Technical Understanding Parallel Leaf Springs

Cheers, hope everyone is doing well.

Starting on a new project, 27 T Touring on a 28 Model A frame. Already have a straight axle sourced, and I'm planning to run parallel leaves front and rear.

I've attached a photo from the book "Tires, Handling & Suspension" by John Dixon.

The paragraph describing the photo is as follows:

"In the case of longitudinal leaf-springs, the roll steer depends on the inclination of the equivalent link AB that describes the motion arc of the wheel center (Figure 5.16.3, considering the figure without the steering system).

This equivalent link is directed toward the unshackled end, is about 3/4 of the length of that end of the spring, and is roughly parallel to it. When the body rolls, A rises on the inner side and falls on the outer, thus tending to steer the axle.

Having point A higher than B gives roll oversteer for a rear axle, B higher gives roll understeer. For a front axle the effects are opposite.

The roll steer coefficient is equal to the AB axis inclination ρA, independent of
the spring length or separation. On the other hand, the spring length affects the influence of load variation on roll steer coefficient. Too high a coefficient, apart from being bad for handling, also leads to harshness on rough roads because of the wheel path in bump and other problems.

Early Hotchkiss axles, pre-1930, were given a negative roll steer coefficient because this resulted in less sensitivity to road roughness. In the early 1930s a positive coefficient was first used (i.e., roll oversteer was replaced by roll understeer) and a dramatic increase in directional stability was found."

So! That being said, I'm trying to understand my spring inclination on the front, rear seems very self explanatory. Fixed end of leaf towards the front of the vehicle, axle center just above front eye, rear eye/shackle mounted higher than front.

Real question arises regarding the front axle. I'm using a straight axle from a 1950 F1, along with the steering linkage and box, so retaining the stock location of the box behind the axle, and fixed end of the spring towards the rear as well, will help to eliminate roll steer (as the book goes on to explain).

However, if I match the factory configuration, they had the front spring shackles mounted towards the front and angled up to give positive caster to the kingpins. Doing this will result in front axle Oversteer (Point A lower than B).

Only alternative I can think of is running the fixed end higher than the front mounted shackle, and then using caster shims on the axle.

Am I overthinking this, should I just mimic the factory setup? Not sure if there's a reason most cars of the era had front spring shackles at the rear (1929 Bentley attached as reference).

Looking to be educated by anyone with experience regarding how to setup front parallel leaf springs for a good handling ride. I could just wing it, but would rather start from a basis of knowledge and understanding on "why" I'm doing something.

 

A Model A did not come with parallel leaves, so the frame is totally not laid out to suit them. It was designed for transverse leaves. You would have to do extensive work to fit parallel leaves.

If your heart's desire is parallel leaves, start with a frame that came that way, like an Essex or any number of other manufacturers from the era.

A truck axle? Come on man, you're getting all thingy about the relative merits and suspension dynamics of parallel leaves then want to use a truck axle? At least use a car weight one!