fork spring preload...

kmx125r

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May 23, 2000
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#1
Hello All,

I visited the Race-Tech site and ran through the suspension settings program for me and my bike (KDX250SR). It was recommended that I use .4038 kg/mm fork springs. I have a pair of .41kg/mm springs on their way to me as I type.

The recommendations also said to have 10mm of preload on the springs. I suspect this means to drop in a 10mm spacer. Is that correct? Any recommendations for material to use? I would assume that the spacer should go in before the spring since the forks are inverted. Any advice would be greatly appreciated.

Thanks.
 

dirt bike dave

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May 3, 2000
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#2
The 10mm spring pre-load is a standard recommendation for many bikes. However, IMO, the KDX250 is a special case. It has very slow steering geometry, and I wanted the bike to turn quicker, so on mine I ran the Race Tech springs with NO preload. Actually, there was about 3-5mm of 'negative' preload, space between the spring top and fork cap.

My thinking was this - no preload lets the forks respond more quickly on rocks and roots (very important with the stock fork valving, which has too much high speed compression). It also helps the front dive down for quicker steering under braking. I also had a WER steering damper and Race Tech gold valves. Combined with the slow steering geometry, headshake was never a problem. My buddy also did this on his KDX250; I ran mine like that for 5 years, he had his for 2.5 years.

No problems at all, worked great for all types of riding, mostly woods. But I'm sure someone who hasn't tried it will be glad to tell me I'm wrong and it would never work. :cool:

If you do decide to use preload, the springs come with a bunch of large washers of the right size, maybe 1 or 2mm thick. Insert enough washers ON TOP of the spring so that the spring is compressed slightly when the cap is installed. You will have to make some measurements. After you put the spring in the fork (fully extended), measure the distance between the top of the spring and the top of the fork tube. Deduct the amount for the fork cap. The distance + 10mm is the amount of washers you will need. You will have to compress the spring 10mm before the threads on the cap will start.

One other tip when you are working on forks: Make sure the top fork pinch bolts are loose. Otherwise it will be very hard to loosen or tighten the fork caps. And the fork caps DO NOT need to be tightened very much (maybe 20 foot pounds max), even though they have a big nut on them.
 
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kmx125r

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#3
thanks...

DBDave,

Thanks for the good info and tips. I will mostly be riding MX style terrain. No big stuff, but a few "gentle" doubles... I do venture into the woods from time to time as well...

I appreciate your help.
 
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