Fork Spring Jig

Questions and comments about converting to beefier forks..
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patreilly
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Fork Spring Jig

Post by patreilly » 10:10 pm Jan 11 2017

I built the fork spring test jig. Used a pc of 3/4" conduit welded to a small plate. I tested a spring using 1 1/2" of preload and the 1 kg = 2.201 # Factor. I loaded spring with 5# which would be 2.2717 kg. The spring compressed 67.5 mm.
Divide 2.2717 by 67.5mm = .03365 kg/mm. What am I doing wrong?

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Tedh98
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Re: Fork Spring Jig

Post by Tedh98 » 10:17 pm Jan 11 2017

what is the purpose of preloading the spring?

I think if you try it without preloading the spring you will get a more reasonable number.

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Re: Fork Spring Jig

Post by patreilly » 11:22 pm Jan 11 2017

Jason, An extremely knowledgeable member, on a previous post suggested not to measure the 1st inch of travel. Made sense to me. It won't be that great a difference (10 X ). Stay tuned, Jason will respond with the correct answer.

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KDXGarage
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Re: Fork Spring Jig

Post by KDXGarage » 02:11 am Jan 12 2017

I can't say I am extremely knowledgeable, but thanks. :grin:

OK, take a base measurement, then add weight until it gets to 1.0" compressed. Write that down. That will be your starting point, not the difference from 0" to 1.0". In my experience, it is just not reliable.

Add more weights until it is 2.0". Write that down.

Do the same from 2 to 3.

Average those two numbers. That will show you how many pounds it takes to compress it one inch, which can be converted to Kg per mm.
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Tedh98
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Re: Fork Spring Jig

Post by Tedh98 » 06:54 am Jan 12 2017

Now that makes sense. There was no way 5 pounds alone
was compressing the spring that much.

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Re: Fork Spring Jig

Post by patreilly » 11:26 am Jan 12 2017

Ted and Jason, You are right! We need stricter measuring stick laws in this country. It is down right dangerous letting a U.S. SAE trained boy (read old man) handle a metric measuring stick. Ted, last night after getting in bed I said, "wait a minute. 70mm is almost 3". That spring isn't compressing any where near that. I went back out and measured again this morning. The spring is compressing 6mm. It would calculate to be a .37mm/kg spring.
I thought and mentioned that I was off by a factor of 10X. That got me to thinking. I was placing a decimal point 1 place off...... Walla that's it. Thanks for sticking with me.
You know I can't even figure out what day it is since they brought in that metric system..... (Jim from Taxi after he didn't show up for work for a week.)

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Re: Fork Spring Jig

Post by KDXGarage » 07:00 pm Jan 12 2017

You can be as good a mechanic as Ladka. :cool:

It has been years since I did a few sets, but the measurements were quite accurate and repeatable.
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Re: Fork Spring Jig

Post by capetowndave » 02:11 pm Aug 28 2019

Thanks very much for this info guys. I'm sitting in exactly same position as patreilly was... also have the 470mm springs in my 2006 200 and scratching my head wondering if they are stock as manual says otherwise and what the rate is... your efforts have given the answer. Many thanks!

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