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I have had a driveline shimmy since my 5 speed conversion and I'm having a go at fixing it. Last year I installed a TKX in my 92RS. I started driving a week ago and it's all good except that it has a high speed shimmy that gets worse on deceleration.I have already had the driveshaft balanced and a new rear U joint . I just measured the rear axle inclination (I know that's not the term used but in a boomer moment I forget the real term) at 2.5 degrees and the eng/trans at 4.7 degrees. I can fix this in two ways; I can adjust the torque arm front position in the trans mount increasing the diff angle, UMI mount, or I can put a shim under the rear trans mount that will lower the E/T angle. Or some combination of both. What do you think? 4.7 seems steep but I believe a real thick shim would be needed to effect any significant change.
4.7° is about normal. You could possibly raise the rear of the trans and cut that down some, but, it's not "wrong".
In a perfect world, the pinion angle would nearly match the engine/trans angle, but maybe ½° lower. Reason being, those 2 angles need to be as parallel as possible under power, and the rear moves upward; so you want to start with it a bit low, sitting still on ramps or whatever. What you have, at over 2½°, is excessive.
I'd adjust the pinion angle if possible. Maybe shim the trans mount up a bit so it's a bit closer (you can probably get ½° or maybe even a whole ° that way); butt you're gonna have to move the pinion upward somewhat.
I read your recommendations and With a 3/16ths shim on the trans mount I'm at 4.4 and I adjusted the torque arm as high as my mount goes and I have 3.7. It seems that this meets your "spec" and I'm at .7 degree difference so I'm going to try it and see how it goes.
"the rear axle inclination" and "the eng/trans" are not u-joint angles. You need to factor in the driveshaft angle to calculate u-joint angles. You could have both "inclination within 1/2* but u-joint angles could be over 3*.
If the vehicle has been lowered, the u-joint angles will be increased due to lowering of chassis in relation to rear-end and also, rear-end being moved off center (track bar).
1. Spicer guidelines are the individual U-joint operating angles should be small as possible, between 1°- 3° and not less than 1°. GM tightens up that spec to between 1.0° - 1.5°. Follow the GM spec. Bottom line is the smaller the operating angle, the faster you can go without issues. 3° correlates to about 5200 rpm max allowable driveshaft speed. 1° correlates to about 7000 rpm max allowable driveshaft speed. I think maybe the 1° minimum is to ensure that the needle bearings turn.
2. To cancel vibrations they want the U-joint operating angles at each end of driveshaft to be equal within 1°. This is the parallel between U-joints and obviously the closer to 0° (perfectly parallel) the better. See video for demonstration of how differing angles cause speed oscillations as driveshaft rotates. Oscillations are bad news and probably what you're feeling. You gotta get those U-joints near parallel.
