I'd like to know what it is then. Swing arm is not a term I've encountered before about leaf springs. All the drawing says is that roll axis is parellel to it and its length is 3/8th of the distance between the chassis mounts. The name sort of suggests its the point around which the rear axle swings, but that would be what's called an instant centre. Which I reckon would be alot closer to the front eye than what's going on in that diagram. I have no idea what it is!

From my understanding of the text, the actual maths behind determining the Roll centre height and roll steer are complicated (They do not go into the equations in the book).
What is presented is a simplified approximation of Roll Centre height and roll axis (Roll steer).
This is done by the roll centre as discussed already (front mount and upper rear mount, vertically above the axle) and the "swing arm" which is derived from the slope of the front 3/8ths of the spring (Q to R), transposed to the roll centre (C to O). The slope of this line then determines roll steer and the end point (C) is the Instant Centre (length C to O = the 3/8 spring length L).
In the text SVSA (Side View Swing Arm) or the single point at which the suspension will rotate around.
I hope that helps.

well it's a pretty dodgy drawing then. their swing arm doesn't use the roll centre in the diagram, it's originating from the centre of the axle, well below the RC!

And I stick by my point that the instant centre (why don't they just call it that?) is alot closer to the spring eye than they illustrate - their illustrated 3/8L is nowhere near to scale.

Even their roll axis term is misleading. In suspension terminology that is normally used to describe the line between fornt and rear RCs. Here they use to describe the axis the rear axle rotates around.

It's like they are trying to confuse/mislead people. At least I guessed correctly they were trying to describe the instant centre... I got something right at least.

real world examples on my 120Ys:

the lowered 2dr sedan is approx 48.5mm lower than standard with an additional reversed leaf added to the top of the pack.

ROLL CENTRE


120Y leaf spring Roll Centre height - low vs stock
stock blue coupe on left - RC is approx 38mm above the axle
lowered yellow 2dr sedan on right - RC is approx at the centre of the axle. String line is distorted from straight by the axle housing.
lowered car's RC is approx 66mm below standard, approx 285mm from the gound with the 185/60 R14 tyre.

INSTANT CENTRE


120Y leaf spring Instant Centre location - low vs stock
'L' length between front and rear spring mounts on chassis is 1140mm.
3/8th L = 427.5mm
= approx 97mm from front eye longditudinally.
Approximate IC locations are at the end of the tape measure.
Stock blue coupe on left IC is approx 80mm higher than the lowered yellow 2dr sedan on right due to a combination of lower RC and decambered front half of leaf.
Effect of lowering is less bind/twist on the spring due to the IC being both closer to the spring eye and the axle's roll axis being closer to horizontal.
That also means less roll steer.

The other thing is that due to the decambered lowered leaf pack, the rear shackle is on a greater angle. This has 2 different (and opposite) effects:
1. it increases the effective spring rate
2. the anti-jacking effect in bump travel will unload the chassis, effectively reducing spring rate.
The overall effect on spring rate will be which of those 2 factors has the greater effect. Which is a mystery to me. But as I can't really do anything about it without affecting other things like ride height, I don't really care. It just is what is is.