Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks

Questions and comments about converting to beefier forks..
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Re: Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks

Post by Chuck78 »

Pictures from Negative_Noel on Thumper Talk:
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'97 KDX220R - purple/green! - KLX forks, Lectron, FMF, Tubliss
'99 KDX220R project - '98/'01 RM125 suspension, Titanium hardware, Lectron Billetron Pro, Tubliss
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Re: Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks

Post by Chuck78 »

More, as well as someone's DIY milling of the triple clamp to gain the necessary steering stem height without reducing the available travel on the chrome tubes:
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.
'97 KDX220R - purple/green! - KLX forks, Lectron, FMF, Tubliss
'99 KDX220R project - '98/'01 RM125 suspension, Titanium hardware, Lectron Billetron Pro, Tubliss
'77 Suzuki PE250 & '83 Suzuki PE175 Full Floater - restomod projects
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Re: Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks

Post by Chuck78 »

I've got too many irons in the fire so to speak, but have gotten the stem pressed out of the bent lower triple that I had, as a test since the triple was not usable as-is. It easily came out with my 10 ton homemade hydraulic press.
I used my oxy-acetylene torch set to heat the VStrom DL1000 triple to remove it's steering stem, since I intend to use the RM or similar GSXR steering stem in the V-Strom triples perhaps, for some street bike fork swap projects. It came out even easier when heating the lower triple with the torches. No prob.
IMG_20230309_221025.jpg
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I got the longer V-Strom steering stem chucked up in my lathe but had to order a new live center for the tailstock, as my old beat up live center gave me about .002" runout.
As you can see from the photo, this will work out just about right, but I'm going to cut the bottom flange stop on the V-Strom steering stem about 1.25mm shorter to allow the stem to press up into the triple further, as it's cutting it pretty close on the shorter 30x47x12 upper bearing + 3mm spacer on top of the bearing, the bearing will be a few millimeters over the threaded portion of the stem. One nice thing about the V-Strom stem, is that it comes with TWO spanner wrench type threaded locking collars, with a toothed washer in between, so I believe the 3mm difference in bearing height I can just use one of these thinner locknuts as the spacer to make up the 3mm difference in height versus the OEM KDX bearing vs the 30mm i.d. conversion bearing.

Also got a nice new diagonal/diamond pattern knurling tool for stem swaps. I assume most people knurl the outside of the steel KDX stem, although I did read commentary of one person stating that their machinist said they had no tool to knurl the aluminum i.d., which I was surprised to hear someone even considering knurling the i.d. of the triple vs the o.d. of the stem.
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I also got a nice face plate for my South Bend Heavy 10 L lathe, so I can just bolt the RM lower triple to the face plate and shim it until it spins true to the bore, and cut the lower bearing seat down in the lathe. I suppose I could also have just made up a press fit stem to insert into the bottom of the triple from underneath with heat to the triple and a chilled shaft fresh out of the freezer, and cut the bearing surface down that way with the specially cut holding tool chucked into the lathe. That might actually be easier to set up, as the bottom of the cast triple is not guaranteed to be a flat surface perfectly perpendicular to the steering stem's machined bore (as well as the fork tube bores).

One other thing - the V-Strom stem will protrude a bit higher than what I'd hoped above the RM's (or DR-Z400E's) upper triple, so an aluminum spacer may be required, or else cut the stem threads down further and shave the top of the stem shorter in the lathe.


Before I determine how much of the RM lower triple's raised bearing seat step I can machine off, I'll need to pull the forks off of my '99 KDX220R rebuild & mod project bike and mock up a stock KDX / RM lower bearing (same size) to check clearance, as this is a "gullwing" type lower triple - as is typical for non-inverted forks' lower triples. The sides angle upwards, so I can't cut too much off to near flush, as the head tube of the frame then could be dangerously close to rubbing the triple.

I also plan to shave 1mm off the bottoms of the lower triples where the fork tubes slide through, as the KDX frame's taller steering stem / head tube height is pretty close to cutting into the travel range of these forks, and internally lowering at the top-out area of the forks, as is typical, won't fix this clearance parameter no matter how much you lower it internally.
'97 KDX220R - purple/green! - KLX forks, Lectron, FMF, Tubliss
'99 KDX220R project - '98/'01 RM125 suspension, Titanium hardware, Lectron Billetron Pro, Tubliss
'77 Suzuki PE250 & '83 Suzuki PE175 Full Floater - restomod projects
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Re: Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks

Post by Chuck78 »

I plan to run near zero preload on the fork springs in order to help "lower" the ride height up front (by allowing more sag with rider seated on bike), since it'll be quite a bit taller with the 97-98 RM fork dimensions vs stock KDX or 96 RM.
Running almost no preload is also highly recommended for a woods bike setup on twin chamber / closed chamber forks, to help overcome the ICS inner chamber spring, which does add some to the initial fork movement's spring rate but is there to control cavitation far better than an open chamber fork.

Stock spring rates in the RM125 are .40kg, and due to being Closed Chamber / Twin Chamber forks and having the ICS spring adding a bit to the initial force required to initiate travel, I absolutely am going to order some Cannon-Racecraft springs in .38kg (what Racetech recommends for me doing singletracks at 30-44yrs old and ***preferring softer spring rates***), and I'll probably also try out the Cannon-Racecraft ICS springs which are listed as a progressive rate starting at 1.2kg/mm and firming up into the 2.-something range. Racetech lists a chart for the ICS springs, and recommends for a woods/enduro/singletrack rider of 160lbs or less, to run their 1.0kg ICS spring. The CRF250X Showa Twin Chamber woods bike forks come stock with a 1.5kg ICS spring. Some people have said that going down to a 1.0 or softer than the fork was designed for just makes things out of balance, as the softer spring allows more cavitation, which alters the way the fork responds, and goes against the design intent and principles of the twin chamber / closed chamber design strategies. RaceTech lists the ICS springs on these RM forks as 2.7kg/mm, which I find VERY HARD TO BELIEVE, but just in case, since I have a stack of these forks now, I'll be able to head to the trail with my Showa fork cap wrench and multiple sets of base valves, since the 98's have the quick change base valve feature at the top of the forks, so I can set up 2 sets of base valves with stock and aftermarket ICS springs, and go for a trail ride with both back to back to test them out.
I have a feeling that the recommended valving which I've stumbled upon and noted in this thread, may behave differently if I go switching the ICS springs out, IF the RaceTech spec of 2.7kg/mm is in fact correct. if it's not, and the ICS spring is much softer, then there won't likely be any drastic side effects to playing with the ICS rate slightly.
Also of noteworthiness, Racetech offers AWESOME products and technical knowledge, but their enduro/singletrack/woods valving recommendations don't always seem to be the best advice... perhaps for motocross and definitely for street/road bikes, they offer a wealth of tuning knowledge, but for off-road trail riding, it seems that they maybe are not the top authority on valving and spring setup, although far better than most of us could do on the first try.

