How it all works
Coil Springs
Here, we'll look at a basic coilover front suspension loosely modeled on the Tacoma and 3rd generation 4Runner as an example of how suspension modifications affect the truck. The principles explained here will work for all coil sprung suspensions.
First off let's clear up a common misconception that the spring rate of the coil can be changed easily. In fact, the spring rate is a fixed value and cannot be changed. Spring rate is a function of the physical geometry (coil diameter, wire diameter, number of coils, etc) and the material properties of the coil and nothing else. However, the effective spring rate of the whole suspension system can be changed relatively easily, but that is beyond the scope of this article.
One myth circulating the internet is that spacers change the preload of the suspension system. In reality, at ride height spacers do not add preload. The definition of preload is to exert more force on a spring and the ONLY practical way to exert more force onto the spring is to add more weight to the truck. This concept confuses most people and understandably so. When a spacer is installed the spring needs to be compressed more in order for the spacer to fit. Naturally, it seems that the spring is compressed more and therefore has more force exerted on it (increased preload). However, this is not the case when the spring/spacer is placed back on the truck. The weight of the truck compresses the spring to the same amount because there is no additional weight on the suspension. Figure 1, below, illustrates this concept.
Figure 1. Three scenarios showing what happens when a spacer is installed.
Scenario 1 shows a coilover assembly when it is removed from the truck. For simplicity, this example assumes that there is no preload in the spring, that is, 20 inches is the free length of the spring when it is removed from the assembly. With a spring rate of 100 lbs/in, a weight of 500 lbs (to simulate the weight of the truck) will compress the spring 5 inches to an overall length of 15 inches, as shown in scenario 2. In scenario 3, a 2 inch spacer has been added and the 500lb weight placed on the assembly. Notice that the overall assembly is 2 inches longer (which gives the lift) but the spring itself is still only compressed to 15 inches.
So you can see that a spacer does not change the spring rate nor does it add preload.
The same spacer principle is also applicable to adjustable shocks like Sway-A-Way's. The coil spring doesn't know the difference between a spacer and a big adjustable nut.
So, in short, a spacer by itself does not make the suspension stiffer, however, the orientation of the A-arms and coilover change, thus changing the leverage on the coil spring. Figure 2, below, illustrates this concept by showing what happens when the IFS is lifted by adjusting the coils.
Figure 2. This illustration depicts an exaggerated IFS lift in which only the coil assembly is changed.
The picture on the left shows the condition that places the most leverage on the coils -- the A-arms are straight out and the lever arm X is at it's maximum. At this position it takes less force to compress the suspension than at any other position. The picture on the right shows what happens when the suspension is lifted and the lever arm Y is reduced. Since the lever arm is reduced when the A-arms are at this position the suspension feels stiffer. Of course there are more things going on than what is described here; for instance, the coil assembly angles change, the tires scrub on the road, the location of the attachment point of the shock, etc. However, this should show why spacers or coil adjustments might make the suspension feel stiffer.
A final note on spacers: it is also said that spacers can cause premature sag in the springs. Well, this is partially true. As said above, at ride height the spring is not compressed any more than normal when a spacer is used. However, when the suspension is completely compressed down to its bumpstops and a spacer is used, the spring is compressed more than it normally would. This can cause the spring to permanently deform, depending on how much, how often, and how long the spring is compressed. Also remember that the spring doesn't know the difference between a spacer and a big adjustment nut so this can happen with adjustable shocks too.
Torsion Bars
A lot of the same principles used in coil sprung suspensions are transferable to torsion bar sprung suspensions. For this reason I will not go too in depth into the explanation of how tbars work.
"Cranking" the torsion bars involves rotating the anchor point of the tbars to provide lift to the truck. Fundamentally, this is the same as using a spacer in a coil suspension. Just like a spacer in a coil spring, cranking the tbars does not add preload, nor does it raise the spring rate. Remember, the only practical way to add preload is to add weight to the truck and the only way to change the spring rate is by changing the geometric and material properties of the tbar.
Since cranking the tbars does not change the spring rate, why do people say cranking the tbars makes the ride stiff? Well, the perceived stiff ride comes from the change in angles of the control arms, just like the coil sprung suspension.
Summary