How to accurately test fork spring rate

5 replies to this topic
  • osheen

Posted April 10, 2009 - 01:28 PM


If you want to check your fork spring rates yourself here is a good way. It does take a little bit of metal fab work, raw materials and a dial indicator.

I took a piece of clean pipe with an OD that was close to the ID of the spring and welded it to a piece of 2" square tubing so I could clamp it in a vice. Using a lathe, I also faced the weld bead around the pipe so the spring sits squarely on the bottom. I also welded a piece of flatbar off to the side of tubing for the mag-base dial indicator to sit on.

With the spring on the pipe, I took a piece of steel roundstock (With a hole in the center same ID as pipe OD) weighing 18.00 kg (almost 40lbs) and set it on the spring. This compresses the spring about 3" and lets the spring settle in to it's actual rate and gives a more consistent reading. Then place the dial indicator under the roundstock and set to zero. In doing this, I lightly tap on the bench to vibrate the whole set-up to make the spring settle in and take any friction out of the equation. The indicator will always come back to the same spot. With indicator at zero, place another piece of roundstock on top of first. My piece happens to weigh 8.74 kg. Watch dial indicator while tapping on bench again. When you get it to consistently come back to same number, write it down.

OK now time for a little math lesson..... Springs are measured by how many kg does it take to compress spring 1 mm.
Please forgive my metric/inch units as my indicator is in inches and everything else is metric.

Example: It took 8.74 kg to compress my spring 0.700" What is the rate ? Convert 0.700" to metric- .700 X 25.4 = 17.78mm.
8.74 / 17.78 = .49 kg/mm

This was my actual test for my 08 YZ450 springs. I was in disbelief since they were supposed to be .47 kg/mm. Thinking I made a mistake, I went back and tested some springs that I previously tested and came back with same numbers as before. I bought a set of .45 springs from Factory Connection and measured one to be .452 and the other .454. So I know my test is good......

Other notes- With my roundstock test weight of 8.74 kg. , a .49 spring gets compressed 0.700" and a .45 spring gets compressed 0.761" . A difference of only 0.061". (I know the numbers seem backwards, but a stiffer spring won't be compressed as far, thus making a smaller number). You have to be able to measure accurately and repeat your own test. A few thousandths of an inch make a difference in the outcome. A broomstick and a tape measure won't cut it.

In chossing a weight amount for the "test" weight, It must be an amount that will use up much of the dial indicator's movement but not too much that it will max out the indicator. The 8.74 kg piece I used just happens to compress the springs in the 17-20mm range, perfect for a 1" (25mm) indicator.

The spring can't be allowed to bow out when compressed. Too much slop in the pipe size or weight holes will cause inconsistent readings.

I am a machinist with a generous supply of materials so It was easy for me to find what I needed. They were remnants that were just laying around. You wouldn't have to use the exact same weight amounts. I weighed mine using our UPS shipping scale.

Whew...... I'm exhausted.... :p

  • SXP

Posted April 10, 2009 - 08:17 PM


That seems simple enough - thanks again:thumbsup:

  • motox88

Posted April 11, 2009 - 05:18 AM


Think you might be able to post a pic of the setup?

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  • RCannon

Posted April 11, 2009 - 06:05 AM


Thats a cool set-up....I think its better if you build it and we'll send our springs to you, with a check enclosed.

  • ace402

Posted April 11, 2009 - 06:47 AM


Thats a cool set-up....I think its better if you build it and we'll send our springs to you, with a check enclosed.


  • RCannon

Posted April 12, 2009 - 07:19 PM


Sorry if I caused trouble here...I did not mean to. I was thinking of how far out of my league building something like this would be.

It would be cool if a company could test these for us.

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