I've read solid arguments that people say that the fork spring rate calculator from RaceTech has too many variables, and that the wide range of results based on the 5 or so different input fields beyond rider weight, is misleading. The argument is that the fork springs really have a narrow range of acceptable variances, as the fork springs need to hold up the bike and rider's weight at a certain stroke height, as that is their job and the valving is then the one that is dependent on compression, travel, plushness, and to some extent bottoming or at least "blowing through the stroke too fast and then encountering harshness deeper in the travel." This does make a lot of sense.
Since I am trying to start out a bit deeper in the travel, I still will go with the Racetech "prefers soft" rate with old geezer inputs (although I look 30-34-ish yrs old, I'm only a few months from surpassing the age range inputted into the Racetech calculator of 30-44yrs old!!!). This way I can have a lot more downtravel in the nasty rocky rutted rooty sections to keep tire contact with the terrain, as well as effectively lowering the ride height of the forks.
I might even try out some of the .36kg/mm Cannon-Racecraft fork springs if the initial effects of the ICS rate + the .38kg/mm fork springs leave a little to be desired.
The stock KDX H-Series fork springs at .35kg/mm or .3475 or whatever they are, I was fine with in my first few years of riding dirt, but once I had just a few hours on the .375kg/mm KLX300R forks' stock springs, more firm fork spring rate that was quite apparent at first on the trail, quickly became much preferred to me as my trail speed at times is quite fast and aggressive nowadays. I think I can get these '98 RM125 forks set up really nicely with a little bit of trial and error, and I stand a darn good chance of getting it really nice on the first try thanks to the valving info I've found and posted in this thread.
Last edited by Chuck78 on 09:01 pm Mar 23 2023, edited 1 time in total.
'97 KDX220R - purple/green! - KLX forks, Lectron, FMF, Tubliss
'99 KDX220R project - '98/'01 RM125 suspension, Titanium hardware, Lectron Billetron Pro, Tubliss
'77 Suzuki PE250 & '83 Suzuki PE175 Full Floater - restomod projects
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Re: Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks

Post by Chuck78 »

I've also found out some other complimentary shock info...


All Showa 50mm piggybacks with independent high speed compression AND low speed compression adjusters on the reservoir:

2001-2004 RM125 shocks are common, DRZ400 or 98-00 RM125 lower spring seats are shorter and best used on these when lowering slightly, to allow full preload range. DRZ are same shock but body has different reservoir mounting position so cannot be used on the H series KDX.

2005-2008 RM125 and also RM250 (250 used KYB some years but these should be Showa) - same shock but 18mm shaft vs previous 16mm shaft, supposedly to allow more oil flow through the shaft damping passages for big hits.

2006-2018-ish KX250F - very similar to the 01-04 RM but longer clevis, the clevis needs the bottoms chopped off and the lower bolt hole redrilled to the same length as the RM shocks. around 2010 or so, a year before they switched to the SFF forks with one side spring one side damping, the KX250F shocks got a different shock reservoir cap that bulges out for a larger bladder capacity. Apparently these are a pain to remove and everyone uses the aftermarket reservoir caps which also have the extended capacity and a standard schraeder valve instead of a needle fitting.

2007-2018-ish RM-Z250 - same as the KX250F shock but fatter 18mm shaft vs 16mm, and a better extra-capacity reservoir cap design, which is an absolute necessity on these due to the extra oil volume displaced by the larger shafts. Same clevis as the KX250F, needs shortened in the same way.

RM-Z 250 shock 2007-2018-ish:
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One thing of note - the Kawasaki model shocks in the OEM parts fiches break down every single part. The Suzuki shock OEM parts fiches only show the shaft and piston and all attached pieces available as one complete assembly.
The aftermarket sealheads between the 18mm shaft 2005-2008 RM shocks vs the 2007-2018 RM-Z250 shocks don't show the same part number interchange despite being the same style of Showa 50mm shock body/cylinder AND the same oversized 18mm shaft, and no Kawasaki models use the 18mm shaft, so I'm not certain on any of the parts breakdowns for the larger shaft versions since they are Suzuki-only. the 16mm shaft is still significantly fatter than the stock KDX shock shaft, but the hollow shaft bores do have a lot to do with the damping abilities as far as flow capacity for the rebound especially. I believe the compression may or may not use the shaft flow as well, I wish I had a better knowledge base on that at this point prior to doing a teardown of these shocks.

All of these shocks in addition to the clevis shortening on the KX250F/RMZ250 versions, will be best to be lowered 4mm or so internally, or at least have the preload backed out, as the KDX linkage rocker will top out on the frame before the shock is fully extended, so the shock will never see the top end of travel otherwise.

I have some KLX300R dog bone suspension links, both the 117mm steel KLX300R/KLX250 links (like the KDX's 112.5mm steel links) that are a direct swap onto the KDX, and a set of identical length 117mm aluminum+brass bushings KLX300R links (thicker) with their longer mounting bolt hardware.
These will lower the KDX around 15/16" or 1-1/16" in the rear, but since these shocks are 1" taller physical shock length, multiplied by the linkage leverage ratio, I'm thinking that these longer links will be very useful in dropping the height down to match the taller forks at around 1" taller. Lowering links generally advertise that 1" lowering won't alter the suspension rate much at all (slightly alters the progressive / rising rate curve however), but the longer links for more lowering will really start to alter the geometry of the linkage significantly.

Being that the KDX came with 5.0kg shock springs and the RM125/250 came with longer links and softer springs at 4.6kg to 4.8kg, I think that the shocks will already be fairly plush mounted to the KDX (longer links and the KDX rocker will perhaps soften the MX valving some?), but wonder if it will soften them up too much with the longer KLX dog bone links, or throw off the rebound? I may fabricate my own links as well if I determine something around 115mm would be more ideal due to this and height reasons. The bushed alloy KLX300R dog bone links sure are nice though, but for my purple 96/97 plastics, I also have tracked down purple 1998 RM125 4.6kg and 1998 RM250 4.8kg/mm Showa 50mm shock springs, so I'm pretty dead set on being able to run those... I'll fab some links if needed...
If I send the shock off to have it professionally revalved, I may send a good working stock KDX shock with it, to mimic the valving traits of the KDX shock, but give me the taller Showa obviously, versus the now too short KDX shock (due to taller fork swap).

