06 kx250f forks

Questions and comments about converting to beefier forks..
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ico135
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06 kx250f forks

Post by ico135 »

I am in the middle of a fork swap, the bike is an 04 kdx 200 and new forks are off a 2006 kxf 250. To maintain the geometry i am lowering the kx forks 1"by installing spacers and will also add new spring perch grooves offset 1" from existing to prevent increasing pre-load. I am also replacing the existing .46 springs with new .42 springs.(single track, 39 year old, B/C rider). I have pulled apart the forks and this is the existing Compression(base) valve shim stack:

30x.10(12)

22x.15

30x.10

29x.10

28x.10

27x.10

26x.10

25x.10

24x.10

23x.10

22x.10

21x.10

21x.10

20x.10

19x.30

19x.30

19x.30

19x.30

22x.30

My plan is to remove 6 of the 30x.10 Lo-Speed shims from the Compression(base) valve shim stack and leave the others stacks as they are. But I defiantly open to suggestions or opinions. I am doing this to improve the handling of the bike but also as a chance to learn about suspension so feel free to lecture/rant/ridicule at will.
ico135
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Re: 06 kx250f forks

Post by ico135 »

Also does anyone know what the factory preload is for those 06 kx250f showa 47mm twin chamber. When i pick up stems monday i am going to drop of the parts to have the perch grooves placed and just had a thought that this would be a good time to increase or reduce the factory preload in addition to the 1".
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06 kx250f forks

Post by SS109 »

Wish I could help but I don't know squat about valving. I can tell you that you'll like those forks as I have the same on my KDX. Something else to consider is triple clamp offset. Stock clamps for the '06 250f is 25mm. Going to the 23.5 or 21.5mm clamps can really help with planting the front end better. Good luck!
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Re: 06 kx250f forks

Post by KDXGarage »

I would ask on ThumperTalk's suspension forum. I would think that not modifying the high speed would not be the way to go. Pull the 29, 27, 25, and not take out 6 LS shims??

Nevermind. I see you are already over there. :mrgreen:

Also, "KX250F", not kxf250. :grin:
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06 kx250f forks

Post by ico135 »

SS109 wrote:Wish I could help but I don't know squat about valving. I can tell you that you'll like those forks as I have the same on my KDX. Something else to consider is triple clamp offset. Stock clamps for the '06 250f is 25mm. Going to the 23.5 or 21.5mm clamps can really help with planting the front end better. Good luck!
Yeah i will look into the offset, thank i haven't even considered that aspect of it yet. Did you send your's off to be revalved and/or shortened?
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Re: 06 kx250f forks

Post by ico135 »

Jason wrote:I would ask on ThumperTalk's suspension forum. I would think that not modifying the high speed would not be the way to go. Pull the 29, 27, 25, and not take out 6 LS shims??

Nevermind. I see you are already over there. :mrgreen:

Also, "KX250F", not kxf250. :grin:
I am new to all this but i'll attempt to explain my logic. And will tell you in advance that all this somewhat ignores the effect of the mid-valve which is a mistake and not realistic. But i'm gonna run with it anyway.

So the harshness of the hi-speed hits(rocks,etc) is a result of two things. The velocity required to engage the hi-speed(HS) shims, and the stiffness of the HS shims.

By removing half of the lo-speed(LS) shims it allows for the HS circuit to engage sooner. So if I left the LS shims alone, and if they were stiff, then the medium to high speed hits would still be harsh because of the required pressure to engage the less-restrictive HS dampening shims would still be significant.

And the way I am envisioning this the stiffness of the base/compression valve is cumulative:
lo-speed hits - lo-speed shims effect stiffness
mid-speed hits - lo,mid speed shims effect stiffness
hi-speed hits - lo,mid and hi speed shims effect stiffness
So by reducing the stiffness of the LS shim set, you actually reduce the stiffness of the overall base/compression valve.

So removing 6 of the LS shims, it reduces the velocity/pressure required to engage the less-restrictive HS valves in addition to reducing the overall stiffness of the compression valve.

Also i would like to make a small change at first considering i have never ridden these forks. That way i get a base line on them and can make further changes as needed once i get a feel for them. So i was thinking this would improve the forks, but still allow me to get a base line. But take all this with a grain of salt because a few weeks ago all i new about forks were that the had a spring and some oil in them. Engineering background and a newly found slight obsession on the the topic has hopefully got me heading in right direction though.
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Re: 06 kx250f forks

Post by ico135 »

Jason wrote:I would ask on ThumperTalk's suspension forum. I would think that not modifying the high speed would not be the way to go. Pull the 29, 27, 25, and not take out 6 LS shims??

Nevermind. I see you are already over there. :mrgreen:

Also, "KX250F", not kxf250. :grin:
And yeah i havent got a ton of feedback overthere. One suggestion was to go to this:

30.1(10)
28.1
26.1
24.1
22.1
20.1
18.1
16.2 clamp

His logic was to remove the crossover saying that " make it a single stage fork stack since you're already dealing with the relationship of the mid valve and the base valve working at the same time. A multi stage base valve just adds to the complexity when troubleshooting."

