Choosing coils
When choosing coils, it's a good rule of thumb to stay around 100 pounds of difference in spring rates of your two coils (example: a 250 over 350). There is no law or rule against having more of a difference (in fact we run 200 pounds of difference on one of our trucks) and it can sometimes be a good thing if you are really running the truck hard and you want a really stiff second rate (once the slider stops moving) to slow down your truck's shock compression. But this, like high pressures of nitrogen in a shock, is a sort of Band Aid, and compression slowing should truly be done by valving of the shock.
Coil lengths should be the same as the stroke of the shock (14-inch stroke shocks get two 14-inch-long coils), but we have seen instances where (usually for clearance reasons during mounting) shorter coils are used on top with the coil retainer threaded further down on the shock. If you must do this, always make sure the lower coil is still full length so the coil slider is not able to fall off the shock's body and onto the shaft during full shock extension.
When choosing spring rates, you must first know the weight of your truck at each corner. A 250-pound spring rate will take 250 pounds to collapse that coil 1-inch. Your initial spring rate (see formula above) is up to you, and choosing softly or stiffly rated coils to have your coilovers ride the way you want is up to you as well. If you're going slow, then you can probably get away with a super soft rate. If you're going fast or jumping, you probably want a much stiffer rate to help slow down/absorb compression.
The specs can usually be found on the side of the coil like on the Eibach coil shown here. The three numbers stand for coil length, coil inner diameter, and spring rate (this coil shows 1600.250.0200: 1600 means 16 inches, 250 means a 2.5-inch inner diameter or shock body diameter, and 200 means a 200-pound spring rate).
When your vehicle is at rest, it's up to you how much shock shaft you want showing. If we might make a suggestion, 6 inches of shaft showing on a 12-inch-stroke shock is a good all-around player. This will give you a lot of extension for crawling around and drooping the suspension out, but will also leave you enough shaft to really compress it during a hard hit.
Check out Eibach's Web site (www.eibach.com) for more spring-rate formulas and information on setting up your own coilovers.
Warnings
Do not weld near the coilover shocks, allowing current to be conducted through them. This could lead to welding the shock together and it no longer compressing, in which case you're done. An arc through the shock can also cause the seals to leak as current jumps through it.
When setting up your coil package, be careful not to scratch the coils. Most shops will swap out your coils if they are in perfect condition and you find you have too stiff or too soft a coil rate.
Before you rebuild your coilover shock or send it somewhere to have it done, make sure it's worth it. You'd be surprised what wasted shocks can be rebuilt into, but sometimes the labor and parts can get close to the price of a complete shock, so make sure a new shock wouldn't make it easier for around the same price.