Front Forks
Two adjustments are available on most front forks, preload and fork oil (it's level and weight). Preload raises and lowers the bike in the front and might make a small change to the front fork's spring rate. Increasing and decreasing the fork's oil level changes the fork's spring rate and changing the weight of the fluid changes the fork's compression and rebound damping rates.
Fork's Spring Rate
The front fork's spring rate is made up of two parts, the metal, coil spring and the air trapped in the fork. For this discussion the metal spring is a straight wound spring, meaning that it's spring rate is constant as the spring is compressed. There is also an air spring inside the fork caused by the air trapped in the fork that is compressed as the fork is compressed. The air spring is variable in it's rate, having a higher spring rate as the fork is compressed and lower as the fork is expanded. The beginning air spring rate is determined by the fork oil's level, the higher the level the higher the air spring rate will be.
Preload
Increasing the preload compresses the metal spring which will raise the bike's front end. Increasing the preload by 1/2 inch may not raise the bike's front by a full 1/2 inch because the air spring in the fork may have gotten lighter allowing the bike to sag just a little. Because the fork's air spring may be lighter at higher preload the fork's overall spring rate may be lower. Most people say that increasing the preload will increase the spring rate, they are wrong. Another group of people say that increasing the preload doesn't affect the fork's spring rate at all, they may be wrong. Raising the preload on the front fork may lower the fork's spring rate because it has no effect on the metal spring but it may increases the volume of the air spring, which will lower the air spring's spring rate. Whether the air spring's rate is lowered or not depends on how the preload adjuster is built. If you raise the preload by taking the top off of the fork and put in a hollow tube you will lower the spring rate of the fork because the hollow tube contains an air space that's now been added to the front fork. More air space equals lower air spring rate. If you use a solid bar you will not add air to the fork and you will not change it's spring rate. If you use a hollow tube to add preload and want the spring rate to be the same as it was, you must add fork oil to lower the amount of air in the fork until it is the same that it was before you added the hollow tube.
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Fork Oil
Most people think that the only thing that fork oil does is change the damping rate. The damping rate is changed, getting higher the thicker the oil is and getting lighter the thinner the oil is. But the other thing that fork oil does is change the fork's spring rate. The spring rate goes up as the oil level is increased and goes down as the oil level is decreased. This is a major part of front fork maintenance that's over looked by most people, that they can change their fork's spring rate just by changing the amount of oil in the fork. An example of how this works is that in the year 2000 I bought a brand new Yamaha VMax and during the ride home I noticed that the front springs were too soft. When I got home I drained the fork's oil into a measuring cup and found that each leg had 10 oz. of fluid. The shop manual called for 22 oz. of fluid in each leg, which I filled the forks with. The ride was much better and the forks were much stiffer. I tried 24 oz. and that was too much and settled on 23 oz of fluid to get the spring rate that I wanted.
The metal spring in our fork is a straight wound spring with a constant spring rate and the force pushing the fork apart is increased by the same amount for each inch the fork is compressed. The air spring has a rising rate so that it's force increases more with each inch the fork is compressed. At full extension the air spring has almost no effect on the spring rate yet at full compression the air spring could be much greater than the metal spring's force. It all depends on how much oil is in the fork.
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