KDXRIDER.NET

This is good info!

Clicky!

1. The formula for figuring spring rate:

RATE 11.5 x Wire Diameter to the fourth power (D4) ÷ by 8 x Number of Active Coils x Mean Diameter cubed.


2. And THIS:

PRELOAD, CONFUSING THE ISSUE (read it)

The 'first inch' part is very good. An excellent reason to change the stoopid preload settings on the OEM KDX.

Please...no one take issue with 'first inch'. Of course it's the first ANYthing, inch or whatever. That's understood amongst the scholars present.......

Ideally, a light preload will let the wheel react to small bumps properly.

Oh yeah!

Which is WHY the OEM preload is SUCH a terrific idea!!!

Wait..that's not right..................

interesting. I went with very light preload on my KDX forks , and stiffer XR springs. I was happy that they were much less prone to dive on braking (especially down steep rocky hills), but they still had problems. I'm no suspension tuna, so I couldn't get them to react well to lots of little bumps AND not bottom on moderate jumps.

After putting on the KX forks, I rode pretty slow at first because I was worried about the whole thing flying apart. The first thing I noticed was how well the front handled the little 4-6" bumps at say 20 mph. it was like they weren't there. I took it to a track and while they're too soft for the big jumps, they were worlds better than the stockers. The stock forks on a modern MX track can easily kill you. Granted a woods bike isn't meant to jump MX style, but I feel a lot safer now.

CC - The general information is good in the article; but the spring rate will vary enough that it is not useful for accurately determining rates on unknown springs due to different types of wire used. Testing using a bathroom scale will actually be very accurate if you can accurately measure the compression distance of the spring or using a pull scale will also work well. The wire alloy, stress relief of the material and whether it is cold or hot wound factor into the spring rate.

Very useful if your looking at cutting down springs to increase their rate. Something that I have not seen discussed on any forums is the use of two different springs in damper type forks like come on the KDX. That is - two individual springs (in the same fork leg) with different rates to vary the control of the fork action. A number of people use to do this before the cartridge forks became common place.

well this has kind of sparked my curiousity into finding out the rate of the three sets of springs i have. I measured the active coils on each one and they came out to 26,27,28. Now would the 26 be a softer spring than the 28 or is it the other way around? Im trying to go by the forumula and it is starting to hurt my brain.

My stock springs are marked .42

where are they marked at??

Right on the end on the side. They are etched in so you may have to look hard.

nope, nothing on any of the three. I am trying to follow the forumula but does anyone know what the "mean diameter cubed" is?

OK Mr. CC!

Take this quiz, give us your answers and you score....

http://www.smihq.org/public/quizzes/quiz90.html

Spring rates are determined by how many pounds of force it takes to compress a spring one full inch.
To rate an unknown spring:
11,500,00 x wire diameter to the 4th power
---------------------------------------------------
8x(spring ID + wire diameter) cubed x active coils

example:
wire diam .362; Spring ID 2.575; active coils 9.2

11,500,00 x (.362)^4
-----------------------------
8 x (2.575+.362)^3 x 9.2

Your rate is 105.9


To figure active coils:
hold spring upright and start from the bottom.
When the flat end coil comes in contact with the first coil, that's zero. Up from there count the number of turns until it touches the other flat end coil. In most cases it won't end up on an even number. Divide the full turn of the spring into 10 units, take this and use it for the fractional determination of the spring coil where the flat coil touches again. You will end up with an active coil number such as 9.6 or 7.8 or 8.5 ....

o ya i knew that.... do you think my dealer would be able to find the rate quickly??

so the mean diameter is the measurement of the wire plus the inner diameter?

If I was you... what I would do and to avoid all this mumbo jumbo is test all the springs on a scale... determine which is lightest to heaviest. Mark them this way and due to your weight, start with the lightest and see where they stand as being right for you. If they are way too light... jump to the heaviest... if they are slightly light, jump to the middle pair.

Of course all this is assuming they are of different rates.

Sometimes you just gotta try it!

i got 23.004, does that make any sence or no?


I did,

11,500,000*(.1875)^4
---------------------------
(8*(1.25+.1875)^3*26)

=23.00467137

.1875=wire diameter
1.25= Inner diameter
26= active coils

**all measurments are in inches***

No... doesn't sound right...

Did you....

hold spring upright and start from the bottom.
When the flat end coil comes in contact with the first coil, that's zero. Up from there count the number of turns until it touches the other flat end coil. In most cases it won't end up on an even number. Divide the full turn of the spring into 10 units, take this and use it for the fractional determination of the spring coil where the flat coil touches again. You will end up with an active coil number such as 9.6 or 7.8 or 8.5 ....

My head hurts from all those #'s. I took the test and guessed randomly without reading the actual question (I read the first 3 and new I was way out of my league) and scored 53%.

i still got 26 on one 27 on the next and 28 on the last. Now with the number of 26 or 27 or 28 i divide that by ten?

Ryan: What I would do... 'is test all the springs on a scale..' too! A BATHroom scale.

HoMedics makes a good one!!

(Like I know this is going to work out)...

Using PVC or some dowel..configure a tool to test your springs on a good (digital) bathroom scale. If you're lucky, your scale will be pound/kg selectable The tool needs to work as a spring guide to keep the spring straight and to provide a positive stop at a given distance when it's compressed.

Ex: Compress the spring 10mm on the scale. Convert the pounds to kg if you need to. A .38kg/mm spring in the this case will 'weigh' 8.36 pounds.



Anyway......

This is fun. I figgered the formula as simply something interesting to post, not anything etched in stone. Obviously what the spring is MADE out of matters..so figuring a spring rate based on # of coils, diameter and thickness leaves a whole lot to be desired.

Or...what IC said.

'What rate spring do I have' is a common question. I figured a little math would shut some of THAT up!!

BTW, 'No... doesn't sound right... '

Yes, it does.

23lbs/inch=.9055+lb/mm= .41kg/mm

Considering the plus and minus of things, margin of error etc, I'd say Ryan measured a .40kg/mm spring. More correctly, a spring that is supposed to be .40, but is in fact .41!!

..obviously I missed something, but I don't know what.

Good job, Ryan!!

BTW#2 'OK Mr. CC! Take this quiz...'

My dictionary don't even have 1/2 those words in it!! Besides, I didn't say I knew anything about it..I just copied and pasted!

Like me?

dang cc your pretty smart. How did you switch that to the .41?? I also have a spring with 27 and then 28 coils and i would like to figure out the weight. Thanks