Showa Twin Chamber 49mm conventional forks 96 97 98 RM125/250 + inverted Showa TC & RM shocks, & DRZ400 49mm forks
Posted: 07:01 pm Nov 20 2022
For quite some time, the fabled 1996-1998 RM125/250 Showa 49mm right side up conventional forks have been on my radar, as many riders, especially older riders, regard them as perhaps the best woods forks of all time...(similar crowds amongst orange Austrian bike riders regard the 1998 orange bike 50mm conventionals in the same light).
These forks contain the same technology as a lot of modern forks still run (aside from the SFF separate function forks with spring in one leg and damping in the other, and the ones that run air springs with no steel coils - to save weight, I'm not a fan, even for my mountain bike bicycles...). Yet they are "right side up" conventionals which keep the chrome tubes more protected out of harm's way and are less prone to blowing seals literally annually....They are also the most burly and rigid/strong of nearly all conventional forks by their massive 49mm stanchion tubes... less flex, but still not the harsh rigidity of similar 46mm and larger inverted forks just by the nature of inverted forks being stiffer and less prone to twisting flex forces from the varied/rugged terrain. Too stiff of fork tubes and you will get the dreaded arm pump a lot more readily from tiring riding in rugged terrain.
I've got both KLX300R 43mm inverted forks and 97-04 KX500 46mm inverted forks, but had never ridden Showa forks off-road yet. The KLX300 forks are an outstanding swap for the minimal effort as they are a direct swap and use the KDX wheel/axle, and suit a 145-165lb woods rider quite well with one rate stiffer on the springs stock and better valving stock (than a KDX's stock springs and compression valving), and the additional work to swap and re-valve and lower the 97+ KX500 open chamber KYB 46mm inverted forks is also a great swap, albeit significantly more work involved (+ more if using 96+ KX125/250 & KX###F forks which all require stem swaps & upper triple stem bushings custom made).
Fast forward a few years into running inverted forks on my KDX220R and my wife's KTM 200 XC-W... first off, replacing my buddy's KDX250 forks due to a tiny bit of rock damage on the lowers/chrome tubes was a bit frustrating - the tubes are significantly more in harm's way and are a critical sealing surface ESPECIALLY on inverted forks.
Then the blown seals ensued... I've spent far too much time using a SealMate tool (thin flat hook-shaped piece of flexible plastic), popping out the dust seals on the KLX and KTM inverted forks to attempt scraping out the residue buildup from the oil seals that causes weeping inverted fork seals perpetually... I've since learned that letting mud dry up on the chrome tubes and then riding them at all will jam some of the clay mud dust up into the oil seals to accelerate the amount of time until the inverted forks inevitably begin weeping and puking out fork oil...
Some people swear by various brands/styles of neoprene seal saver products that are a pre-filter sock that goes over the inner and outer tubes at the fork seal area in order to prevent dirt from ending up on the inner sealing areas of the oil seals causing leaks, but there are a lot of critics of these as they will require frequent removal and cleaning, otherwise they will hold in abrasive grit which will subsequently accelerate the wear on the chrome tubes in the travel range. There are sewn versions which must be slid on during fork rebuilds, there are velcro versions, and there are even zippered versions. None of these seem as good as just running right side up forks with accordian pleat fork boots aka gaiters.
The stock KDX forks with some very basic revalving (swapping out of some of the largest diameter compression stack shims) work great for casual woods riding, but some riders in rutted and rocky terrain cannot cope with the excessive underhang protrusion of the fork legs 3.5" below the axle, as this catches on roots/rocks/ruts substantially, and will throw the rider off track (or off the bike!) as bashing that part of the fork will give unexpected steering inputs to the front end... Others also complain that the stock 43mm conventional forks flex too much. On the opposite end of the spectrum, some riders say that inverted forks by the nature of their design, are too stiff and lacking in desirable flex, which will directly result in ride harshness in nasty terrain as well as riders experiencing arm pump. This inverted fork rigidity and resistance to twisting flex will also add slightly to the need for a steering damper in nasty rocky terrain...
This is what drove me to look into these somewhat legendary Showa 49mm conventional twin chamber Suzuki RM forks.
