stock fork spring - did some rate measurements - Wrist Twisters
 
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post #1 of 8 Old 03-04-2012, 10:04 PM Thread Starter
TEXAN
 
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stock fork spring - did some rate measurements

NOTE: This is by no means authoritative.

I used a simple set-up to derive the spring rate for a single, stock fork spring from a 2006 919. Now, this is a dual-wound spring, so it behaves like two springs in series until the softer spring fully collapses. That is to say, the spring equation is:

Ktotal = 1/(1/Ksofter + 1/Kharder)

I was not able to compress the spring far enough using my simple setup (drill press, digital weight scale) to collapse the softer coils, but I thought, maybe, the data could be useful to someone.

so, here is the data:

stroke (inches) | force (lbf)

0.00 | 000
0.25 | 10
0.50 | 19.5
0.75 | 30
1.00 | 40
1.25 | 49.5
1.50 | 60
1.75 | 70
2.00 | 82
2.25 | 92

The spring rate stays very consistent for the first two inches of stroke at 40 lbf/inch, or 0.714 kgf/mm. Of course, if you dial in max preload with all OEM parts, this takes up at least 25mm of the spring stroke. That gets you that much closer to collapsing the softer spring coils.

Like I said, it's not great, but I thought it might help someone. Setting up an assembled fork tube would allow me to test the spring through the entire stroke, but honestly, I don't care. New springs are practically mandatory regardless of the numbers.

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post #2 of 8 Old 03-04-2012, 11:14 PM
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did you go any further into the harder part of the spring?

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post #3 of 8 Old 03-04-2012, 11:52 PM
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Somewhere on here is a post with the rate - pretty sure it was .75 or similar so your calculations appear pretty good! I may have been LDH who measured them.

Anyway they are too soft!

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post #4 of 8 Old 03-05-2012, 12:14 AM
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ha, you think they were soft? you should check out the f4i spring rates. i was told they were around mid .60's or so. MUSH!

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post #5 of 8 Old 03-05-2012, 04:17 AM
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Quote:
Originally Posted by Pvster View Post
ha, you think they were soft? you should check out the f4i spring rates. i was told they were around mid .60's or so. MUSH!
I think an f4i would be a lighter bike than a 919 though.

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post #6 of 8 Old 03-05-2012, 06:46 AM Thread Starter
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Quote:
Originally Posted by nd4spdbh View Post
did you go any further into the harder part of the spring?
No, I went as far as I could with it, that was it.

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post #7 of 8 Old 03-05-2012, 06:56 AM
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While similar, the springs in the 04s and later are a bit different.
It matters not.
Mush is mush is mush.

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post #8 of 8 Old 03-05-2012, 07:55 AM
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Quote:
Originally Posted by AllanB View Post
Somewhere on here is a post with the rate - pretty sure it was .75 or similar so your calculations appear pretty good! I may have been LDH who measured them.

Anyway they are too soft!
Check out thread Dan Kyle Racing Suspension

D K / L DH did a rig type check and posted results.

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