Measuring Fork Spring Rate Trick
 

I've bought a few used bikes and knowing what the spring rate is in the forks and shocks has always been important to me. I came up with this trick, and I am surprised with how accurate it is.:D
For the fork springs I remove the forks. Remove the small air bleed screw on the fork cap and place the bottom of the forks in a clamp to keep them vertical and balanced.
Measure the distance from the bottom of the fork to the seal with the forks extended all the way until there is no pressure on the spring.
Then, measure the distance from the bottom of the fork to the seal with the forks with a weight on top of the forks. I used an Olympic 45lbs. weight which ways 20.4 KG, bounce the weight a little to overcome friction.
Figure the difference in length. On my 4357 WP forks from my 2003 EC250 the difference was 2.25of an inch or 57.15 mm.
To figure the spring rate KG / mm divide the mass of the weight by the difference in fork length in mm. For me it was 20.4 kg divided by 57.15, which equals .356 kg/mm.
This seems a little light for even stock springs. Usually in the past the numbers have been with in a .01 kg/mm so I believe they are just light springs.
I'm thinking I will have to go up to .44 kg/mm springs for my 200lb body on the EC250. but that seems like a big jump. Maybe I'll try .42 for woods riding.
Hope this can help someone in the future.
http://i1180.photobucket.com/albums/...ps9ce64c6a.jpg

I really didn't trust my results so I used a spring calculater.
http://www.racingsuspensionproducts....ing%20rate.htm
Outside Dia. 1.5"
coil Diameter.187"
35 active coils

And I got a rate of .40KG/MM which seems more reasonable.

I even tried the original test with the fork cap unscrewed and the outer fork leg all the way down. I still got 2.25 inches for the difference.
I wonder if the spring weakens with age? Cold?

You're probably better off removing the spring completely from the fork leg. Slide it over a dowl/broomstick handle, and then throw your weight on the spring. Use your calculations.

Ya, I tried it with the outer fork tub all the way down so the only thing it was riding on was the damper tube. Same result.

Tried doubling up the weight just as a control?

Stock springs are usually around the .42 to .44 range on the gassers. Not uncommon for most to go a little firmer again.

I measured the other fork and it was a little firmer, .37. I'm going to try doubling the weight to see if I get better results. I ordered new springs, .44, so when I get the old springs out I'll get a better idea of why they are measuring low and when I put the new springs in I'll show you how this is supposed to work.

i weigh 195 lbs, and i put .46 springs in my forks, and a 5.6 kg rear spring in the back end. my ec 300 rides like a 50 horsepower couch! never bottoms on either end and yet it is smooth and supple all the time. don't be afraid of buying the springs that will support your weight. bigger springs don't equate to being harsher, in fact just the opposite is true. it doesn't really matter where your riding, you still want to spend your time in the top 1/2 of your available travel, not in the bottom 1/2, by having to weak of springs to hold you up.

So I got the new springs in my forks, along with new bushings and SKF fork seals (which I'm really impressed with). I measured the new .44 kg/mm springs the same way and they worked out to be .436 kg/mm. The new springs according to the spring calculator worked out to be the same rate, .435. My guess is the old springs must have lost some tension with age.

Is this going to be whipped into shape for the spring Theilman ride?