I'll attempt to compile all the info I've saved on these things here in this space. I held off thus far because I didn't want to generate any additional interest in the form of competition for me trying to buy a better set off eBay when one may pop up... But at this point, I have 4 sets of the RM125W (W= 1998 model year) forks, and 2 spare chrome upper/inner tubes, and yet I still have ZERO chrome tubes that I consider great useable condition... 24 years of wear and tear has chipped and damaged the chrome on ALL of these parted out RM's chrome inner tubes, although I may have 4 tubes out of the 10 total that may be "useable" with some fine sanding / micro/spot wire brushing / acid etching & neutralizing / epoxy filling / fine sanding of the small gouges and rust pits... FRUSTRATING! re-chroming is available for around $585 shipped/taxed, and if you look in the right places, new OEM tubes are still available for $700/pair same as Race Tech charges for rechroming a pair of fork tubes. That's quite spendy unless you're okay having as much money into your suspension upgrades as a "good deal" (in the year 2022 at least) on a running riding rebuildable KDX...
I also have found that virtually all of these beautiful gold anodized Showa fork outers are made of 7000 series aluminum, which when the gold anodizing is chipped or scraped off, leaves 7000 aluminum very susceptible to some nasty corrosion from salt etc, which unlike 6061 aluminum, will appear to rot the aluminum to pieces in layers. Do a web search for "Showa corrosion" and you will find this all across the realm of dirt and street bikes running Showa forks, not just these Showa 49mm forks.
I believe from reading up on the DRZ400 forks, that scientifically, this is known as intergranular degradation or something to that effect. It is of utmost importance on these Showa Twin Chambers or the almost equally awesome and almost identical appearing (externally) DRZ400 49mm open chamber conventionals, that any of this Showa aluminum corrosion be sanded/scraped/wire-brushed out, neutralized, and then coated to prevent this from spreading like a disease. Brownell's Clear Epoxy paint (intended for firearms) has proven to me to be a great and tough single stage rattle can finish that I've used on shocks and street bike gas tanks (over top of single stage paint which is not gasoline resistant). I think this stuff or some Cerakote or other finish, or powder coating, would do really well to preserve these Showa outers.
I was reading up on 2005+ Yamaha YZ S.S.S. suspensions (and a few Kaw KX-F forks 2006+ which are also S.S.S.), and it is recommended that the closed chamber inner chamber seals on those things be replaced EVERY 6 MONTHS per RaceTech, WOW! I formerly regarded KYB S.S.S. forks as the best inverted woods forks one could have. I now think that for woods riding with some more favorable traits for high speed whoops type riding etc as well, running Showa Twin Chambers with the lightest inner damping chamber blow-off springs available, 1.0kg Racetech or 1.2kg/1.6kg progressive rate Cannon-Racecraft inner chamber springs, may top the KYB S.S.S. if this 6-month S.S.S. inner chamber seal life is in fact true. The Showa Twin Chamber inner chamber seals also do wear out in time and leak into the outer chambers of the forks (where the oil for lubricating the fork slider bushings is), but 6 months... wow.
So obviously you may be wondering why the more modern closed chamber (also referred to as twin chamber) forks are "better?" Well, it depends on the rider's choice of terrain, skills, and riding style, as to whether or not the more advanced nature of these forks is beneficial or even noticeable, but the twin chamber (closed chamber) design is more resistant to cavitation on higher speed riding sections.
Cavitation is when the fork oil gets blended with the surrounding air or nitrogen charge inside the suspension assemblies, from washboard sections and whoops or more particularly, aggressive high speed riding through these conditions resulting in a lot of deeper fork travel movements in rapid succession. Cavitation completely alters the viscosity of the suspension oil, as it becomes an emulsified mixture of oil and tiny air bubbles, which will also completely alter the damping rates on both compression and rebound. A design that resists cavitation will thus provide more consist damping traits when the rider is blasting through a lot of rough terrain or whoops with great speed. The tradeoff is the initial plushness of the forks.
The inner chamber springs on the damping assemblies add resistance to the initial suspension travel movements, so this will take away from the plushness of the suspension's initiation of movement and damping slightly. Some of the Showa T.C. motocross forks come with quite stiff internal chamber springs (2.4kg/mm or even 2.7kg/mm) to very strongly resist cavitation before the spring allows the compression valving assembly to blow off as far as I how I understand it's terminology in the TC's.
