Extended shackles are a low cost method of increasing rear vehicle lift between 30-40mm, and they give you an extended range of travel with increased droop. In some circumstances they enable the installation of longer leaf springs, which brings better suspension.
What is a shackle & why would you want to extend it?
A shackle is a very important part your suspension, connecting the leaf spring and the vehicle frame. It enables the leaf spring to operate through length changes as your suspension performs. The leaf packs, or leaf springs, are placed centrally under the axle of your vehicle.
Shackles also support your vehicle's weight, given they are the primary part of the suspension. As a leaf spring flexes, its distance from the main eye to shackle eye fluctuates. When you upgrade your suspension or use a lift kit, they will probably be supplied new shackles.
The pros & cons of different shackle lengths
If you begin with factory fitted shackles, there are a number of factors affected when you extended the shackle... let's look at the pros and cons:
Pros of extended shackles:
The axle rotates downwards at the shackle side.
Pinion angle is enhanced
Shackle mount gets enhanced leverage
Suspension down travel is enhanced (up travel could be hindered a bit)
Suspension height is enhanced
Cons of extended shackles:
Caster angles will alter, which can cause steering issues at front axle
Approach & departure angles reduce in relation to caster alterations
The big question: Does an extended shackle effect your wheel travel?
Yes. As a leaf spring compresses, it gets longer as the spring arc flattens. Then as the leaf spring relaxes, it gets shorter as the spring arc returns to its default position. It's the shackle which enables this motion to occur, and the adjusting length very much impacts the total spring movement.
An extended shackle enables the spring to drop farther down while in its default position. It may also enable the spring to flatten more as it compresses... however this can be limited by the frame side mount or a rear crossmember. As it compresses, the shackle will be going away from the axle.