Copied from a spring site:
Shackle angle determines the rate as well. A shackle with a 3 degree layback incorporates a stiffer installed rate than a shackle at 25 degrees layback. In addition, as the spring is put under load the shackle angle increases; therefore, the spring rate decreases under travel. To determine the effective angle of a shackle, pull a string from the center of the front eye to the center of the rear eye of the spring, and then a line from the rear eye through the shackle pivot point. Measure the angle derived from the two lines. You can decrease the spring rate by increasing the angle, or increase the rate by decreasing the angle. Also, excessive torque of the shackle bolts will increase the installed rate.
LANDRUM SPRING and many of the top chassis manufacturers recommend that the shackles never extend straight back. This will produce a lock-up or binding effect and the car will perform erratically and unpredictably.
Shackle rigidity and length play a roll in the installed rate as well. If the shackles are long and do not have any middle support or bracing, then the shackle tends to absorb more of the twisting instead of the spring; therefore, your lateral resistance is less. Also, the longer the shackle length, the slower the arc movement which decreases the rate of the spring change. The opposite holds true, the shorter the shackle, the faster the rate of change.
