Fork Force Spreadsheet

MACE

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#1
I'm working on a spreadsheet to find the static force (wheel rate)the fork exerts as a function of linear displacement. As variables I enter the spring rate in Kg/mm, the preload in mm, the oil height in mm, the male slider OD in mm and the total available travel in mm.

The oil height, travel and OD all factor into the "spring" provided by the air chamber. Q: about how much does the oil rise when the spring is inserted? I have to know this to get the starting air volume. If someone has an accurate scale weight of a fork spring, I could back out the volume displaced by the spring that way as well. You'll save me a partial teardown if you have these numbers handy. Right now I'm just using a SWAG of the spring displacing 1/2 the oil level.

It is also interesting that springs typically are measured in kg/mm when kg is a unit of mass, not weight. I had to squint a bit to get the words to match the music.

It's interesting to see just how little the force on the wheel varies with the ranges or rates we use. I guess that makes sense since the Heavy springs for my YZ are just 10% heavier than the stockers.

And harking back to my old "Question Authority" thread, my pet soft-spring-with-lottsa-preload setup has almost identical static wheel rate as Jer's reccommended .48 springs with zero added preload. My setup is softer for the remainder of travel and so easy on my hammered wrists.

Anyway, I can share the spreadsheet when it's done if anyone is interested. I think I might add fork rake and bike rider weight as a variable as well as that would be the next step in approaching the dynamic system.......... Or maybe I'll just go riding:cool:
 
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#2
stock fork spring out of 2000 KX250 ( claimed .43kg ) weighs out at 17.6 oz.

I use .29 lbs per cubic inch with tool steels , what are you using for spring materials?
 

MACE

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#3
Thanks P Taylor. I expect most steels have similar densities, but would look it up anyway.

I just got the idea that the best plan is to simply fill my forks up to the tip top through the air bleed hole and then reset my level and measure how much fluid I have to remove;) .

I'm not accounting for the volume of the spring and damper rod in the air chamber but I'll live with that error for now.
 

Jeremy Wilkey

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#4
Mace,
Very cool.. Like all things in science.. We prefect them as best we can for what we know then we test and refine.. I've found that, What we don't know is not the problem. We begin with what we can test and work from there..

Jer
 

MikeT

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#5
Mace,
P_Taylor is very accurate with his .29 per cuin. Steel wieghs 490#/cuft.

490/1728 = 0.283 so he is accurate as long as your not launching the space shuttle. ;)

To get the volume of oil displaced, I'm not sure how that will help you. I think you might have to do that tear down because the entire spring won't be totally submerged in the oil at rest. The weight of the spring and unit weight of steel will allow you to obtain the volume of oil displaced ONLY when the spring is completely submerged. I mean once you find out the volume of oil displaced, you can then find out how much that same steel weighs...... Somehow I don't think thats what you want though. I have written several complicated engineering spreadsheets, so if I can assist you just send me a email or PM.

MikeT
 

MACE

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#6
Oh, you can do it...

but as you start to estimate the air volume displaced by the damper rod and the rubber bumpers and all the other stuff in there you are better off just measuring the actuals (as you said).

I expect the algebra involved would be pretty simple. Just too many estimations to bother.

One thing I don't know how to do on Excel is looping and conditionals. I'd like to be able to input the total travel (x) and have it create a table with displacement from 0 to x in 10 mm increments. Right now I just have a fixed size table and the answers are unstable when the table value exceeds x.

Gawd, I'm a dork nerd....
 

BRush

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#7
Originally posted by MACE
One thing I don't know how to do on Excel is looping and conditionals. I'd like to be able to input the total travel (x) and have it create a table with displacement from 0 to x in 10 mm increments. Right now I just have a fixed size table and the answers are unstable when the table value exceeds x.
I'm pretty up to date when it comes to driving Excel, particularly in writing VBA macros to do stuff like that. One method is to place a form button (i.e. "Calculate") on the main worksheet and tie it to a macro that generates a new, sorted table when you press it. If you need any help to get pointed in the right direction, let me know or drop me an email.
 

MikeT

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#8
In Excel there are several ways to do the same thing. If you are still stuck. Send me an email an we can talk. I'm sure I should be able to assist.
 

MikeT

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#9
Mace, Check your email

Get back to me.