UrS4

General question.

I have a lowered 06 GT with 1-piece Alu DS. I think my on car adjustable rear LCAs are wearing out.

What is the pinion angle supposed to be and how does one measure it?

Right now, I could adjust the pinion angle with the LCAs, but if I go to a fixed length (still have stock UCA), will that throw the pinion angle off enough that I may develop a DS vibration?

808muscle

Are you experiencing any vibration now? If not dont touch anything. To measure PA you need a PA gauge. You can get them from home depot or Lowes for under $10. Do a search and you will see how to use it. It can get very confusing thats for sure.

Why do you think your lca's are wearing out?

UrS4

New metallic rattle that is coming from the spherical joint rod ends on both rear LCAs.

jpplaw

When you installed the LCA's you have now, did you adjust them? If not they should be at stock length. If you had no problem with pinion angle then, you'll have no problem with new non-adjustable LCA's.
Measuring angle with a 1 piece DS is kind of easy. Get an angle gauge. Load the suspension. The DS should look pretty much level with the pinion. Measure the transmission output flange angle (a small carpenters square will help), then measure the pinion flange angle. The Pinion should have 2-3 degrees more angle (pointing down toward the ground) than the tranny.

157dB

http://iihs.net/fsm/?dir=77&viewfile...easurement.pdf

UrS4

Well, this might be embarrassing. When I installed the LCAs, I still had the stock DS now that I think about it. I adjusted the panhard bar to center the wheels based on the rear quarter panels and adjusted the LCAs so the wheel was in the same location fore and aft within the wheel well on both sides. I do not believe any adjustments were made to the LCAs with the DS installation to address the pinion angle. I did notice a small vibration or hum in what I thought was the rear of the DS when above 80 mph right after install. In reviewing other DS posts back then, most were saying that I should expect some increase in NVH as the stock two piece was used to eliminate that increased NVH. Over the years since the DS install, the hum has lessened. After lowering the car a month ago, the hum now occurs at 90-95 mph. I don't drive any faster on the street to know if it becomes worse above those speeds.

Maybe since the rear end drops more during the spring install than the front I have inadvertently improved my pinion angle.

Norm Peterson

With LCAs featuring sphericals (of any sort) and/or bushings of material stiffer than OE rubber, you almost certainly don't need as much PA as with the stock stuff. The reason is that you aren't having to compensate for as much bushing compliance.

I think I set my Currie LCAs about 1/16" shorter than the OE LCAs, which is worth about 0.3° less PA. If anything, there's a little less vibration at 70-ish mph. I might eventually try taking out another turn (LCAs 1/16" shorter), but it's not on this month's to-do list.


Norm

UrS4

Update.

So I got my replacement adj Delrin rear LCAs back on and finally got around to tackling the PA. I have an Alu 1-piece DS. Lowered on Steeda Ultralites. I have noticed since the DS install years ago a slight hum and slight vibration at about 80/90 mph. Didn't worry me too much but I want to get into HPDE this summer and would like to not have my DS break.

Wanted to post my measurements and see if my math is correct. Car was raised with ramps under the front tires and the rear was supported at the axle with jack stands and the car was nearly level, maybe slightly nose high.

With the new rear LCAs set to stock length, 18 inches from the center of each mounting bolt, I got under the car. Using a digital angle finder and placed on the rear part of the DS just in front of the pinion flange, it read 3.24 degrees with the end of the DS lower than at the transmission so I'm taking this as a negative 3.24 degrees. Placing the angle finder on the bottom of the pinion flange it read 0.60 degrees with the pinion flange angled up, so I took this as a positive 0.60 degrees.

So PA=DS-PF So -3.24-0.60= -3.84 degrees.

I understand that the Pinion angle should be -2.0 degrees.

So I lengthened the rear LCAs to about 18 and 3/4 inches thinking that the pinion would rotate downward against the fixed stock UCA.

Getting under the car again. DS was again -3.24 degrees angled down towards the rear diff. Measuring the pinion flange showed 1.20 degrees with the front of the diff pointing down towards the ground. I took this value as a negative 1.20 degrees.

So -3.24-(-1.20)=-2.04 degrees. I highly doubt the angle finder is accurate to 2 decimal places so I figure its -2 degrees.

Took the car for a test drive and there is no hum or vibration at 80-85. Didn't push my luck going any faster.

UrS4

Any help on if my math is correct?

jpplaw

Your math is correct. With a one-piece, you really should be measuring the transmission output flange instead of the driveshaft. May not make much of a difference though.