So I was not having much luck in researching how to install the IRS torsion bars from scratch. People I asked never directly responded or send me to the same adjustment info for already installed torsion bars.
I then started mixing up my search queries a bit and finally got some info, including a reference to Bentley which I must have overlooked when I was going through the suspension pages.
Stock torsion bar recommendation for IRS:
- 21° 20′ +50‘ (which is supposedly in the Bentley Manual)
1973 and later all have Long 26-9/16″ Torsion Bars
1969-1972 should have Intermediate 24-11/16″ Torsion Bars.
The longer and the thicker the diameter of a torsion bar, the softer the ride. The shorter and the thicker the diameter of a torsion bar, the firmer the ride. (source)
This page goes into a shitload of Torsion Bar Position Math and even has a built in calculator for raising and lowering.
Rob and Dave’s Aircooled Volkswagen Pages
Given how much time I’ve spent on this site, I’m shocked I haven’t read the page on rear suspension. I’m sure I have but it just hasn’t come up as a top search result for my current work.
If the car is completely level, the correct angle is between 20 degree 20 minutes and 21 degrees 10 minutes.
If you don’t have a suitable protractor, measure the distance between the ground and the axle ends of the spring plates, so you can judge the effect of the adjustments. You can get the adjustment pretty good just by taking note of the height from the floor to the rear end of the spring plate. Each torsion bar has 40 splines on its inner end, and 44 on its outer end. VW designed it this way so you can make fine adjustments to the angle of the spring plate (ride height of the rear end of the car)
Now one thing I came across is a second reference to double swing plates.
The IRS cars have a double spring plate with the brake hub attachment sandwiched between the two plates. The IRS models also have a dog-leg shaped diagonal arm which is bolted to the brake hub assembly and rotates on a fitting on the torsion bar tube. The spring plates have elongated holes so you can set the rear suspension toe-in (actually it should be 1/2 degree toe-out on the rear axle). You might even find an alignment mark on top of the axle fitting (source)
The spring plates I removed from Murbella are in fact single swing plates and not double.
So I’m starting to think she’s not as original as I suspected she was, at least in some systems.