Originally Posted by ATL ZJ
There are a lot of ways to set up coilovers. Many right and a few wrong. A lot of it is personal preference, especially when it comes to the softness or firmness of the ride.
Before you start you'll need to make some decisions:
1. Amount of travel desired (i.e. 14" total)
2. Split of travel up/down, and ride height (i.e. 6 up, 8 down)
3. Approximation of corner sprung weight (although not critical- this can be measured)
4. General idea of mounting locations, rake (angle) etc.
5. What kind of wheeling you plan to do
I'm gonna do this in a story format so it's not all about me, but everything here is based on my experiences... it's all fictional and forgive me if it's too lame and basic. With that disclaimer, here goes...
Let's take Ryan's ZJ (rstrucks). I have no idea what his CSW (corner sprung weight) is, but let's focus on the front and say sprung weight is 900 lb on each front corner. Again this is just a guess. Let's say Ryan wants to keep his existing ride height. He wants as much travel as he can realistically get. He doesn't really want to cut his hood for shock hoops to poke through, and when he droops out his suspension he measures that he only has about 9" of droop (from his desired ride height) before his driveshaft yoke (or trackbar, longarm, etc. etc.) starts to bind. He has measured his uptravel (from his existing ride height) and it's about 5" or so.
So although he was hoping for 16" coilovers, he realizes that he can't use all that uptravel or droop, without doing surgery on his pretty 5.9 hood, and decides that 14" would be a better fit. He knows that he will be running about 35% uptravel (5/14) and about 65% droop travel (9/14) on a 14" shock. He wants his ride to be a little firmer than it is with regular coils he has now. They seemed to have gotten soft after adding a front bumper and winch. Let's also assume that Ryan plans to mount these coilovers more or less straight up and down, rather than raking them inboard or toward the front or back of the rig.
Now that Ryan has determined how long of a shock he needs, he is ready to do some calculating. He's done some reading and has read that it's customary to run two springs with lengths equal to the total shock travel. So he is expecting to run two 14" springs. Has read that a 250% step up, or increase in rate from the primary rate to the secondary rate is pretty normal. So he uses an online calculator from his favorite shock manufacturer, and determines that for his front shocks, his rates need to be somewhere around 160 over 240. The closest he can find in 14" springs is 150 and 250, so he orders up his springs and shocks with the valving recommendations of his favorite shock builder.
He gets all his parts in, and builds his shock mounts. He decides to tack it all up at ride height, using the shocks without springs on them for mockup. He measures to have about 5" of shock shaft showing, although he could have easily taken the rig to full bump and mounted the shocks at full compression to maximize his total travel. Then Ryan pulls the shocks off his rig, and charges them with nitrogen while they are fully extended. He installs his springs, and runs the upper adjuster all the way down until it preloads the springs slightly. He notes the position of the dual rate slider in between the springs, and adjusts the dual rate stopper all the way up and out of the way. He installs the shocks with springs on them, and lets the rig down off the jack to rest on the springs. It's about perfect. The ride height is almost exactly what he wanted. He adjusts the dual rate stopper to be a few inches above the dual rate slider, about half his total uptravel, as a starting point for his step up. Ryan tightens his bolts and packs up to go wheeling the next weekend.
Ryan wheels it and loves it. It seems smoother, etc blah blah. The biggest improvement is in rock gardens, but on gravel roads in between trails it bottoms out on waterbars. Ryan thinks that maybe his dual rate stopper is too low, so he adjusts it down an inch and keeps wheeling. It seems better, but the suspension still feels really soft at first. He adjusts the dual rate stopper down even more, so that the dual rate slider is almost riding on the stopper all the time. Something doesn't seem right. He wonders if his rates are correct even though the calculator said they would be.
Ryan gets home and does a little more reading. He comes across a simple formula for combined spring rates. (Rate 1)(Rate 2) / (Rate 1 + Rate 2). In his case (150)(250)/400 or 37,500/450= 93.75. So his initial rate is 93.75, and when the dual rate slider hits the stopper, and the rig is riding only on the bottom/main spring, the spring rate jumps up to 250. That works out to be a 266% stepup, because of the variation in springs available vs. what the calculator said he needed for a 250% stepup. Ryan decides he wants a stiffer initial rate (before the slider hits the stopper) and starts looking at springs again. He thinks about buying a stiffer top spring, like a 200 or even a 250, that is also shorter, to let him maintain the same ride height. He starts to run the calculations and then realizes that with a shorter spring, he would have to lower the upper adjuster down past where it currently is, to avoid having slack and having the springs bounce around (and on certain shocks, have the lower coil seat fall off). So he just orders up some stiffer springs (let's say 250 lb springs) of the same length. He realizes he will only now have a 200% stepup because his upper and lower springs are the same rate.
Ryan gets the new springs on and the upper adjuster back where it has to be, lets the rig down, and it's taller. He expected that, and decides to live with it, and try it out again. He ends up liking the ride, but he wishes his rig was not quite as tall. So he starts reading up on triple rates, which he'd decided not to buy with his shocks, and realizes that with a triple rate, he could run shorter stiffer springs as he had wanted. Unfortunately, to install triple rate sliders and springs, the dual rate stopper is in the way. He has to either take his shocks apart and bleed and reassemble them, or send them back to the manufacturer.
So after a crash course in shock rebuilding, he ends up running a 12" long 250lb spring over a 14" 250 lb spring with a flat triple rate coil. He ends up liking the overall firmness of ride and the ride height is finally correct. He wished he did it all that way in the first place but he learned a lot in the process.
Ryan thanks for being our guinea pig in this story. I'm sure you would figure it out faster than this. (edit to make the calcs work out for 900 lbs CSW.)