Geometry dictates that it will always be less than half. Not sure how you can get more based on the condition of the springs. <img src="/forums/images/graemlins/confused.gif" alt="" />

I got more flex with longer shackles. I wouldnt use blocks.
If i had the money, i'd do new springs.

A lot depends on the relative stiffness of the front and rear suspension and the vehicle's weight distribution:
- http://www.4crawler.com/4x4/ForSale/Shackles.shtml#FAQ4

Yes, simple geometry would say lift = front-length/full-length * shackle-length, modeling the spring as a simple rigid member. But, like the way they adjust race car weight bias by "jacking" one corner or the other, the longer shackles can do the same as well as changing the load on the front and rear half of the srping pack itself. I have seen anywhere from 0" lift from a 1.5" longer shackle to 2" of lift from a 3" longer shackle. So if you are looking for exactly X" of lift, a shackle is not a good way to go, since it is so hard to predict the exact lift a given shackle length will provide.

Thinking about it more, there is also the 3+ " from the bottom of the spring to the center of the axle, which will move in a different arch during the installation of the shackles... could impact that as well, I guess. You're also suggesting the spring rate could change as it shifts relative to the vehicle.... interesting. Never thought of that.

Not so much the spring rate changing, but you are changing the loading on the front and rear halves of the spring. It is like putting a bottle jack in between the frame and rear spring eye (with the shackle removed). Start lifting and you have the frame raising but also you have the rear of the spring bending down. How much of each happens affects the lift.

And with a longer shackle, you may also have the factor of shackle angle. A shorter shackle may be kicked back farther while the longer shackle stands up more vertical, so you would see more lift than the simple length change. Basically just a lot of variables involved. That is why I tell folks I can make the a shackle X" longer than stock but I can't make them a shackle for Y" of lift.

Well, after a lot of thought I think for now I'll do nothing. From the various responses I think the *best* option for an inexpensive and temporary fix would be to add the chevy leaves in to the pack. I don't think I really like the block plan, and shackles, though the easiest and quickest option affect other areas of the suspension that I don't really want to get in to right now. So for now I think I'll just be happy that the sag is significantly less than before, and wait until I can do the whole she-bang (BJ spacers, shocks, OME springs). I figure better to do it right the first time around. Thanks for the input though, very much appreciated.

Oh, you could make a shackle for X" of lift, but it may take a few tries... <img src="/forums/images/graemlins/smile.gif" alt="" /> <img src="/forums/images/graemlins/smile.gif" alt="" /> <img src="/forums/images/graemlins/smile.gif" alt="" /> I suppose you could use a good CAD modeling program, but that would take knowing all the weights, dimensions, and spring properties of the target vehicle. Making three or four sets of shackles just might be cheaper. <img src="/forums/images/graemlins/lol.gif" alt="" />

Yes, that is true. Would probably take 2 or 3 tries to dial in a lift height for a given shackle length and vehicle. But the rough rule of thumb of 1:2 is good enough for most folks.

I guess that is one advantage a lift block has over a shackle, a more direct relationship between block height and lift, at least initially.