Racetech calculator says for B or C class enduro racer on singletrack terrain, age 30-44, 155lbs-ish no gear, who prefers softer suspension, to run 4.6kg rear shock spring and .38kg fork springs, so the shock spring on the KDX is not nearly as appropriate as the RM125 or RM250 springs according to that. Teknik Motorsports tells me I want .40kg fork springs, basing that just on holding up my bike + body weight and letting the valving do the rest. I'd lean towards the .38kg considering I'm trying to bring the height back down a bit and also allow more down-travel for dips and ruts in the terrain/rocks/etc to keep tire contact with the terrain over dips.
'97 KDX220R - purple/green! - KLX forks, Lectron, FMF, Tubliss
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Re: Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks

Post by Chuck78 »

(both photos show my buddy David on a mostly stock KDX200H picking the easier lines through the rocks on trail 11 at Redbird Crest OHV trails in Daniel Boone National Forest Kentucky, my (currently-) KLX-forked purple KDX is parked as I was snapping these photos to show people the gnarly terrain that can be found at these amazing and seldom visited trails)
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At Redbird Crest OHV trails in Southern Daniel Boone National Forest Kentucky or the Buffalo Mountain / Rock House trails in the Hatfield-McCoy trail networks of Southern West Virginia, I do always notice that in the really rocky sections that we we ride in, that I'm always banging my toes on boulders and big loose rocks, as well as smacking the skid plate a fair bit. I know that a firmer suspension would keep me up away from the rocks a bit further with more compression damping and/or .40kg springs, but I really like the plushness in this terrain of running .38kg fork springs, and backing off the preload adjuster on the KDX shock even more on our trip to Redbird last week. I think this was one of the factors contributing to me making it up sections that I'd never made cleanly before on the most challenging technical areas, but made rather cleanly on this most recent trip.
springs 1 step on the softer side and a good woods valving, plus the added ground clearance of the taller forks and shock, will definitely be an upgrade and fit in well with my suspension setup/upgrade strategies here...
IMG_20230316_162825~2.jpg
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(pardon the blurriness, I had to zoom in substantially on the camera to get the up close perspective, no time to run on foot up there while he was crawling through this section)

All the while, I absolutely love the way my bike handles with the stock KDX shock rebuilt and with preload set pretty soft/saggy, and the stock-ish KLX300R 43mm inverted forks. My other KDX220R badly needed a complete rebuild anyways, so I figured why not just go all out with the upgrades on this machine and go for some serious 49mm Showa conventional forks and a shock to match...the 49mm Showa RM conventionals or similar external appearance DRZ400E forks seemed the answer. I absolutely hate how often mud gets packed up into the seals on all inverted forks in our riding group's bikes... the ultimate conventional forks seemed the obvious answer, plus less chance of suffering rock damage to the chrome tubes since inverted forks are right down there in all the boulders and rock gardens we ride through on my favorite terrains...
'97 KDX220R - purple/green! - KLX forks, Lectron, FMF, Tubliss
'99 KDX220R project - '98/'01 RM125 suspension, Titanium hardware, Lectron Billetron Pro, Tubliss
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Re: Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks

Post by SS109 »

Man, you really dive deep in to it!

I'll give you a little food for thought. RaceTech has always seemed pretty spot on with their spring recommendations IMO/IME. For me, a 45+yo, desert, B rider, they recommend .38 up front a 4.74 rear, However because I didn't shorten my forks internally, and have them pushed up in the clamps, I went .40 up front to help keep the forks from hitting the fender/lower triple (really scary thought!) on big G-outs. At this same time I set the preload at zero on the springs. All I can say is that it works great and it soaks up the trail junk great. The stiffer spring with zero preload allows that first bit of spring movement to be really soft yet it holds the bike up in the proper sag range so the valving can do it's work for great handling everywhere. As I'm finally learning, it even handles riding the whoops very well yet still eats up the small stuff.

As to dialing back the preload on the shock spring, there is a contingent over at GasGasrider.org that swear by doing that very thing. I plan to try it on my Gasser once it's completed. They normally run at the stiff end of the spring range for their weight (if they are borderline they'll step up to the next stiffer spring IIRC), set it for proper sag to make sure it works for their weight, then dial back the preload adjuster until it is only preloading the spring 10mm. They say it really plushes out the rear but still allows it to take the big hits without hard bottoming. Of course, like I said, I haven't tried it myself yet so take it with a grain of salt.
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Re: Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks

Post by Chuck78 »

Another deep dive for ya, Kelly!
Doing a little bit of number crunching...

'95+ KDX rear ride height with a taller shock:
448mm length '01-'04 RM125
-vs-
420mm length KDX H-series shock.

So a 98-04 RM125 shock = 28mm longer than the KDX shock.

SS109 said his H-series KDX with an 01-04 RM125 shock rode significantly taller than stock.
He also said the RM125 shock was so much longer, that it couldn't install at full length, and that the suspension rocker linkage topped out against the frame and he had to compress the shock (if I recall correctly) around 1/4" total approximately, to get it to just mount on the KDX rocker linkage.

1/4" = 6.35mm. Round up to approximately 7mm for a touch of clearance.
95-06 KDX have 11.8"/300mm advertised rear suspension travel.

95-06 KDX shock travel is 127mm
01-04 RM125 shock travel is 140mm

That's 13mm additional shock shaft travel over stock.

So we'll figure we need to at minimum, lower the 50mm Showa RM / KX250F / RMZ250 shocks 7mm internally.
(KXF and RMZ Showas have substantially longer clevises, which need shortened and redrilled to match the RM/DRZ400 clevis length)


28mm longer minus approximately 7mm internal lowering spacer = 21mm taller than stock still.
28mm longer minus a 10mm sealhead lowering spacer 18mm taller than stock.

***The math below is *approximate guesswork* due to the complex engineering nature of rising rate linkage designs, although I had read heresay that the KDX has a fairly linear rising rate linkage.

300mm stock rear wheel suspension travel out of a 127mm travel shock, 300÷127 = 2.3622 *average* suspension ratio, rear wheel travel vs shock travel.

This isn't a terribly accurate method to calculate the difference in rear wheel ride height, but I think it'll be close at least:

21mm taller shock (after 7mm internal lowering spacer) x 2.3622 = 49.6mm taller rear ride height...
Wow, that's 1.95" taller.

18mm taller shock (after a 10mm sealhead lowering spacer) x 2.3622 = 42.52mm/1.67" taller ride height vs stock.

@SS109, does that sound like how much taller your rear fender sits? Granted @ 1.95" taller is at the rear fender position at the very back directly above the axle where wheel travel is measured, seat height would be less than 1.95" taller if the forks were only 1" taller or less, splitting the difference at about 1.5" taller.



