What are the pros/cons of different shackle lengths?
In relation to stock length, there are a bunch of things affected when you change shackle lengths.
A longer shackle will move one end of the leaf spring further away from the frame, doing several things.
It rotates the axle down at the shackle side of the spring.
It decreases castor angle of the front axle affecting the steerings stability at high speed, and reduces the steering ability to return to center after a turn.
It increases pinion angle
It increases leverage on the shackle mount
It increases droop (down) travel
It decreases approach/departure angle
It adds lift to the suspension in the amount of half the difference between the new shackle and the original shackle. Example: shackle is 4” from bolt to bolt; replacing it with a shackle 6” from bolt to bolt will provide 1” of lift)
Heres a few reasons you really shouldn't run extended shackles from a handling driveabillity point of view. In other words dont over do it.