It is very possible to make these twin chamber forks quite plush however. The Honda CRF250X and CRF450X models are a 4-stroke woods bike version of the motocross bikes ending with the R designation, and these bikes also run a much more plush version of the 47mm inverted Showa Twin Chamber forks, and these bikes run 1.5kg/mm inner chamber springs. Many people say these bikes are quite plush, although I have read of a few complaining that they still want something more plush than the stock X models offer.
Race Tech's page for inner chamber springs recommends their lightest offering, the 1.0kg/mm inner chamber spring, for a 160lb woods/enduro rider for both Showa Twin Chamber and KYB closed chamber forks. I believe that this 1.0kg/mm inner chamber spring + premium SKF low-friction seals or the even better dual compound SKF seals for 49mm Showas + correct spring rates per rider weight (.38kg Cannon-Racecraft main springs for me @ 154lbs) will in fact result in a fork that can be nearly as plush in the rocky rooty rugged nasty terrain as plush-valved open chamber forks, but also will be better at providing more consistent damping than open chambers when maxed out in the fast whoops and riding quite aggressively at higher speeds inducing lots of suspension travel in rapid succession.
HOWEVER.... just because these forks are more modern technology, and "better," doesn't necessarily make them "better" and beneficial in any noticeable way to many casual woods riders, or riders who only ride primarily in the slow rocky nasty terrain. There is something to be said for a good properly valved / properly sprung open chamber fork, even the older more basic versions with just a base valve and rebound valve and no compression damping floating mid-valve on the underside of the rebound piston. These basic old school open chamber forks, like 1991-1994-ish KX125/250 inverted forks, KLX300R forks, 1988 and/or 1989 KX conventional cartridge forks, and the stock 1995-2006 KDX forks are rather easy to set up very plush for casual trail riding or even for nasty rocky rooty rugged technical terrain... One doesn't necessarily need the cavitation-resistance of the closed chamber design if they aren't blasting whoops and other sections at excessively high speeds. In that case, I would steer towards 2000-2022+ DRZ400 49mm Showa open chamber conventionals as being the "ultimate" woods bike fork swap for someone wanting conventionals. They are notoriously plush and awesome in their valving, and about as rigid in terms of twist/flex as one would want in the woods without going to what some consider harshly rigid and twist/flex resistant in the fatter tubed inverted forks...
(*bike photos above with the 49mm RM Showa forks are borrowed from elsewhere on this forum! NOT MINE! I'll add photos of mine in 2023 after some winter shop work getting the triples machined for the stem swap, and rebuilding/revalving a set of these for woods)
These forks contain the same technology as a lot of modern forks still run (aside from the SFF separate function forks with spring in one leg and damping in the other, and the ones that run air springs with no steel coils - to save weight, I'm not a fan, even for my mountain bike bicycles...). Yet they are "right side up" conventionals which keep the chrome tubes more protected out of harm's way and are less prone to blowing seals literally annually....They are also the most burly and rigid/strong of nearly all conventional forks by their massive 49mm stanchion tubes... less flex, but still not the harsh rigidity of similar 46mm and larger inverted forks just by the nature of inverted forks being stiffer and less prone to twisting flex forces from the varied/rugged terrain. Too stiff of fork tubes and you will get the dreaded arm pump a lot more readily from tiring riding in rugged terrain.
I've got both KLX300R 43mm inverted forks and 97-04 KX500 46mm inverted forks, but had never ridden Showa forks off-road yet. The KLX300 forks are an outstanding swap for the minimal effort as they are a direct swap and use the KDX wheel/axle, and suit a 145-165lb woods rider quite well with one rate stiffer on the springs stock and better valving stock (than a KDX's stock springs and compression valving), and the additional work to swap and re-valve and lower the 97+ KX500 open chamber KYB 46mm inverted forks is also a great swap, albeit significantly more work involved (+ more if using 96+ KX125/250 & KX###F forks which all require stem swaps & upper triple stem bushings custom made).
Fast forward a few years into running inverted forks on my KDX220R and my wife's KTM 200 XC-W... first off, replacing my buddy's KDX250 forks due to a tiny bit of rock damage on the lowers/chrome tubes was a bit frustrating - the tubes are significantly more in harm's way and are a critical sealing surface ESPECIALLY on inverted forks.