So if we're at 1.95" taller at the rear fender, and 117mm KLX250/KLX300R suspension links would lower the ride height approximately 7/8"-15/16", and the '97-'98 RM forks are approximately 1-1/2" taller overall when installed in a KDX frame's taller head tube
(longer steering stem required, eats up the available travel on the non-inverted forks' chrome fork tubes - forcing the lower triple to be moved down further to accommodate longer steering stem), then I think we have a winner here... KLX links and 7mm internal lowering spacer, will equal a similar ride height increase that taller MX forks will give the front end...{UPDATE} - the '97-'98 RM forks are a lot more than 1" taller, more like 1-5/8"+ taller than stock KDX, & '96+ KX forks are as well - so the solution is to disassemble the forks after determining how much rider sag or race sag they give with rider seated on the bike, and then determine the proper amount of preload spacers needed, and remove some or all of the preload spacer stack height and make a top-out spacer that same height to install above the top-out spring, & this will then effectively internally lower the fork, but also reduce the travel. We don't want to go too drastic here, and I'd probably limit this to allowing 11.2" or 11.6" of fork travel still. The MX forks generally have 12.2" advertised travel, which realistically equates to around 11.6" travel with top-out spring forcing the forks to ride 1/2" into the "full travel" range. KDX advertises 11.6" travel KLX forks advertise 11.2" travel.


A Zeta 10mm sealhead lowering spacer + KLX300 pull rods aka dog bones aka suspension links should then lower the RM125 shock ride height to 3/4" taller than stock. Now THIS I think is where I really should be aiming for. Nothing too drastic, but 3/4" of ground clearance gained will help, but won't raise the center of gravity much, and will still allow me, at 5'10" & 32" inseam, to comfortably touch my toes or the balls of my feet down on the ground.

I'll have to hang the rear end from the shop ceiling and drop the shock bolt and measure the total available extension on the shock install height with the rocker bottomed out or topped out against the KDX's frame, and then I can fine tune the internal lowering spacer length needed if trying for maximum shock shaft travel. {UPDATE} - i think lowering the shock 10mm internally to only have 3mm more travel than the stock shock will be totally fine, as 117mm suspension links @ 4.5mm longer than stock will provide the wheel/swingarm with more leverage than stock links, and will initially give more shock shaft travel vs wheel travel, to the point where the entire available shock travel could not even be utilized, therefore I think this is the target I need to aim for, and then, say, if the rear height increase is 3/4" but the forks are 1-3/4" taller, we'd then lower front end 1" by lowering the forks internally 1/2" and externally another 1/2" by sliding the triples down 1/2" on the now "11.7" travel" modified height forks, to achieve a balanced ride height to maintain the KDX's 26.5° steering head angle, and at 3/4" ride height gain, we've only slightly lengthened the wheelbase up front due to the steering rake angle vs ride height.
I might try for 5/8" internal lowering on the forks then, and 5/8" of fork tube slid up through the triples, to get closer to matching the '94-'02 KX125's steering rake angle of 26°, which at having the rear about 1/4" higher than the front vs stock stance, should give me about 26.25° steering angle. Very nice for tight woods riding.
The 96-98 RM triple clamp offset (22mm? iirc) + the forks' axle lug offset equates to nearly the same trail geometry as a stock KDX, so we're not changing the bike's geometry much at all, only softening the initial rear suspension leverage but also running a more stiffly valved MX shock. I feel the valving should be fairly close with this combination.

I'd prefer the rear to be a bit taller than the RM125 / KX250F / etc forks height increase, so that I can back out the preload on the shock spring some to lower it via more seated sag. That'll allow more traction in the nasty uneven terrain when the wheel drops down in between rocks and ruts/roots/etc... So I might end up making some 115mm suspension links, although those KLX300R links are VERY NICE being thicker aluminum with bushings at the pivot bolts, vs the KDX 112.5mm and the other 117mm KLX250/KLX300R steel links, and I think the longer links will also soften the suspension action slightly and also perhaps soften the MX shock valving up enough that the high speed compression and low speed compression clickers might get me right into the ballpark of where I'd be happy with shock valving. The rebound might need some tweaking though.
Last edited by Chuck78 on 07:29 pm Nov 22 2023, edited 2 times in total.
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Re: Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks

Post by Chuck78 »

I'm still on the fence of whether or not I can reasonably run the .40kg/mm RM125 fork springs. I'm leaning towards .38kg Cannon-Racecraft springs due to what I've been reading about twin chamber forks' internal chamber pressure springs adding to the initial force needed to initiate travel. generally the answer is to run the fork springs at nearly zero preload to compensate for the small coil spring in the damping assembly. I also may just switch from what Race Tech alleges are 2.74kg internal chamber springs to a progressive rate Cannon-Racecraft Showa/KYB Twin Chamber ICS spring that is a 1.2kg-2.2kg progressive rate... Those are way cheaper than fork springs as well. That should make initial travel require less force, giving more bump compliance.


These forks as they sit stock, just compressing on the shop floor, really do not feel like typical harsh MX forks to me... I might even just put some fresh oil in a set and bolt 'em up to my 99 with an RM125 shock and KLX300R suspension links, and back out the compression clickers, and take 'er for a good test ride! I think it might surprise me, as I've read of a lot of woods riders being pleased with these forks bone stock.

I'm very pleased with my KLX300R .375kg/mm fork springs now.
Teknik Motorsports tells me that for a KDX220R at around 154lbs rider weight and slightly more with hydration pack, saw, phone, & a few tools), I need .40kg fork springs and 4.73 to 4.8kg shock springs on a KDX. I've got what I believe are two 4.9kg RM250 springs and one 4.6kg RM125 spring, all purple 1998 models that fit 98+ RM and KX250F 50mm Showa shocks.
Teknik is apparently of the way of thinking that the spring needs to support the rider's weight, and is less dependent on riding style, as the springs need to keep the rider's weight held up to the proper height in the fork travel regardless of terrain, age, riding style, etc.

Racetech has lots more options for tailoring spring rates to your needs, and tells me:
(inputs - singletrack trail riding, age 30-44, Novice/B-Class, standard stiffness, 159lbs (added a few lbs for water pack and saw), .38kg fork springs and 4.64kg shock springs. I have one KX125 1998 purple shock spring that is probably a 4.6kg (what RT says I need on the KDX for my 153lb rider weight without gear), and 2 springs that measure slightly thicker, one off an RM250 and one off an RM125, same thickness, probably the RM250 4.9kg or optional rates in between, 4.75-ish.
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Re: Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks

Post by SS109 »

Chuck78 wrote: 09:58 pm Mar 29 2023SS109, does that sound like how much taller your rear fender sits? Granted @ 1.95" taller is at the rear fender position at the very back directly above the axle where wheel travel is measured, seat height would be less than 1.95" taller if the forks were only 1" taller or less, splitting the difference at about 1.5" taller.
Yeah, that sounds about right. I can take a measurement if that would help? Remember, I only weigh 140 without gear and I'm currently running .38 up front and a 4.8 rear. I can get the rear to bottom out on a fairly regular basis and the forks will touch the fender on the nastiest g-outs once in a great while. I think you would be fine with the same but you never know until you try it!
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Re: Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks

Post by Chuck78 »

SS109 wrote: 12:29 am Mar 30 2023
Chuck78 wrote: 09:58 pm Mar 29 2023SS109, does that sound like how much taller your rear fender sits? Granted @ 1.95" taller is at the rear fender position at the very back directly above the axle where wheel travel is measured, seat height would be less than 1.95" taller if the forks were only 1" taller or less, splitting the difference at about 1.5" taller.
Yeah, that sounds about right. I can take a measurement if that would help? Remember, I only weigh 140 without gear and I'm currently running .38 up front and a 4.8 rear. I can get the rear to bottom out on a fairly regular basis and the forks will touch the fender on the nastiest g-outs once in a great while. I think you would be fine with the same but you never know until you try it!

