Fork Spring Preload???

My stock preload also measured 22mm. I currently run 15mm with 21lbs springs. Adjust for taste.

For a sanity check you should have about 3/4" static sag, another 3/4" when your on it, perhaps a little less with the heavy springs. To stay on the trail you nees some downward travel available, ya know?
and like MAC said, adjust for taste.

My stock springs/spacers had about 25mm preload. I replaced them with .38 race techs and cut new spacers to give 10mm per instructions. I like this set up much better, especially on rutty rocky downhills. I can just point it and go and carry more speed than before. I weigh about 155.

The race techs are a little longer than stock so you're going to have to cut a new spacer or you'll end up with way too much preload (30mm or so) with the stock spacers.

I know what you mean about adding preload to make up for weak springs like Kawasaki apparently did. I haven't done the calculations, but my feeling is the total force needed to compress .41s with 10mm preload will catch up and pass the .35s with 22mm pretty early on in the fork stroke.

I don't know what I'm missing...but I've figured and refigured the stock preload. It's a lot closer to 30 than 25mm. Ridiculously too much in any case.

Braahp sent me a fork spring calculator awhile back. I don't know if the equations involved take into account the INcreased rate as the spring compresses or not...maybe just a linear compression of X&so many kg/mm....in any case:

Comparing .41@10mm and .35@22 shows the .41 passing the .35 (force-wise) at approx. 65mm travel.

At 30mm preload for the stock spring, that doesn't happen until 110mm travel.

BUT.......

It's the spring rate INcreasing as it's compressed (rate INcreases, load capacity DEcreases) that's a big factor with the different springs. With a whatever rate spring and 10mm of preload, you're close to the rate of the spring. Take that same rate of spring with 30mm of preload, and the rate goes up considerably...AND THE LOAD CAPACITY GOES DOWN.

Use exaggeration to make the point. Take a spring 1/2" long that's rated at .35kg/mm and a 72" long spring that's rated at .40. Compress them both 1/4". You're not going to be seeing .35kg/mm any more on the first spring..and the second spring will easily be .40...for a LONG time to go, too! Another 1/4" and what do you get? Effective failure of the first spring to suspend anything, and the .40 hasn't broken a sweat.

One of the reasons for considerable preload is to ensure you still have SOME after the aftermarket cold-wound spring changes length over time. Certainly a bandaid over a gaping wound..but that's the way it is.

Which is why hot-wound springs are superior. Ask jeremy (mx-tech) 'bout that. (He's the one that educated me on the issue).

dirt bike dave said:

IMO, a soft spring with excessive preload will result in an overly harsh ride on small sharp obstacles, yet still be too soft for the big hits

Click to expand...

That's been my impression also.

Yet, I think (not sure) that Braahp prefers a somewhat softer spring with more preload. I know he was working with that awhile back:

http://www.dirtrider.net/forums3/showthread.php?threadid=43664&highlight=fork+springs+calculator

What's it all come down to? Adjust to taste, of course. My spring/preload choice comes from trying multiple setups and choosing my preference. If someone else doesn't like it....that's nice, but I honestly don't care! ;)

The point of filling a fork tube that is supposed to suspend stuff with a piece of solid steel completely escapes me.


...imo..........:)

Exactly...adjust to taste. I ended up going with the .39kg springs and stock spacers. Works best for me in these ol' TN hills. Doesn't fair to well on a MX track but the woods are where my heart lies. Owenlo if you want the fork spring calculator just email me at RippinR@citlink.net and I'll be glad to email it back.

i believe the rule of thumb,sag 20-30 % of total travel.i too agree it's what u like far as 10,15,20 mm whatever but if the sag is wrong u have the wrong spring or preload #'s need a look.that has been my exp.

Let me add that I'm using stiffer than ordinary Gold Valve shim stacks in addition to my lighter spring more preload setup.

Canyncarvr - I'm no expert, but I think your logic is a little off on the spring rate. Springs are usually designed to achieve a constant spring rate (except for the progressive rate ones). If they are 10 lb/in, 10 pounds will compress them one inch, add another 10 pounds and they compress another inch. Theoretically, this would work until the spring bottomed out. So, theoretically the preload does not effect the spring rate, it just changes the amount of weight needed to begin movement in the fork.
My guess is the preload will affect the sag a lot, but not make much of a difference in the spring rate.
How this affects the overall handling -well I'm still trying to figure that out.

Originally posted by FLBob
So, theoretically the preload does not effect the spring rate, it just changes the amount of weight needed to begin movement in the fork.

Click to expand...

Thus making the small-stuff, harsher, no?

A few cents' worth here: pre-load affects ride-height [=sag] and not much else.

Also take Gearloose's point, though, that an excessively preloaded spring will be harsh on small stuff, in so far as it's running a kind of negative sag.
B.

Maybe my disconnect is, I suspect std fork springs are somewhat progessive, and that 'progessive' springs are only more progressive. Perhaps just because a spring is not labeled progressive doesn't imply is has no progressive qualities. That could explain the difference FLBob's very correct 'theoretical' explantion, and the trend in riders reports that high preload appears to result in harshness in the first 1-2" of travel. ? :confused:

Preload can change the "stiffness" of a spring altogether. And I have yet to experience this harshness others speak of using gretaer amounts of preload.

Click here for the latest addition to the JustKDX home page Tech Tips. Hopefully it will answer most of your preload questions. It still needs some format changes and the picture added but there should be enough there to get you started.

David