Then the blown seals ensued... I've spent far too much time using a SealMate tool (thin flat hook-shaped piece of flexible plastic), popping out the dust seals on the KLX and KTM inverted forks to attempt scraping out the residue buildup from the oil seals that causes weeping inverted fork seals perpetually... I've since learned that letting mud dry up on the chrome tubes and then riding them at all will jam some of the clay mud dust up into the oil seals to accelerate the amount of time until the inverted forks inevitably begin weeping and puking out fork oil...
Some people swear by various brands/styles of neoprene seal saver products that are a pre-filter sock that goes over the inner and outer tubes at the fork seal area in order to prevent dirt from ending up on the inner sealing areas of the oil seals causing leaks, but there are a lot of critics of these as they will require frequent removal and cleaning, otherwise they will hold in abrasive grit which will subsequently accelerate the wear on the chrome tubes in the travel range. There are sewn versions which must be slid on during fork rebuilds, there are velcro versions, and there are even zippered versions. None of these seem as good as just running right side up forks with accordian pleat fork boots aka gaiters.
The stock KDX forks with some very basic revalving (swapping out of some of the largest diameter compression stack shims) work great for casual woods riding, but some riders in rutted and rocky terrain cannot cope with the excessive underhang protrusion of the fork legs 3.5" below the axle, as this catches on roots/rocks/ruts substantially, and will throw the rider off track (or off the bike!) as bashing that part of the fork will give unexpected steering inputs to the front end... Others also complain that the stock 43mm conventional forks flex too much. On the opposite end of the spectrum, some riders say that inverted forks by the nature of their design, are too stiff and lacking in desirable flex, which will directly result in ride harshness in nasty terrain as well as riders experiencing arm pump. This inverted fork rigidity and resistance to twisting flex will also add slightly to the need for a steering damper in nasty rocky terrain...
This is what drove me to look into these somewhat legendary Showa 49mm conventional twin chamber Suzuki RM forks.
I'll attempt to compile all the info I've saved on these things here in this space. I held off thus far because I didn't want to generate any additional interest in the form of competition for me trying to buy a better set off eBay when one may pop up... But at this point, I have 4 sets of the RM125W (W= 1998 model year) forks, and 2 spare chrome upper/inner tubes, and yet I still have ZERO chrome tubes that I consider great useable condition... 24 years of wear and tear has chipped and damaged the chrome on ALL of these parted out RM's chrome inner tubes, although I may have 4 tubes out of the 10 total that may be "useable" with some fine sanding / micro/spot wire brushing / acid etching & neutralizing / epoxy filling / fine sanding of the small gouges and rust pits... FRUSTRATING! re-chroming is available for around $585 shipped/taxed, and if you look in the right places, new OEM tubes are still available for $700/pair same as Race Tech charges for rechroming a pair of fork tubes. That's quite spendy unless you're okay having as much money into your suspension upgrades as a "good deal" (in the year 2022 at least) on a running riding rebuildable KDX...
I also have found that virtually all of these beautiful gold anodized Showa fork outers are made of 7000 series aluminum, which when the gold anodizing is chipped or scraped off, leaves 7000 aluminum very susceptible to some nasty corrosion from salt etc, which unlike 6061 aluminum, will appear to rot the aluminum to pieces in layers. Do a web search for "Showa corrosion" and you will find this all across the realm of dirt and street bikes running Showa forks, not just these Showa 49mm forks.
I believe from reading up on the DRZ400 forks, that scientifically, this is known as intergranular degradation or something to that effect. It is of utmost importance on these Showa Twin Chambers or the almost equally awesome and almost identical appearing (externally) DRZ400 49mm open chamber conventionals, that any of this Showa aluminum corrosion be sanded/scraped/wire-brushed out, neutralized, and then coated to prevent this from spreading like a disease. Brownell's Clear Epoxy paint (intended for firearms) has proven to me to be a great and tough single stage rattle can finish that I've used on shocks and street bike gas tanks (over top of single stage paint which is not gasoline resistant). I think this stuff or some Cerakote or other finish, or powder coating, would do really well to preserve these Showa outers.
