Well there has been a lot of talk about 4 links so I am not gonna talk about that you can figure that part out with the oh so difficult search function <IMG SRC="smilies/rolleyes.gif" border="0">
when it comes to the quarter eliptic springs themselves many people use long chevy leafs succesfully. if you take a spring pack cut one end off and bolt the axle mounting portion to your frame. Then the spring rate is half of what it was with the complete spring pack in the semi eliptic orientation.
You want to make the springs as long as you can without sacraficing clearance, so that they are less likely to break.
You also want to choose a pack that has thin leaves because these are also less likely to break. I know that is hard to believe, but if you don't believe it then clamp a piece of 1/8 x 1 HRS strap and a piece of 1/4 x 1 hrs strap in a vice, leave about six inches sticking out. You will be able to bend the 1/8 piece over about an inch without it taking a permanent set. You will only be able to bend the 1/4 inch thick piece over about 1/2 an inch before it takes a permanent set. In a semi eliptic setup the leaves also have to control axle wrap and the movement of the axle, so the manufacturers had to compromise and make thicker leaves. In a four link the spring pack doesn't have to do any of these things so thinner is better.
if you take a stock spring pack apart and lay the leaves side by side instead of on top of each other they will form an approximation of a triangle. This is because the leaf spring was designed for constant stress. At the top surface of each leaf all along the length of each leaf the stress is approximately the same, with this geometry. As you add or subtract leaves keep this in mind to avoid adding a "kink point" where your spring pack might bend or break.
These aren't rules that you need to follow to the letter, but people with quarter eliptic setups with thick leaves and short spring packs, with lots of short leaves in them often break springs.
Also it is OK to have the center pin hole out in the working length of the spring, if you need to do that to get leaves the length you want. The stress concentration factor for a 3/8 hole in a 3 inch wide leaf spring is pretty small. No big deal if it gets you the length of spring you need.
I haven't seen an attachment of the spring eye to the axle that I like very much there are lots of pictures of this detail on the net. Make sure the shackle is long enough that the spring stays more or less above the axle throughout the range of suspension motion. 45 degrees maximum of shackle angle at any point and you would be doing as good as most setups I have seen.
don't hinge the springs so that they fold away from the frame, most people who have done this have undone it.
Use limiting straps when your inexpensive shock decides to stop doing the job of a limiting strap it can be a mess. Coilovers handle this better because they are much beefier shocks.
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