I'm just wondering what are the advantages to having shackles on the rear leaf springs? These are the metal bars/bolts at the rear of the leaf spring that hold them to the car. The shackles use longer metal bars with several holes/mounting points to raise or lower the distance between the leaf spring and it's mounting point on the frame. I've seen these before on lots of other leaf spring cars, and always wondered what they do?!

You need to think about how a leaf spring works. Because it's curved, when it compresses. it also increases in length. So the shackle allows for this increase in length.

By changing the length of the shackle, you are changing the leverage points which changes the dynamics of the spring action.

Chris

In other words, you need shackles. Which is why all 1200s have them.

Are there advantages to shorter-than-stock shackles?

Some people use longer shackles to raise the back end slightly.

Adjusting the geometry of the rear suspension is a nice tweak. In the stock setup the rear spring eye is quite a bit higher than the front spring eye. This makes the axle move forward and rearward as it moves vertically. What this does is set the car up with some roll understeer in the stock configuration.

What happens is the rear axle steers the car a small amount as the body leans in the corner.

One of the nice tweaks I have done on several 1200's (and my brothers Ford Pinto, many years ago) goes like this. Add a 1" lowering block (or have the springs de-arched 1"). Increase the rear shackle length a hair over 2". This causes the spring to be much more parallel with the ground and does not change the ride height.

With the rear springs more parallel to the ground, the rear axle has less roll steer. It makes the car more stable on bumpy roads. Really shows up on long sweeping corners that have lots of bumps. Most of the tiny steering corrections you normally make just go away.

Its a good mod Gary_P , and cheap to do as well.