of the tranny is it will at least slightly raise the exhaust for a little more ground clearance at the collectors and just behind. Good to know I was complicating it and don't have to re do the perches. As always thanks CP. I will get my parts order together and hopefully be ready for another test drive next week.

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[PlumCrazyChris]

The springs are supposed to be softer than stock, but have a different arch to keep them from winding up.  I cannot predict how much the pinion will rise.  I put a monster stroker with killer torque in.  I am sure that will have an effect on the rear springs!  I am planning on putting on a pinion snubber to help too.  The car is not drivable yet.  I need to finish the rear rebuild and try to get it in in the next couple of weeks.  Rims and tires come next!  Lots of questions coming up on that soon.  Thanks for the help Neil.  This was my first attempt at a rear end regeometry and your support sure did help.   

Hey Jordan,  I'm doing the same thing with my car.  I used the Dr. Diff relocate kit and need to weld on my perches too. 

So did you end up just matching the downward angle of the trans output to the pinion on the diff?  If so, how did that work out, any vibrations?

[cuda346pk]

Ok CP, I rechecked my angles and wearing my old man glasses it looks like both angles are actually 5*. Summit did offer a 6* so I got it and a 4* so I had both on hand. I have to replace the seal and bushing in extension housing so I will be pulling shaft and seal once I get the 2 special tools and parts and I will check everything again then as I will be able to get my gauge on the actual end of the extension housing and the face of the yoke u-joint mounts. Question is will I be better going with the 6* so I can point my pinion down by 1* and set my trans at 1* down or even 0* to accommodate the rise of the pinion? I have a box of parts to return to Summit so I figured it was best to get the extra set of wedge shims so I didn't get held up, then I will return everything at once.