Yeah if you don't mind measuring, that'd be greatly appreciated! Stock rear fender? UFO version? Maier MX KDX fender?

I'll definitely be running the thicker shock springs of the 3 purple '98 RM springs I have here, 2 of them are a 4.9kg or maybe a 4.7kg. White paint dot code for 1998 RM's, no clue where to find a chart stating the rates, but they list stock rate and optional rates in the parts fiches, just not correlating paint dot code or wire diameter.
Stock '98 RM125 was 4.7kg (62211-36E20), optional "firm" rate was 4.9kg (62211-36E30), optional soft rate was 4.5kg (62211-36E10).
Last edited by Chuck78 on 02:15 pm Mar 31 2023, edited 2 times in total.
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Re: Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks

Post by SS109 »

I have the Maier MX fender. Measurement with bike under it's own weight is 35.5" at center of rear axle up to fenders lower edge.
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Re: Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks

Post by Chuck78 »

SS109 wrote: 01:08 pm Mar 30 2023 I have the Maier MX fender. Measurement with bike under it's own weight is 35.5" at center of rear axle up to fenders lower edge.

SS109, I'm not exactly sure how you measured to get 35.5"???? posting photo below. center of axle to bottom of rearmost frame section seat frame - fender support tubing underside.
Last edited by Chuck78 on 01:27 pm Apr 01 2023, edited 1 time in total.
'97 KDX220R - purple/green! - KLX forks, Lectron, FMF, Tubliss
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Re: Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks

Post by Chuck78 »

SS109 wrote: 01:08 pm Mar 30 2023 I have the Maier MX fender. Measurement with bike under it's own weight is 35.5" at center of rear axle up to fenders lower edge.

SS109, I've got only OEM fenders here on both 220's. Can you check that measurement again, with the suspension topped out, from center of rear axle up to the bottom of the end of the frame tube, I believe there's a plastic cap in the end of each rear seat frame tube, the underside of the horizontal portion of the rear end of the seat frame tubes behind where they bend and turn down.

I've got 26-1/4" center of axle stright up to the underside of the last horizontal bit of seat frame tubing, pardon the tattered rear fender and sub-par dual sport tires, this is my old retired and mostly stock '99 220 that's slated for a rebuild ASAP:
KDX-rr-susp-ride-height.jpg
KDX-rr-susp-ride-height.jpg (77.2 KiB) Viewed 4501 times
That'd be a more accurate comparison as I have no Maier MX fender. THANKS!
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Re: Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks

Post by Chuck78 »

Here's the valving info that I meant to post here a while back:

negativenoel @ https://www.thumpertalk.com/forums/topic/544779-97-suzuki-rm125250-forks/?do=findComment&comment=14181921 wrote:
MID VALVE LOW SPEED COMP:
{starting stack from the mid piston face}
13.2x.10 mm : bleed shim
[6] 20x.10 mm
collar/sleeve 8mm o.d. - modified in length to create mid valve float
11x.10 mm
11x.20 mm
Base washer
Running .21 shim float on the sleeved shim stack { most important procedure of the build }any more than .25 float should require a check spring and retainer to be fitted {otherwise noisy}. i sanded that collar/sleeve to the required height always checking the size all around it with the vernier.

as follows from the piston face:
REBOUND
[2]20x.10 mm
10.2 mm
20x.10 mm
17x.10 mm
14x.10 mm
10x.10 mm
9x.20 mm
11x.2
12.5x2.5 washer
High speed dual stage race tech shim stack is as follows
20x.10
11x.10 :crossover shim
[6]20x.10
18.3 x.10 that's all i had,call it an 18 mm o.d.
16x.10
14x.10
13x.10
12x.10
11x.10
10x.10
9x.10 on a FMGV 2320 gold valve
The figure in brackets being how many of that shim ie:[6] and the rest is only a single shim.
running 420ml of Motorex 5wt in each leg

Image Image Image


I'll probably be leaning towards running that valving but perhaps with just a touch softer mid valve or some additional sleeve extender shims, based on what negativenoel had to say about not being able to run any more than .21mm of float.

Looking into getting some .37kg Eibach 983.037.1 springs, although it seems that they are only available from Australia at Tekinic Motorsports (for $$$$) and EibachDirect.com, the other Aussie sources wanted outrageous shipping, and Racetech lists the .39kg versions as avaialble, but the stock .40kg springs are probably worn in to give a realistic rate of .39kg by now, so no point...

Racetech recommended oil level for my inputs is 405cc's, which is the minimum of their spec range.

I'll also likely be ordering some of the Cannon-Racecraft inner chamber springs in their variable rate of 1.2kg-2.2kg

5WT oil or 0w20w Mobil 1 (reportedly very slick and works well in place of 5wt suspension fluid)

very minimal preload on the fork springs if possible (2.5mm?)

SKF Low Stiction Seals and wipers

I'll also be looking at the stock base valve piston vs the Race Tech FMGV 2320 Gold Valve, and comparing the shape of the piston's ports. If the stock is not as smooth of corners on the openings, I'll consider dropping $155 for the Race Tech Gold Valve kit to reduce potential for cavitation with the more rounded ports.



and more general valving and setup info:

Chuck78 wrote: 07:39 pm Nov 20 2022
Best advice on twin chamber / closed chamber forks is to use low stiction SKF seals on these forks
(-negativenoel on KDXRider.net)

From Heart_Of_Darkness on Vital MX:
The stock pistons have large ports, they're not a restriction.


I've been reading about Twin Chambers & cavitation vs plushness, ReStakor has amazing writeups about this - but it seems as if you can get away with a softer TC ICS spring if yourbase valve ports are shaped in a way to minimalize cavitation - so perhaps the FMGV 2320 (97-98) & FMGV 2040 could still perhaps be beneficial? I'll have to tear a pair of these down to check the stock port shapes soon. See ReStakor cavitation articles
-Chuck78


A good woods re-valve is a two stage base with greatly softened high speed, a somewhat softened mid with a float in the .25-.30 range, and correct rate fork main springs with minimal preload (there's a lot of preload from the factory).

