2003 kx250 forks

[bebite]

Has anyone ever fitted 2003 KX250 forks onto a 1990 KDX200? If yes, what is all that is required?

[SS109]

Pretty much the same as any other conversion. Swap stems and use the complete KX250 front including wheel and axle.

[bebite]

I have an opportunity of getting the whole front end from a 1995 yz125. Any comments on how those perform compared to the stock 1990 KDX forks?

[Chuck78]

I have an opportunity of getting the whole front end from a 1995 yz125. Any comments on how those perform compared to the stock 1990 KDX forks?

Compared to old school 43mm damper rod forks, those are a pretty fair upgrade, and are very similar to '91-'95 KX125/250 forks (& '91-'96 KX500). That was the last year of Kawasaki and Yamaha running 43mm open chamber forks. Suzuki had already begun working with Showa on Twin Chamber forks '94-'95 but they were still small diameter (43mm iirc), and I believe Honda shortly after as well. The early closed/twin chambers were a new design and parts are not as common, so in that era, the KYB 43mm inverted cartridge forks are the best choices.
However, if you are a heavier rider with a fast aggressive riding style, 185lbs-190lbs is a reasonable rider weight limit for 43mm inverted forks due to where they flex when pushed really hard. 140lbs-165lbs riders will find these forks excellent when set up with proper re-valving for off-road riding. The 43mm tubes on lightweight early inverted forks will flex more readily when pushed really hard and fast by a heavier rider, and cause a lot of damping irregularities in the form of stiction.



I was digging for you, trying to find out some more info on those forks, but all I could quickly turn up in a few internet search results pages was confirming my suspicion that they were still 43mm forks. They may likely be mid-valve forks, as the KX models first debuted the mid valves in 1995, and 46mm diameter tubes in '96 (same as the YZ), so they may be a bit more advanced with a second compression shim stack on the underside of the rebound piston in the middle of the fork. This adds a second compression damping mechanism in addition to the base valve - which on open chamber forks is located in the botttom of the fork tubes. The mid valves have a large gap .15-.30 or greater between the damping inletband face shims on the shim stack, a "float" gap that allows damping oil to flow freely with no restriction at low fork compression speed movements and only starts adding resistance and deflecting the shim stack on very high speed movements. Basically it adds extra damping resistance on the hard deep stroke hits (high speed riding or jumps / dropoffs). The prior forks with no mid valve are much easier to set up with just a RaceTech Gold Valve base valve piston set, and do just fine for tight slow and mid speed woods riding, but aggressive racers or desert riders etc, those who see a lot more high speeds, will appreciate the mid valve damping. I believe the mid valves on some of the earlier KYB forks are a bit tougher to gain access to internally if wanting to make changes, so some just simply make all valving changes to soften the base valve only.



YZ fork info from my search:

-1991 to 1995 : 43mm open chamber

-1996 to 2003 : 46mm open chamber

-2004 : increased to 48mm but still open chamber

-2005 : 48mm closed cartridge fork introduced (AOSS design only used in '05 model year)

-2006 to 2018 : 48mm AOS closed cartridge fork (Air/Oil Separate, a closed chamber or what Showa calls Twin Chamber design), Yamaha markets them as the KYB S.S.S. forks - "Speed Sensitive Suspension."

Twin Chamber/Closed Chamber forks are sort of the ultimate, but also, not necessary for most woods riders, as the advantages only outweigh the added maintenance intervals and more difficulty valving plush for off-road riding for aggressive racers.Open Chamber forks with no mid valves will perform just as well or better for most average woods riders, and are simpler to rebuild.

You'll quite possibly need to do a stem swap with the KDX, but with more swap bearing size availability in the KDX frame i.d. / bearing o.d.
in a 28mm and 30mm bearing i.d. to fit most MX type fork swaps typical of KDX, and 12mm bearing height, if the steering stem is the correct height for the KDX frame (some Yamaha and Honda stems are much closer than most '92+ KX stems and all Suzuki RM stems), then you may be able to swap the forks just with a "conversion bearing" size - mostly it's just the upper bearing that's the different stem diameter size, 7-5/8" is roughly the KDX/KLX/KX500 stem height from the lower bearing seat to the top of the frame, so around 7-1/2" from the lower bearing seat to the bottom of the threads roughly is what you'll need there. the 12mm tall 28mm and 30mm conversion bearing sizes will require a 3mm spacer which All Balls sells or can easily be turned on a lathe by any machinist.

If you want an easy swap, get a pair of Honda XR400R forks (all years) and front wheel with it's larger 17mm axle, + brake caliper (or any '95+ KDX caliper), or same parts from the almost identical Suzuki DR350 last gen '96-'99, the gold leg/silver axle lug Showa 43mm right side up forks. Do a Race Tech Gold Valve base valve piston upgrade and their custom valving spec'd for you, and you will be quite happy most likely, as these are excellent forks for most trail riders. They come with .39kg (DR350/350SE & 1st year or two of XR400) and .40kg or .41kg springs stock (all later years of XR400R), which will suit 145-195lb riders pretty well. They have external damping adjustment, and are lighter than the KDX forks by quite a lot. I'm not sure about the steering stem height, but they have aluminum steering stems, so if they are close to the same height as the KDX, then I'd most definitely run the Honda triple clamps. Being 43mm fork tubes same as 89-92/95-06 KDX200's, the KDX triples will work also 89-92 KDX's have awkwardly positioned handlebar mounts integrated into them, so the XR400(?) or 95-06 KDX triples are preferred


I hope this information overload is helpful for you.

