Rear shackle angle

this chart is what I set our leaf springs up
Sliders or shackles
.

When at the track- sitting on the line- bring the "R"s up to load the chassis
the car will raise the rear- if set up like the chart
while under load, the shackle will arc sightly forward,

the more angle used will make a different shackle arc , causing the rear of the spring , to move upward toward the chassis,
and the rear of the car to squat and lose some of the cars reaction time




Out of curiosity, what is the source of this diagram? I don't recall seeing this in the chassis books I have.