*****Check those forks (you'll need to disassemble them) to see if the bottoming cup at the base is installed correctly. If it wasn't (and it's easy to not install it correctly) then the damping rod cartridge hammers it every time the forks compress. I had a set that happened to. It's very dangerous if you don't catch it. -gotanubike on KDXRider

Some have used this thread on DR-Z 400 49mm open chamber conventional fork valving as a basis for modeling woods valving on the Showa 49mm twin chamber valving on the RM fork swaps, although the compression piston diameters are different, the valving and shim sizes are not going to be directly compatible without some adjustments for the added flow of a larger piston etc:
https://www.thumpertalk.com/forums/topi ... ns/page/8/

also look at the Thumper Talk thread with a subject/title regarding How to make Showa Closed Chamber forks plush for woods riding. I'll post that link later if it's not in this jumbled aggregation of my notes elsewhere already.

------------

Drop the oil level down quite a bit lower then what RT recommends for the RM twin chambers, and work your way up. Measure your fork springs as my set had negative preload and if you do too then make some spacers to bring it up to 0-3mm.

I'm running this right now for woods but I'm always playing with the base/mid.

20X.1

18X.1
16X.1

14X.1

13X.1
12X.20

11X.20

11x.25
11.35X.40

Rebound - Stock
Midvalve
20X.1 (3)
08X1.35 (collar)
11X.20 (2)
Float - .0mm (??? really ???)



And here's another shim stack that a rider simulated through the ReStakor software:


burkeee @ ThumperTalk https://www.thumpertalk.com/forums/topic/1061716-96-rm250-woods-stacks/ wrote:
I've been doing lots of reading and also playing with the demo version of Restackor to get ideas on how certain changes effect the damping profile. For the base valve I'd just like to soften the whole thing up to re-center the clickers on my near full soft setting. I'll add a crossover to soften the LS and remove some from the taper to soften HS.


('96 shim sizes same size as KDX base valve compression pistons, 20mm pistons are 3mm smaller than 23mm '97-'98, use 17mm face shims vs 20mm)
BV Stock BV Mod

17x.1 17x.1

17x.1 11x.1
14x.1 17x.1
12x.1 12x.1
10x.1 9x.2
9x.2 11.4x.4 (4)
11.4x.4 (4) 12.54x2.54
12.54x2.54


Same thing for the rebound, I just want to make the whole thing softer and recenter the clickers. Accomplished by moving the crossover closer to the face to soften the LS and removing a few from the taper to soften the HS.

Reb Stock Reb Mod
17x.1 (3) 17x.1
9x.2 9.2
17x.1 17x.1 (3)
15x.1 15x.1
13x.1 13.1
11x.1 9x.2
9x.2 11.4x.4 (2)
11.4x.4 (2) 12.54x2.54
12.54x2.54


On the mid valve is where I'll try to help with the deflection off roots and rocks. I'll pull one of the face shims, which will also add some float.

Mid Stock Mid Mod

13.3x.1 13.3x.1
17x.15 17x.15
17x.1 (4) 17x.1 (3)
8x1.7 (collar) 8x1.7 (collar)
10x.15 10x.15
12x.15 12x.15

0mm float 0.1mm float (??? again, no mid valve float, initially??? seems odd.)

EDIT - I'm digging up a lot of good woods valving from searching ThumperTalk for Honda CR250R woods valving, as the '97-'02 CR250R, '97-'04 RM125, '97-'00 RM250, and '02 CRF450R all use the same Showa fork base valve pistons... This expands my search quite a lot for searching what shim stacks have worked for others in these "early showa 20mm piston forks." (the pistons are actially 23mm o.d., but the face shims (largest shims to match piston ports) are 20mm.
Also some good news for woods valving is that so many bikes 1998+ use the same Showa 50mm shock pistons, so I'm finding a lot of good woods valving stacks from CRF250X's (woods models), 06+ KX250F's, 97-08 RM125 and 97-00/05-08 RM250, 08-??? RM-Z 250, and even the DRZ400E. Most of these bikes except the RM-Z's have a similar spring rate on the rear shock as what I'll need, so they should be fairly close. The 2001+ shocks have the high/low speed compression adjusters, and a lot of the valving is handled through the small shim stack in those adjusters, so I may be just going the 09+ KX250F shock body in order to run the Lanier compression adjuster assemble in order to get a more wide range of external adjustments, especially since I'm going into a relatively untested custom shock swap territory. SS109's suspension guy did a great job on setting up the RM125 shock for his KDX, but I'm really trying to learn this in depth myself, although it will of course involve a fair bit of trial and error...




I might also experiment with a V-Strom steering stem and see if I can turn down the upper bearing area from 30mm down to 26mm, and cut M26x1.0 threads above that for commonly available steering stem nuts and a more commonly available 26x47x15 upper bearing. As it is now though, aside from the not so common upper bearing size of 30x47x12, the extra 3mm of space allows a better steering stem lock nut setup on the swap without infringing upon available travel area of the chrome tubes (don't have to have the upper triple as high above the frame), using the two V-Strom thinner stem locknuts with a slotted/toothed lockwasher locking into the slot on the stem, with 1 locknut below the Kawasaki upper bearing dust shield, and one above it. As opposed to a 15mm stock height bearing being nearly flush with the top of the frame, and a single thicker RM/DRZ400 locknut on top of that.

The upper triple clamp stem bore area is 25mm, so this wouldn't require machining a spacer at all, unlike all other fork swaps which require machining a special spacer for the KDX stem to the more modern sized upper triple clamp's stem bore.

I also was wondering perhaps if a KDX250 steering stem or a KLX300R steering stem would be a direct press fit or not... if I can find one of those for dirt cheap, I may buy it to press apart and measure. The KDX250 stems are aluminum! Also the other option may be the KX Guru Racing steering stems for the KX500's (same stem bearing diameter/spacing dimensions as the KDX), which are aluminum stems that are made in the slightly larger press fit area diameter (.003" larger than the KDX stems) so that they can be pressed into many more modern fork swaps' triple clamps.
Last edited by Chuck78 on 01:29 am Feb 06 2024, edited 3 times in total.
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Re: Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks

Post by Chuck78 »

Well we're getting near the end of the season here, and my 99 that's a waiting rebuild is sitting right in the middle of my basement shop... I'm really looking forward to late January tearing into this thing full bore, at which point I might as well build up two sets of the same suspension since I have several of these '98 RM125 forks. I might even pick up a 97-98 or 99-02 KX125 chassis to build a KDX220 hybrid out of and use these forks. With some steering stop trimming, putting them on a chassis that was designed for a 46 mm inverted fork (upoer tubes are significantly fatter), I could probably manage to have the tightest lock to lock turning radius of any bike out there, when running conventional forks were inverted forks previously were. That's some very highly beneficial goat trail singletracking goodness there!



