I'm not too sure how much further we need to go down the rabbit hole - the good books on the subject are locked up behind a pay wall and I can't get at them (for less than $100 USD - I hate that. So the next best thing is to call the guys at Tech Support at Spicer and lay it all out for them. Aside from the horrific elevator muzak, the tech was nice and knew what he was talking about.
They have a selection of calculators:
https://spicerparts.com/calculatorsFirst up - The Operating Angle calculator -
https://spicerparts.com/calculators/driveline-operating-angle-calculatorFeed this thing with my angles (drive = 3.4 down, shaft 1 = 6 down, shaft 2 = 3 down and driven =0) so this leaves the following operating angles, first joint = 2.6, middle = 3, differential = 3. The details says that if I run the first joint and the last joint in the series no more than 1 degree different and nothing over 3 degrees...so all good. On the edge, but not against the rules. It's just operating angles - nothing fancy.
If it's outside of these parameters, the calculator tells you that death will ensue.
Next up - The Torsional Analysis calculator-
https://spicerparts.com/calculators/torsional-analysis-calculator This tool will give an estimate of various predicted torsional and inertial effects that can damage many of the driveline components in a vehicle. You need more data - length of the shafts and distance the diff is off the centre line.
So to the above data we add, shaft 1 will be 26" long and shaft 2 is 22" long. The diff is 1.875" off the centre line. Bung this in putting all the off set to shaft 2. Then set the shaft RPM to 3000 for the first run.
Inertial Effects-Drive Degrees 6.39 Rad/Sec
1229.2Inertial Effects-Driven Degrees 9.45 Rad/Sec
2684.3Torsional Degrees 7 .69 Rad/Sec
1776.0* Max Allowable Inertias -
1000 Rad/Sec/Sec
Inertia Effects (Drive) - usually caused by a large operating angle at drive end of driveshaft.
Inertia Effects (Driven) - usually caused by a large operating angle at driven end of driveshaft.
* Max. Allowable Torsional -
300 Rad/Sec/Sec Torsional - usually caused by large, unequal operating angles or out of phase driveshafts.
So at 3000 RPM, it's above the max allowable at the transmission and over twice allowable at the differential...let's not talk about torsional limits. Yikes.
I ask the tech - he says "If the calculator runs, it's going to be OK - it won't fail" - I reply that it may be like driving in a paint shaker...he doesn't respond to my witty retort. Hey, it was Monday morning...I understand.
So then I change a few things. Basically, I ran a quick and dirty sensitivity analysis - without changing the drive or driven angles. First, I varied the speed. 3000 RPM is around cruise on the highway - so not really smooth (hard to know what all this means, but most things in my car are not well isolated for NVH and I'm not all that sensitive to it, but let's be realistic). So I up the RPM to 5K...calculator still works.
Inertial Effects-Drive Degrees 6.39 Rad/Sec
3414.6Inertial Effects-Driven Degrees 9.45 Rad/Sec
7456.5Torsional Degrees 7 .69 Rad/Sec
4933How about 6K
Inertial Effects-Drive Degrees 6.39 Rad/Sec
4917Inertial Effects-Driven Degrees 9.45 Rad/Sec
10737Torsional Degrees 7 .69 Rad/Sec
71037K - BANG - it gives the warning about death.
OK - let's play a little with the offset. If I split the distance between the shaft at 3K (well, 1" to the front and 7/8" to the rear) things look great at 3k
Inertial Effects-Drive Degrees 4.89 Rad/Sec
719.7Inertial Effects-Driven Degrees 5.27 Rad/Sec
835.9Torsional Degrees 5.08 Rad/Sec
774.9Wonderful - let's do this. No idea what it all means, but inertial effects are below tolerance and the torsional thing is only double what it should be...plus a little.
REV it up - 5K BANG
6K
Well, if it wasn't going to do 5...I tried it anyway. WTF. I tried every which way to move angles and offsets and it all blows up. Nice. I found the optimal solution by looking at it. It still won't work well, but it will work. Just don't drive it fast. Ya, right.
So what about a one piece. I did this and the shaft angle is 4.6º (3.4 and 0º for drive and driven) - so I can't make it past the first calculator.
The angle results from the engine being higher than the diff...and I can't tilt the diff up 3º without having to cut apart the rear subframe for the 4th time.
Now looking at CV joints instead of U-joints.