A spring shackle is a device found on leaf-spring equipped vehicles. It mounts to one end of the leaf spring and allows it to flex and move while keeping the tire on the road. Without a shackle, the spring would not be able to move and the tire would be pulled off of the road's surface when a bump or obstacle was encountered. The spring shackle can also be lengthened and give lift or a greater amount of ground clearance to the vehicle.
The leaf spring is attached at the front and rear by a long bolt passing through the spring's eyelet as well as a mounting bracket. One end of the spring is held closely to the vehicle's chassis and cannot move, the other end of the spring has a spring shackle mounted between the chassis mount and the spring's eye. This is nothing more than two flat pieces of steel with several holes drilled through to allow different mounting heights. The shackles allow for movement of the suspension by pulling in or pushing out as the suspension travels through its up and down cycle.
By choosing to mount the spring at the furthest mounting hole from the vehicle's chassis, the spring is lowered and subsequently raises the ride height of the vehicle. While this does not increase the clearance between the vehicle's axle and the ground, it does increase the amount of clearance between the vehicle's fender and the tire. This increase allows for more clearance when the body flexes on the chassis and enables the vehicle to traverse rougher terrains.
Certain spring shackle designs also incorporate a swivel to allow the vehicle to rotate as the spring flexes. This type of spring shackle is usually used in severe off-road applications. This type of spring shackle may also incorporate a hinged set of shackles which when fully flexed, open to give the shackle twice its length, resulting in twice the travel allowed by the spring.
The disadvantage of running a leaf spring mounted at the maximum lift on the highway is that this allows the vehicle to sway on the springs. The leverage over a lengthy time period will begin to pull and push the leaf spring until it becomes loose and sloppy. This in turn will allow the chassis to sway over the springs and lose some of the control that was engineered into the vehicle. While this sway may appear minor, it can have an adverse effect on the vehicle's handling in emergency turning situations.