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I used the Spicer Drive Line Calculator and came up with these numbers.
Trans down 2.5 Drive shaft up .5 Rear Diff.up 3
Operating Angle #1 is 3
Operating Angle #2 is 2.5
Do these numbers sound ok ? Car is a 68 Chevelle and it is lower than stock. Thanks, Tony
Last edited by tmcmillan5; Dec 27, 2020 at 02:42 PM.
On the face of it, the angles sound OK. Once you drive the car, you will know more. The issue is that under load, the pinion will rise, this will increase the front and rear operating angles. How much or how little depends on the bushings that you are using in the rear suspension.
With trans down 2.5*, pinion would be up 2.5* for drive angles to be parallel (keep u joints in sync). Move pinion down 1.5* to allow for axle housing rotation under load (as mentioned above). Not too concerned about operating angles since it's not a 4wd with significant differences in the height distance between trans output and pinon. Hope this makes sense.
Test for vibration. Mount camera underneath if you want to capture how much axle housing rotates. You don't want it to over rotate past parallel drive angles.
A couple of additional thoughts -- pinion rotation under load occurs both on acceleration AND deceleration, just in opposite direction. So allowing for some rotation under accelerative load by changing the pinion angle relative to the power train angle WORSENS the angles under deceleration. And, that rotation is notably worse with a leaf spring rear suspension than with a 4-arm set up. Drag-race-only cars will often set things up so engine/tranny/pinion angles are equal but opposite under hard acceleration and live with any deceleration issues that may cause. For a street car, my perspective is - if you have the ability to adjust the rear pinion angle (adjustable length upper arms usually) - then I'd set the pinion angle equal to the drivetrain angle - and then drive the car and see what you see. Go from there....
I set my engine/tranny angle at 2.5 degrees down, pinion at 2.5 degrees up - and my u-joint angles measured out at 1.7/1.8 degrees -- smooth as silk, once I got the DS balanced.
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Drive Line angle #'s 
