OK, installed the one degree shim, and road tested. With the pinion rotated up an additional degree, I should be starting out around 3-3.5 degrees down. I did not check with the angle finder. It is not an exact linear adjustment, because as you rotate the pinion up, you are slightly decreasing the driveshaft angle, which makes the change greater then the angle of the shim. Anyway, results:
The vibration now does not start to around 70-75 mph. So, I am going in the right direction.
The vibration is no longer responsive to torque, i.e. up to 70 mph in drive or overdrive, no vibration, including shifting back and forth from drive to overdrive at 65-70 mph.
So, I am thinking I still need to rotate the pinion up slightly, maybe pull the 1 degree shim and try a 1.5 degree shim. I should have ordered some 1.5 degree shims
OR, I can slightly space down the skid plate with a thick washer, decreasing the driveshaft angle, effectively lessening the angle difference between the pinion and driveshaft.
I know it sounds crazy talking about 1-1.5 degree changes, especially considering we are talking about a Jeep CJ, but it makes a huge difference
You have to remember, I, Jeep CJ owners, have about the worst combination of drive line factors to overcome:
1) short wheel base, steeper driveshaft angle, even stock, compared to a full size truck
2) lifted, increasing the driveshaft angle
3) soft springs, a bit more pinion wind up
4) 4L60 transmission is pretty long, decreasing the length of the stock driveshaft, increasing the drive line angles
5) 4.10 gears/33" tires means a higher driveshaft RPM, compared to a stock 3.31 gear/30" tire combination
The last point is interesting. Spicer has a chart telling you max operating angles of u-joints at a given shaft RPM. More angle, less RPM. This explains why sometimes you can throw a 4" lift/33" tires on a Jeep, keep the stock 2.73/3.31 gears, and have no vibration. The driveshaft is not spinning that fast. Put in deeper gears, it starts vibrating due to shaft speed. Even without a CV, you can have deep axle gears (4.88/5.13), with large enough tires (38"+), you can drive probably up to 60-65 mph because the shaft is not spinning that fast. Put smaller tires on, it will shake you to pieces at lower speeds.
I kind of proved this. No shim, so I had probably a 3.5 degree angle on the pinion u-joint at 65 mph, vibration (lower shaft speed)
One degree shim, say a 2.5 degree pinion u-joint angle, vibration pushed back to 70-75 mph (higher shaft speed)
So, say a 1.5 degree shim, 1.5 degree pinion u-joint angle, vibration might be pushed back to 80-85 mph (even higher shaft speed)
Even with a "CV" shaft, if it is operating at a steep enough angle, even with the pinion u-joint at 0 degrees to the driveshaft, given enough shaft RPM, it will vibrate. Example, say you have a 25 degree driveshaft angle, transfer case output shaft at 5 degrees down. So, 20 degree operating angle, each u-joint in the CV is operating at 10 degrees. Given enough shaft RPM, it will vibrate. IIRC, Spicer wants a MAX 3 degree angle, or say even 5 degrees. A stock CJ is at that. You lift them, you exceed the angles. A CV most of the time will help, with say a less then 6" lift. You get much past that, with deep enough gears/high enough shaft RPM, it will vibrate.
Theoretically, if I swapped in 3.54/3.73 gears, I would lower my shaft RPM, so the vibration would not present itself until probably over 90 mph.
When I say vibration, mine is slight. It is not shaking the whole Jeep, but I can feel it a bit in my butt/floor pan. With the amount of lift, and other factors, I know it should be 100% smooth. It was, before the springs settled slightly and I reinstalled the full hard top. Just needs a bit more adjusting
Picture of the steel 2.5 degree shim vs the aluminum 1 degree shim. I prefer steel, but I can not find a steel shim less then 2 degrees.
The other issue - this shim is NOT pinned to the leaf pack like the steel shim. It simply slips around the center pin head. Because it is one degree, it is pretty thin, so I still get 100% engagement between the center pin head/hole in spring perch. If it was say 2 degrees or more, it would be thicker, and a taller center pin head, or center pin head spacer, would be needed to get full engagement.
Thickness difference. Ittsy bittsies sometimes make a huge difference:
Installed, looking from above:
IF I was looking for another project to do right now, longer "anti-wrap" style spring perches would be a good idea. The stock perches are still fine, not rusty or rounded, but a longer/thicker perch would help eliminate some pinion climb. Long term solution - shim the pinion to where it is good, record the angle. Weld on new perches to the correct angle. Re-do the transmission mount/spacers to allow for some minor drive line angle adjustment there, where it is easier to adjust. Remember, softer springs will settle a bit more depending on the amount of crap you have in the bed. If you set it up "perfect" empty, when you load it up, the angles could be off a hair. I have been adjusting this one with a full 20 gallon tank of gas and about 200 pounds of crap in the bed. I would rather have it "perfect" for how it is loaded on long cross country trips, and maybe have a hair bit of vibration when 100% empty
When I had the Superlift springs, it took 1,500 pounds of gravel to get the suspension to compress 1", so no chance of enough of a drive line angle change to cause a vibration, loaded or empty. The RE springs are softer, so they will compress more easily. I would rather have the softer springs, better ride quality, and have to deal with a bit more angle tuning
This is probably why the factory offered different spring rates