Which i defiantly think it has merit and I am considering taking the suggestion of removing the crossover, but going with a slightly different shim stack.
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Re: 06 kx250f forks

Post by KDXGarage »

I sure as heck would not pull 6, then start using that as a baseline. The second you get back on the internet asking for help, they will tell you something way different than what you are planning and you can do all the work for baseline 2.0, then start again. Since you mentioned engineering background and obsession, then I will put you in the "future never satisfied, always tinkering" category. :grin: Take notes and don't be afraid to make big changes to get a feel for it.

Be aware of posts from ten years ago on THEN suspension theory, as it may not be the same as 2018 way of dealing with 12 year old forks. :grin:
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06 kx250f forks

Post by SS109 »

ico135 wrote:
SS109 wrote:Wish I could help but I don't know squat about valving. I can tell you that you'll like those forks as I have the same on my KDX. Something else to consider is triple clamp offset. Stock clamps for the '06 250f is 25mm. Going to the 23.5 or 21.5mm clamps can really help with planting the front end better. Good luck!
Yeah i will look into the offset, thank i haven't even considered that aspect of it yet. Did you send your's off to be revalved and/or shortened?
I have a local tuner I like to work with. He understands my terrain and what I'm looking for in my suspension. No shortening of the forks. I slid the forks up in the clamps. I'm currently about 20mm above the clamp up to the cap. No problem with hitting the fender or bottoming but I don't do 80ft MX jumps either.
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Re: 06 kx250f forks

Post by KDXGarage »

That would be a great test to see if you can slap plastic or not with the front tire. I won't sign up for it. :shock:
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06 kx250f forks

Post by SS109 »

It's not a worry I have and I do jump it and ride hard. Even on the worst G-outs I have never gotten the travel less than 1.5" of bottoming which gives about .75" of clearance to the fender. Like I said, my tuner knows what I'm doing and what I need. It works perfectly.
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ico135
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06 kx250f forks

Post by ico135 »

SS109 wrote:
ico135 wrote:
SS109 wrote:Wish I could help but I don't know squat about valving. I can tell you that you'll like those forks as I have the same on my KDX. Something else to consider is triple clamp offset. Stock clamps for the '06 250f is 25mm. Going to the 23.5 or 21.5mm clamps can really help with planting the front end better. Good luck!
Glad to hear rides great and good to know about the 20 mm. I have been toying with the idea of only shortening .5" and not getting spring perches re-grooved for now. I hate to ask for back to back favors from machineist friend and i am getting more and more exited to get project wrapped up. So this would give me a little extra pre-load, but going from .46 to .42 spring that may not be a bad thing. Then would probably still have to bring up in risers some but not much.
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06 kx250f forks

Post by ico135 »

Thought i would do a little update on this.

Ended up buying a 20 ton press from harbor freight. Got both stems pressed out in about 10 minutes. Just wished i would have bought press a few weeks back when i replaced the linkage bearings. Oh Well.

The kdx stem looks to be about .01 smaller than the kx stem so i am going to drop stem off to machinist friend this week. He said he can add anywhere from .01-.02 so should be good to go.

I got my eibach .42 spring in the mail and installed. I scored a set from a damaged box, but new, for 59 dollars and apparently they make most of race tech's springs so should be set. I removed a few of the base valve compression shims as well so we will see how that works out. Did have a damaged bushing in my Pivot Works rebuild set, but they were awesome when called and they will have me a replacement here Monday.

So project is moving along.
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Re: 06 kx250f forks

Post by KDXGarage »

Cool. Good luck on it. :bravo:
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06 kx250f forks

Post by ico135 »

So got stem back, pressed in, and new bearings and races. I have everything mounted back up and the lower triple clamp doesnt seem to be seating as close to the frame as i expected. I know people say you have to build up the steering stops. but for some reason i was thinking each side to prevent turning angle, not up to actually make contact with the steering stop block. Image link below, Any thoughts?


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Re: 06 kx250f forks

Post by KDXGarage »

As best I remmber, ...

so KDX stem? It is taller in the clamp area than the newer clamp. This causes what you are having a problem with. One of the things RB does is to "lathe down / cut down on a lathe" the thickness of the KDX stem in that area.

The bearing did not go down all the way because the stem was too thick in the area that is now above the new clamp.

I hope this makes sense.
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Re: 06 kx250f forks

Post by ico135 »

Jason wrote:As best I remmber, ...

so KDX stem? It is taller in the clamp area than the newer clamp. This causes what you are having a problem with. One of the things RB does is to "lathe down / cut down on a lathe" the thickness of the KDX stem in that area.

The bearing did not go down all the way because the stem was too thick in the area that is now above the new clamp.

I hope this makes sense.
Alright i have a second kdx stem still in the triple clamp. May press it out and get some measurements on it to see if this could be the issue. Jason what year kx front end do you have? Is you steering stops on level with the stop block?
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