Here is one for you. For instance lets say on a new KDX 200. If one used a 40kg spring with 10mm preload vs. a 39kg spring with 20mm preload(ie. stock spacer on KDX).... what would be the actual difference? You see what I'm asking? Which setup would end up being "stiffer" or "plusher"? Just how does this work out in technical terms? Lighter spring-more preload or slightly heavier spring with less preload?
spring/preload...not your average question
- Thread starter Braahp
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Whoa nellie......... thats some heavy duty stuff in that link! Interesting stuff. In a nutshell.... I wonder if I'm actually getting a stiffer rate with the 39kg spring with more preload than the 40kg spring with less pre. Points to ponder...Hmmm.
this is hard to explain without pictures. but, depending on how much preload is added, the softer spring can be made to react like the heavier spring at different areas of the stroke. picture this, a graph of force vs. suspension travel, with the reaction force created by compressing the spring on the y-axis and the amount of suspesnion travel on the x-axis. for this graph, the line will be straight with a positive slope equal to the spring rate of the given spring. no matter what, a .44 kg/mm spring will alway have a lesser slope thnan a .46 kg/mm spring.
now, here is were the preload comes into play. by adding preload to the .44 spring, the line begins to shift in the positive y-direction. so, the spring now produces an additonal x amount of force at every level of travel, which is directly related to the amount of preload. evently, the line for the .44 spring will intersect with the line for the .46 spring. from the point of intersection, the spring will be stiffer in the negative x-direction, and softer in the positive x-direction. for example, i ride an 01 520 mxc that has .42 kg/mm spring in the forks stock. by adding 5 mm of preload, i can get the stock springs to be stiffer from 0 - 1/4 travel and softer for 1/4 - full travel than a .44 kg/mm spring. in my opinion, this is a perfect woods setup, but not good for motocross. i could add enough preload to make the .42 equal to the .44 at mid stroke, but this would make the .42 spring too harsh in the upper stroke. it would aslo cause the race sag of forks to lessen.
added preload to a softer spring will always have its advantages and disadvantages. it will make the upper part of the travel stiffer, but the latter part travel will still be softer than the next stiffer spring, unless you add an enormous amount of preload which can then cause the problem of coil bind. but like i said, i was able to add 5mm of preload to my stock springs and it worked for me. the next joe might not like it. plus, i saved 80$. if you ride mainly motocross, i would definately recommend getting the next stiffer spring. for woods, i think it is actually better than the next stiffer springs.
i'm ready for the questions.
now, here is were the preload comes into play. by adding preload to the .44 spring, the line begins to shift in the positive y-direction. so, the spring now produces an additonal x amount of force at every level of travel, which is directly related to the amount of preload. evently, the line for the .44 spring will intersect with the line for the .46 spring. from the point of intersection, the spring will be stiffer in the negative x-direction, and softer in the positive x-direction. for example, i ride an 01 520 mxc that has .42 kg/mm spring in the forks stock. by adding 5 mm of preload, i can get the stock springs to be stiffer from 0 - 1/4 travel and softer for 1/4 - full travel than a .44 kg/mm spring. in my opinion, this is a perfect woods setup, but not good for motocross. i could add enough preload to make the .42 equal to the .44 at mid stroke, but this would make the .42 spring too harsh in the upper stroke. it would aslo cause the race sag of forks to lessen.
added preload to a softer spring will always have its advantages and disadvantages. it will make the upper part of the travel stiffer, but the latter part travel will still be softer than the next stiffer spring, unless you add an enormous amount of preload which can then cause the problem of coil bind. but like i said, i was able to add 5mm of preload to my stock springs and it worked for me. the next joe might not like it. plus, i saved 80$. if you ride mainly motocross, i would definately recommend getting the next stiffer spring. for woods, i think it is actually better than the next stiffer springs.
i'm ready for the questions.
What is the tube diameter and oil height for the Kaydex?
I have a program that will estimate the effects of rate, preload and oil height on total wheel rate.
If you take oil level (the air spring) out of the equation, you will find that preload ONLY ALTERS THE STEADY STATE RIDE HEIGHT. The next mm of travel from steady state will be at whatever rate the spring geometry defines. If the spring is softer the force on the spring is less and vice-verse.
Where the higher preloaded spring will feel stiffer is if the wheel drops into a depression (like a whoop) and extends beyond the steady state length. The spring extends as far as it can - but the preload makes it act like it already has extra load on it so the first several mm of travel may well have more force with the soft spring / heavy preload.
Bottom line, soft spring / heavy preload will jolt the bars less when you run over a root. Soft spring / heavy preload will jolt the bars morein whoops or wheelying over puddle.
These are my observations. Personally I run soft springs (for my weight) and heavy preload.
I have a program that will estimate the effects of rate, preload and oil height on total wheel rate.
If you take oil level (the air spring) out of the equation, you will find that preload ONLY ALTERS THE STEADY STATE RIDE HEIGHT. The next mm of travel from steady state will be at whatever rate the spring geometry defines. If the spring is softer the force on the spring is less and vice-verse.
Where the higher preloaded spring will feel stiffer is if the wheel drops into a depression (like a whoop) and extends beyond the steady state length. The spring extends as far as it can - but the preload makes it act like it already has extra load on it so the first several mm of travel may well have more force with the soft spring / heavy preload.
Bottom line, soft spring / heavy preload will jolt the bars less when you run over a root. Soft spring / heavy preload will jolt the bars morein whoops or wheelying over puddle.
These are my observations. Personally I run soft springs (for my weight) and heavy preload.
Braahp - to make a 39 spring stiffer than a 40 over the full range of travel it will take at least 6mm of preload. if you understood my explanation and would like some more information, i have a spreadsheet that calculates the reaction force of the fork spring throughout the travel and graphically depicts the reaction force vs. travel as i spoke off of in the explanation. if you are familiar with excel, i can send you the file. email me at ml36_ktm@yahoo.com
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