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There are 2 kinds of springs. Progressive, and Linear.
With progressive springs, as they compress the lighter rate coils compress, and force higher rate colis into action. Effectively increasing spring rate as they are compressed. Stock 240sx springs are progressive. (most stock springs are.) This type of spring is good for street driving b/c they are comfortable over small bumps, but tighten up as you start turning, or braking harder.
Linear springs are the same spring rate no matter how much they are compressed. Almost all coilover systems come with linear rate springs. Linear springs are much better for sport driving, b/c they are much mroe predictable, and make tuning your suspension much easier.
You can only preload springs with threaded shock body coilovers.
To preload a spring, you compress it b/w the upper and lower spring perches by adjusting the lower collar with the spanner wrench.
With linear springs:
When you put the weight of a corner of a car (probably around 700lbs) over your spring it compresses to absorb that weight. With 400lb spring, it will compress about 1 3/4". So, if you start with the lower collar so that it is just tight enough to hold the top of the spring against the upper perch this would be 0 preload. Raise the collar 1 3/4", and you now have 700lb's of preload. (arbitrary #'s for the sake of illustration.) Since you have preloaded the spring, the same weight as the car's corner, when you put the car on that spring, it will not compress at all, and will simply sit on top of the spring. If you were to hit a bump, the force would still compress the spring the same amount it normally would if you had set the spring to 0 preload. Now if you set the spring to say 900lbs of preload, when you hit the bump (say its a bump that generates around 250lbs of upward force), the spring will compress much less b/c the spring is already beyond the load specified to absorb the bump. Since there is only 50lbs beyond the preload, the spring will only compress about 1/8th inch, and the car will likely be launched over the bump.
You will likely never ever preload a spring beyond the weight of the corner of the car it is on for that exact reason. Springs are there to absorb bumps, if there are any bumps at all, this is a bad idea.
What good is preloading the spring then?
It can provide you with more suspension travel in certain situations. In the ideal world, having the spring at 0 preload would place the piston inside the shock exactly halfway through it's stroke (middle of shock) once the car's weight is resting on the spring. This is almost never the case. by preloading the spring, you can adjust the amount of rebound, or compression travel you have in the shock. If you preload like 200lbs, this will give you more compression travel, and will help keep your car from bottoming out when cornering. You have to be careful though, b/c you don't want to sacrifice too much rebound travel, or you could cause all sorts of other problems.
Progressive springs:
With progressive springs, preloading the spring does all the same things as linear springs, but it also increases the initial rate of the spring. So say you have a spring with a 200lb/in initial rate, and a 400lb/in max rate. (all progressive springs are rated this way.) Preloading 200lb's will compress the spring just under 1". Since the rate goes up with a function, it will have increased to probably somewhere around 250lb/in by the time you reach a 200lb preload. So now your initial rate is 250lb/in, with a max 0f 400lb/in.
You will likely never deal with this though, b/c very few coilover systems come with progressive springs, and remember you can't preload without adjustable spring perches. (coilovers)
-Charles
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