Commonly asked questions like, "What's the difference between spring rate and preload?" and "Can't I make the spring stiffer by cranking in more preload?" require delving a little deeper into the mechanics of spring forces.
A stiffer spring would also start at zero force, but its compression rate would increase at a steeper angle, as in Figure 2. Notice that at 20mm travel, the softer spring requires only 10kg force, while the stiffer spring requires 20kg. The stiffer spring at 1.0kg/mm is both twice as steep and twice as stiff as the softer spring, which is 0.5kg/mm.
Let's back up and define what spring rate and preload really are. "Spring rate" reflects the stiffness of the spring and is measured in kilograms per millimeter or pounds per inch. One of the ways to test spring rate is to first measure the spring's "free length"-the uninstalled length-and then put weights onto it, measuring the resulting compression with the addition of each weight. "Straight-rate" springs maintain a constant rate of compression throughout their travel. If you are testing a straight-rate spring and you plot these points on a graph, you will end up with something that looks like Figure 1.
Now let's take the original spring and install it in the fork. As it's installed, it gets compressed, or preloaded, a small amount. "Preload" (or "preload length") is the distance the spring is compressed from its free length as it's installed with the suspension fully extended.
Just a note here on the difference between preload and preload adjusters: All bikes that I am aware of have preload. Some bikes do not have external preload adjusters, but they do have preload. All forks can have preload adjusted internally by changing spring spacer length, though sometimes it takes special spacers. Forks that have external preload adjustment have preload even when set at the minimum adjustment.
The "preload force" is the initial force the spring exerts on the end of the fork tube with the fork fully extended. Referring now to Figure 3, you can see that when preload is added to the spring, it effectively shifts the curve (line) to the left proportional to the amount of the preload force. In this case, the preload length is 35mm and the resultant preload force is 17.5kg at zero travel. In other words, with this 0.5kg/mm spring and a setting at 35mm preload, you would have to put more than 17.5kg force on the end of the fork tube to create any movement at all.
For a straight-rate spring, the relationship between force, spring rate and travel is described by the equation: F = K x L (or force [F] equals spring rate [K] times length [L]).
When you tighten the adjusting collar on a shock or increase the preload length by tightening the adjuster on the fork, you are indeed increasing the initial force exerted by the springs. This decreases sag, making the bike ride higher. It does not, however, increase the spring rate.
For example, you can achieve a targeted sag on the fork even with a spring that is too light (or soft) if you use a lot of preload. You can also achieve that same sag with a spring that is way too heavy by using very little or no preload. Let's look at just one fork leg on paper. Refer to Figure 4 and notice that the softer spring has 35mm preload and therefore has 17.5kg force at zero travel (fully extended). Let us assume the front end has 30mm of static sag. At 30mm of sag, the total spring force is 32.5kg. This means that each fork spring must push up 32.5kg to create a 30mm sag figure. Any combination of spring rate and preload that gives 32.5kg force at 30mm travel will create the same sag. Notice the stiffer, 1.0kg/mm spring has 2.5mm preload and, at 30mm travel, also creates 32.5kg force. This means they will both have the same sag; however, they will perform totally differently.
The quality of the ride will suffer with a spring that is either too soft or too stiff. The spring with a rate that's too soft will dive and bottom easily because the spring doesn't provide enough additional force as it gets deeper into the travel. The spring that has a very stiff rate will feel harsh, like it's hitting a barrier or very stiff spot.
A few more measurements will show if your spring rates are in the ballpark. Set the sag to standard settings (see Technicalities, August '95) and then measure the "free sag." "Free sag" is the amount the bike settles under its own weight. Use exactly the same procedure as when checking static sag, but without the rider on board. Street and roadrace bikes require 0 to 5mm of free sag on the rear, but should not "top out" hard. "Topping out" occurs when the suspension extends to its limit. It should barely have enough force to top out without the rider on board. If it takes a lot of force to compress it at all, you can bet it needs a different spring. On the front, expect to see 5 to 10mm of free sag.
When the static sag is correct and the free sag is less than the minimum recommended (e.g., it tops out hard), you need a heavier spring rate with less preload. A lighter spring is recommended when the free sag is more than the maximum recommended.
Most bikes, but not all, are set up with fork springs that are too soft for aggressive riding. Keep in mind that personal preference, conditions and type of riding come into play when setting up suspension. Racers generally use higher spring rates with less preload than street riders. When in doubt, consult a good suspension tuner.
Static sag for a particular bike and rider combination is dependent only on spring rate and preload. Springs are position sensitive and only care where they are in their travel, not how fast they're being compressed. Damping settings, on the other hand, are dynamic forces. In other words, damping forces only occur when there's vertical suspension movement. This means they do not affect static sag, as sag is measured when there is no movement.
Just a couple of final thoughts about air and oil level. The addition of air into the fork tubes on models with air valves will have a huge effect on both sag and harshness. We don't recommend the use of air as a tuning variable because harshness generally increases a lot for the relatively small benefit in bottoming resistance. It is almost like adding spring preload. The use of additional air is quite effective on touring bikes for temporarily changing the load-carrying capacity, like when packing double. Oil level changes will affect the total spring-type force, but only as the fork gets to the bottom of its stroke and, therefore, does not affect sag.
Now, of course, we haven't even talked about damping and how this affects the overall ride and handling characteristics of a sport bike-but that's another story.