Best of luck with your project. '89-'92 KDX's greatly benefit from fork swaps, and '93-'94 with their obscure (for parts sourcing) 41mm KYB inverted forks as well.

[bebite]

Thank you for the detailed explanation Chuck 78. I did some reading on this site before I bought the 2 KDX's I now have (89 and 90) and saw alot of "negative" comments about the stock forks. I was expecting them to be terrible but quite honestly I think they perform well most of the time for my time of riding. All my riding is tight single track. Mainly second gear riding. The only time I find they dont work well is large bumps and series of stutter bumps. Its almost like they lock up. If I go ahead with the yz125 forks I'll post some updates. Tx again.

[Chuck78]

Thank you for the detailed explanation Chuck 78. I did some reading on this site before I bought the 2 KDX's I now have (89 and 90) and saw alot of "negative" comments about the stock forks. I was expecting them to be terrible but quite honestly I think they perform well most of the time for my time of riding. All my riding is tight single track. Mainly second gear riding. The only time I find they dont work well is large bumps and series of stutter bumps. Its almost like they lock up. If I go ahead with the yz125 forks I'll post some updates. Tx again.

At any rate, I'd recommend some slight shim shuffling in the base valves, and on a 43mm fork, it may be a good idea to add Race Tech Gold Valves for a slightly improved flow on the base valve pistons, AND theor online digital valving software access to get a custom tailored valving stack recommendation.

On the KX forks, '94 saw larger base valve pistons than 91-93, & mid valve compression added in '95. So the '95 YZ probably have the better OEM base valve piston as well.

Make sure to watch instructional videos on fork revalving. The base valves have an aluminum threaded post with a nut locking the damping assembly onto the valve body/plug, and the post is then staked/peened over, acting as a permanent locking nut retention. You have to file off the end including slightly into the nut, before removing the nut. Then use blue medium loctite 242/243 or similar upon reinstall. You'll want to remove a few of the larger valving shims around the face shim (largest diameter at end of stack) and add them to the other end where they function only as spacers. You can take the pyramid shaped shim stack arrangement and leave say two 20mm etc face shims, step down to a 14mm or 13mm etc shim as a "pivot shim," and then jump back to the previous order of the stack back up to say an 18, 17, 16, 15 etc down to 12mm or smaller. Some will say just remove the odd numbered (diameter number) shims, etc to make a motocross stack softer, but that's going really soft. You'll be adding them back to the end of the stack anywhere after the smallest shim, in an opposite pyramid sizing arrangement (just as spacers). The deeper you get into the stack, the "higher speed" the damping action is that those shims become active during. The first few shims are for lunch w speed and high speed damping. A "two-stage" stack with a pivot shim after the first 2 or 3 face shims will allow a bit more initial plushness, allowing the face shims to deflect more to flow more oil before the face shims contact the 18mm or whatever nect shim is after the pivot shim. Then the fadt the fork travel hit, the further down into the pyramid stack the oil-flow-induced deflection occurs.

Here's a Race Tech mid valve drawing. Rebound shim stack is on top (usually the rebound is good enough as-is, and mid valve compression stack at bottom. The float gap is always there under compression, and can be increased by using less valving stack shims, or decreases or rendered null by stacking more shims to add a lot of extra (unwanted) compression damping.

Some people will use just a face shim on the mid valve along with some "clamp shims" (the smallest diameter of the stack), and the light coil spring under the stack, to effectively convert the mid valve into just a check valve, offering basically no significant damping effect, and only as a check valve to close off flow one direction but allow damping oil flow the other direction.
This shows a 2-stage mid valve compression stack, as the third shim is significantly smaller diameter than the two face shims (largest diameter), and acting as a "pivot shim" since the stack jumps back up in diameter to the usual pyramid stack format beyond the small "pivot shim" that makes it a 2-stage stack. The "float" or gap between the face shims and the piston allow free flow initially up to a certain fork compression velocity, then the face shims deflect significantly iup to a certain point due to the pivot shim allowing more deflection of the face shims, and then the stack pyramid below that kerps things from getting too aoft and mushy by offering more resistance on the deeper harder faster suspension hits.
It takes a lot of trial and error of shim shuffling to get it perfect as a DIY'er, and even pro suspension tuners will often offer free revalves after trying out their initial revalve job, until the customer is satisfied. I've known of many people who paid good money for a pro revalve, but were very unhappy with the results. That's why a model-specific expert is very good to use, like Jeff Fredette for KYB forks. Buy your Gold Valves from Fredette Racing Products Off Road. He knows exactly how to stack the KYB shim stacks for our bikes. Or just learn to DIY it. ThumperTalk forum is your best friend for revalving shim stack info...

FMGV_3930C_REB_ASSY_2015_SHOWA_AIR_2_ST_OPEN_BENT_TXT_600.jpg (86.21 KiB) Viewed 7777 times

I hope this is all fairly understandable if you're unfamiliar.
If you can easily get into the mid valves, that's a more critical area to soften up and possibly remove at least 1 shim without replacing it, to allow more stack "float" gap.