Using Adj. Lower Control Arms to Set Pinion Angle?

[Socrates]

I have UMI adjustable lower rear control arms and have welded/boxed the upper arms on my 66 GTO. I need to raise the diff pinion angle up 1.5 degrees to match my new T56 transmission install. It seems most folks using adjustable upper arms to set the pinion angle, though I presume using adj. lowers would work just as well to this end, right? Are there other geometry complications at hand by shortening the lower arms a hair that I should be aware of in terms of strip traction or track/street handling? Thanks!

[The WidowMaker]

no issues for the amount you need. the proper way would be to do a little of each (upper/lower) in opposite directions to keep the wheel centered, but the amount youre going to need wont make a difference.

Tim

[old66tiger]

Problem with using adjustable lowers is that unless they are double ended, you can only adjust the arms by 1/2 turn increments. Double adjustable uppers are the bomb as you can adjust in fractional increments to get the axle where you want it. I ended up with both for my 66 GTO as my axle did not sit in the frame straight and i had to tilt the rear end up to get the right pinion angle. Also, when you adjust in the lowers, it will pull the axle more forward in the center of the wheel well and could mess up the drive shaft length. Good luck!

[JRouche]

I need to raise the diff pinion angle up 1.5 degrees to match my new T56 transmission install. It seems most folks using adjustable upper arms to set the pinion angle, though I presume using adj. lowers would work just as well to this end, right? Are there other geometry complications

Might sound kinda silly but thats the main reason I went with a parallel 4-link, so I could dial in the pinion angle when the time came. I didnt have a set ride height or drive train for that matter. So I needed some adjustability in the end.

Im not sure how much you will need to shorten the lower arms to get the correct pinion angle. But if its not much I dont see too much of an issue. You will suck the tire forward a lil, might look a lil odd if its too much.

For handling I dont see too much effect. You will get a sharper pinion angle change with suspension travel due to the shorter arm. Are they level with the road surface? Or pointed up or down now? All depends on how much you need to shorten the arms and if they are level. JR

[The WidowMaker]

if i did my math correct, you need to move the lca mounting pt .25" forward which will move the axle centerline .125" forward. not enough to be worried about. 4 turns on a 3/4-16 single adjustable and 2 turns on a double.

[Socrates]

You guys rock! Thanks for working the math out, WidowMaker, that gives me a great place to start.

old66tiger, thanks for the driveshaft info. I'll be sure to get the angle figured out before ordering the custom driveshaft.

JRouche, the lower are is pretty close to parallel as it sits now. Shortening the arm may actually help make it more level and I think should also push the instant center farther forward.