So I've got 2 RM125 01-'04 shocks which are a well known good swap onto our bikes but are over an inch longer than stock and require at least 7 mm internal lowering just to fit in the frame without topping out the rocker into frame contact, not to mention back that this would put the rear right a couple inches taller than stock. As I mentioned before I'll be using internal lowering and KLX 300 suspension link dog bones which will act as a 1 inch lowering link as well as slightly softening up the Motocross valving at least for a better baseline that will be plenty rideable in the woods.

The 05-08 RM125/250 shocks (did '04 RM250's switch to Showa from KYB in '04? or '05 when the RM125 did?) are very similar Showas, but have longer clevises and a different shaft needle rebound aduuster assembly + 18mm shaft vs 16mm.
'07-'16-ish RM-Z 250/450 are basically the same, but with much firmer spring rates and valving tailored to their linkage design.
06-16 KX250F/450F are the longer clevis variety but use the 16mm shaft like the 01-04 RM125.
Honda CR 125/250 & CRF 250/450 also use very similar shocks but I believe the body configuration is quite different.

Forgive me if this is rehash from previous in this topic.

I may now build a hybrid of the shocks as I'm diving even deeper into the radical ultimate modifications brainstorming, IF I can get a cheap '09-'16 KX250F shock. The shock body appears virtually identical externally, but apparently has a slightly different thread diameter in the high speed/low speed compression clicker assembly, which even has its own little shim stack and check valve in it.
With this '09+ Showa era, I can get a more readily available aftermarket Lainer Suspension compression adjuster assembly, which allows you a lot wider range of compression adjustments and a true high-speed versus low speed external compression adjustment, despite the stock shocks being labeled as that, they both basically do almost the same thing, and stock is more like a low speed compression versus low mid speed compression adjustment on both the Showa and KYB shocks with this feature.

KYB_kit-AV-2.jpg
KYB_kit-AV-2.jpg (304.01 KiB) Viewed 2851 times

I'm also looking at a replacement for my purple 1998 RM250 4.8kg shock springs that I was adamant about obtaining.
A 4.8kg purple '98 RM250 shock spring weighs 3lbs 0.1oz.
Diverse Spring's Superlight Chrome Silicon 4.8kg red springs weigh 2.6lbs with torsion bushing included. I'd have to get a bulk length of purple heat shrink tubing and wrap that spring to get the aesthetic I'm after 😂, as red is the only color they offer them in, Showa & KYB large body applications...
RCS / Renton Coil Spring titanium 4.8kg springs weigh 2lbs...
Raw Ti finish matches a green/purple Kaw just fine, & drops 1lb of centralized weight! I just worked a ton of overtime last week, so I'm likely to make another big purchase of titanium hardware, and I might see a total weight reduction of 12 or 13 lbs if I go through with all of it on the KDX. That offsets swapping on larger better forks and shock, skid plate, bark busters, handlebar bag, regulator rectifier, etc...
Titanium springs are a lot more lively, they react a lot faster, which some pro riders really like, but some suspension tuners hate because they aren't familiar with valving for them. I would think that I'd perhaps need a bit more rebound damping, but I think the RM125 MX valving would really work out fairly close with a Motocross shock that had a bit lighter leverage ratio against it, meaning that swapping onto the KDX, the MX valving would probably be pretty close to a woods valving with the extra leverage that the KDX rocker linkage has on it I'm hoping. The '01-'04 RM125 came with 4.8kg, 4.7kg, 4.6kg, & '04 4.6kg again on the stock shock springs. The KDX was 5.0kg stock. I get by with 100 mm of sag on the stock KDX spring just fine, although Race Tech tells me I need a 4.6 kg spring. I think with the longer KLX300 aluminum 117mm links, and a 4.8 spring, it'll soften everything up to be darn near perfect for my 160lbs-ish frame + gear/water/tools 👍
'97 KDX220R - purple/green! - KLX forks, Lectron, FMF, Tubliss
'99 KDX220R project - '98/'01 RM125 suspension, Titanium hardware, Lectron Billetron Pro, Tubliss
'77 Suzuki PE250 & '83 Suzuki PE175 Full Floater - restomod projects
'77 Suzuki GS750-844cc, '77 GS400/489cc & '77 GS550/740cc projects
'62 GMC 1000 Panel Truck
'88 Suzuki Samurai TDI/Toyota swaps
'88 Toyota 4x4 pickup
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Chuck78
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Re: Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks

Post by Chuck78 »

For those wanting a bit more straightforward swap requiring less machining (like myself who has yet to learn how to cut threads on a lathe
'97 KDX220R - purple/green! - KLX forks, Lectron, FMF, Tubliss
'99 KDX220R project - '98/'01 RM125 suspension, Titanium hardware, Lectron Billetron Pro, Tubliss
'77 Suzuki PE250 & '83 Suzuki PE175 Full Floater - restomod projects
'77 Suzuki GS750-844cc, '77 GS400/489cc & '77 GS550/740cc projects
'62 GMC 1000 Panel Truck
'88 Suzuki Samurai TDI/Toyota swaps
'88 Toyota 4x4 pickup
User avatar
Chuck78
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Supporting Member II
Posts: 609
Joined: 06:20 pm Nov 30 2016
Country: USA
Location: Columbus, OH

Re: Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks

Post by Chuck78 »

For those wanting a slightly more straightforward stem swap while still retaining an aluminum stem, still requiring some machine shop work but not as much (for those such as myself who have yet to learn how to cut threads on a lathe! Let alone metric threads on an old US-made SAE lathe), KX Guru Racing makes an aluminum steering stem that is the press fit diameter of most modern triples, at least newer Kawasaki Showa triple clamps such as a 2014 KX450F Twin Chamber SFF 48mm inverted fork, basically the best of the best modern coil spring fork technology if you don't mind an inverted persuasion. This may work for the 96-98 RM & DRZ400 triples after machining down the lower step, but will still likely require an upper triple clamp bushing (very easy to make one on a metal lathe).

http://www.kxriders.com/forums/index.ph ... 396.0.html

Image


I've emailed Oscar @ KX Guru Racing for some dimensions on the press fit portion as well as press fit height. He sells these for $150, but I would pay that to not have to cut down a VStrom stem and rethread to smaller upper bearing i.d. and more importantly to not have to use a heavy steel steering stem!
Also, to remind you, this will use KX500 OEM bearing sizes which are also KDX/KLX bearing sizes!
These could be useful in fitting modern KX250F Showa Twin Chamber forks and Showa Twin Chamber SFF forks as well... The 2011-2016 or 2018 Showa Twin Chamber SFF 47mm and 48mm inverted forks in my opinion are truly the best forks ever built to date. They of course need revalved for woods however... and a lighter ICS spring. BUT... with only one side doing damping, you only have the need for one ICS spring (the equivalent of the nitrogen bladder in our shocks, also ICS springs don't exist on open chamber forks, only closed chamber / twin chamber). Only one ICS spring and only one base valve and mid valve means you have half as much resistance, and therefor you will have a much more plush ride with less stiction, once revalved for woods. Truly the ultimate. I will still hold my stance for the 96-98 RM125/250 right side up dual spring etc versions though, due to my hatred for leaking inverted fork seals packed full of mud/clay dust!!!!
'97 KDX220R - purple/green! - KLX forks, Lectron, FMF, Tubliss
'99 KDX220R project - '98/'01 RM125 suspension, Titanium hardware, Lectron Billetron Pro, Tubliss
'77 Suzuki PE250 & '83 Suzuki PE175 Full Floater - restomod projects
'77 Suzuki GS750-844cc, '77 GS400/489cc & '77 GS550/740cc projects
'62 GMC 1000 Panel Truck
'88 Suzuki Samurai TDI/Toyota swaps
'88 Toyota 4x4 pickup
User avatar
Chuck78
Supporting Member II
Supporting Member II
Posts: 609
Joined: 06:20 pm Nov 30 2016
Country: USA
Location: Columbus, OH

Re: Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks

Post by Chuck78 »

I'm getting a better idea now of the Showa RM125 > H-Series KDX rear suspension setup mods and ride height after alterations.

The H-series shock has about 130mm travel with bump stop fully compressed, and I'm finding the advertised rear suspension travel to be listed as 11.8" (300mm), so:

300 ÷ 130 = 2.30

So we have an average leverage ratio of 2.3:1.

Many bikes are closer to a 3:1 average leverage ratio, as witnessed by the KDX having a slightly thicker spring / higher rate spring vs most average 2-stroke bikes (lighter than 4-strokes) which have longer links and softer lower rate springs and longer pull rods / dogbones / suspension links. The KDX's initial rate is probably closer to 3:1 and increasing in firmness deeper in the stroke to 2:1.

So the 10mm thick Zeta Racing Showa/KYB 50mm shock "30mm lowering spacer" on these Showa shocks on the H-series KDX will only lower the bike around 23mm, & the 117mm KLX300 suspension links will lower the KDX around 0.92" roughly, 23.37mm, so the ride height of the taller Showa shock will be around 46.37mm lower after those two mods, as well as the 117mm KLX pull rods softening the initial suspension movements to make the stock MX compression valving a bit more suitable for woods.

If I custom make a 7mm internal lowering spacer for the Showa shock, or mill down the Zeta spacer, @ the KDX220R's 2.3:1 average suspension leverage ratio, I'd be at -16.1mm + the 117mm KLX300 pull rod lowering effect of 23.37mm, for a total reduction in ride height of 39.47mm.

I forgot to measure the exact length difference on the KDX vs RM shock earlier tonight, but it was right around 1" longer overall. So if the linkage rocker would be able to travel to full length (it can't utilize the last 1/4" of the Showa shock's shock extension), we'd be at 2.3" (58.4mm) taller ride height.

The lowering spacer and KLX300 "KDX lowering links" will be 46.37mm reduction off the 58.4mm taller ride height, 58.4 - 46.37 = 12.03, so I'll be 12mm taller than stock in the rear, 0.474", so just a bit under 1/2" taller than stock.

1" longer shock @ 25.4mm roughly
25.4 x 2.3 = 58.42mm taller ride height over stock
@ 7mm lowering spacer (vs 10mm Zeta lowering spacer):
58.42 - 39.47 = 18.95.
So in the rear we're looking a ride height increase of either +12mm (using 10mm Zeta lowering spacer, or +19mm (using a 7mm shock lowering spacer).

As installed on the KDX frames, the distance from the center of the front wheel axle up to the bottom of the lower steering stem bearing is over an inch longer. The available travel range on the chromeinner fork tubes between the dust seal and the lower triple clamp wil be right around 11.6" with the forks into the upper triple down as low as acceptable with the 49mm diameter portion of the base valve top cap outer hex area just flush with the tops of the triples (chrome tubes about 2mm down in the triples). I was assuming these MX forks had a typical 12.2" of travel as most modern MX bikes do, and so I was worried I'd have to reduce the travel and lower these internally. Then I made a nice discovery:
28689918922_2bef51eb15_k.jpg
28689918922_2bef51eb15_k.jpg (849.88 KiB) Viewed 1221 times
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52616502484_54cef4e9b7_k.jpg (538.75 KiB) Viewed 1221 times

Well... After coming up at a loss trying to find '98 fork travel specifications listed, I found the 1997 and 1998 brochures on flicker amongst a massive collection of bike brochure info from Tony Blazier. Guess what? It lists the 97 & 98 RM125/RM250 forks at 11.4 inches fork travel!

Most fork travel is advertised with including the unrealistic specification of having the top out springs fully compressed, which basically never happens unless your rebound is set extremely soft/fast and you unload the front end rapidly off a jump. Perhaps the 11.4 inches IS NOT including the compression of the top out springs? But that would be the first I've actually seen real world travel advertised, meaning the amount of suspension travel with the complete bike having the front wheel and tire and fork hanging in the air as if on a stand.
I'll have to actually physically measure these myself to confirm for real world travel, as this would be a huge help as the forks may not need any internal lowering at all if I can slide the triples down as much as 0.80" / 20.3mm down on the fork tubes with no worries of risking the fork dust wiper seals bottoming out on the triple clamp, or the tire bottoming out on the fender bolts.



Here's a link to the source of those OEM Suzuki brochure photos, Tony Blazier's Flickr photo album collection with a TON of KX, KDX, RM, etc literature scans of sales brochures, ads, magazine articles and reviews, and other motocross and other unrelated categories:

https://www.flickr.com/photos/tblazier/ ... 9395909367
'97 KDX220R - purple/green! - KLX forks, Lectron, FMF, Tubliss
'99 KDX220R project - '98/'01 RM125 suspension, Titanium hardware, Lectron Billetron Pro, Tubliss
'77 Suzuki PE250 & '83 Suzuki PE175 Full Floater - restomod projects
'77 Suzuki GS750-844cc, '77 GS400/489cc & '77 GS550/740cc projects
'62 GMC 1000 Panel Truck
'88 Suzuki Samurai TDI/Toyota swaps
'88 Toyota 4x4 pickup
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