I was reading up on 2005+ Yamaha YZ S.S.S. suspensions (and a few Kaw KX-F forks 2006+ which are also S.S.S.), and it is recommended that the closed chamber inner chamber seals on those things be replaced EVERY 6 MONTHS per RaceTech, WOW! I formerly regarded KYB S.S.S. forks as the best inverted woods forks one could have. I now think that for woods riding with some more favorable traits for high speed whoops type riding etc as well, running Showa Twin Chambers with the lightest inner damping chamber blow-off springs available, 1.0kg Racetech or 1.2kg/1.6kg progressive rate Cannon-Racecraft inner chamber springs, may top the KYB S.S.S. if this 6-month S.S.S. inner chamber seal life is in fact true. The Showa Twin Chamber inner chamber seals also do wear out in time and leak into the outer chambers of the forks (where the oil for lubricating the fork slider bushings is), but 6 months... wow.
So obviously you may be wondering why the more modern closed chamber (also referred to as twin chamber) forks are "better?" Well, it depends on the rider's choice of terrain, skills, and riding style, as to whether or not the more advanced nature of these forks is beneficial or even noticeable, but the twin chamber (closed chamber) design is more resistant to cavitation on higher speed riding sections.
Cavitation is when the fork oil gets blended with the surrounding air or nitrogen charge inside the suspension assemblies, from washboard sections and whoops or more particularly, aggressive high speed riding through these conditions resulting in a lot of deeper fork travel movements in rapid succession. Cavitation completely alters the viscosity of the suspension oil, as it becomes an emulsified mixture of oil and tiny air bubbles, which will also completely alter the damping rates on both compression and rebound. A design that resists cavitation will thus provide more consist damping traits when the rider is blasting through a lot of rough terrain or whoops with great speed. The tradeoff is the initial plushness of the forks.
The inner chamber springs on the damping assemblies add resistance to the initial suspension travel movements, so this will take away from the plushness of the suspension's initiation of movement and damping slightly. Some of the Showa T.C. motocross forks come with quite stiff internal chamber springs (2.4kg/mm or even 2.7kg/mm) to very strongly resist cavitation before the spring allows the compression valving assembly to blow off as far as I how I understand it's terminology in the TC's.
It is very possible to make these twin chamber forks quite plush however. The Honda CRF250X and CRF450X models are a 4-stroke woods bike version of the motocross bikes ending with the R designation, and these bikes also run a much more plush version of the 47mm inverted Showa Twin Chamber forks, and these bikes run 1.5kg/mm inner chamber springs. Many people say these bikes are quite plush, although I have read of a few complaining that they still want something more plush than the stock X models offer.
Race Tech's page for inner chamber springs recommends their lightest offering, the 1.0kg/mm inner chamber spring, for a 160lb woods/enduro rider for both Showa Twin Chamber and KYB closed chamber forks. I believe that this 1.0kg/mm inner chamber spring + premium SKF low-friction seals or the even better dual compound SKF seals for 49mm Showas + correct spring rates per rider weight (.38kg Cannon-Racecraft main springs for me @ 154lbs) will in fact result in a fork that can be nearly as plush in the rocky rooty rugged nasty terrain as plush-valved open chamber forks, but also will be better at providing more consistent damping than open chambers when maxed out in the fast whoops and riding quite aggressively at higher speeds inducing lots of suspension travel in rapid succession.
HOWEVER.... just because these forks are more modern technology, and "better," doesn't necessarily make them "better" and beneficial in any noticeable way to many casual woods riders, or riders who only ride primarily in the slow rocky nasty terrain. There is something to be said for a good properly valved / properly sprung open chamber fork, even the older more basic versions with just a base valve and rebound valve and no compression damping floating mid-valve on the underside of the rebound piston. These basic old school open chamber forks, like 1991-1994-ish KX125/250 inverted forks, KLX300R forks, 1988 and/or 1989 KX conventional cartridge forks, and the stock 1995-2006 KDX forks are rather easy to set up very plush for casual trail riding or even for nasty rocky rooty rugged technical terrain... One doesn't necessarily need the cavitation-resistance of the closed chamber design if they aren't blasting whoops and other sections at excessively high speeds. In that case, I would steer towards 2000-2022+ DRZ400 49mm Showa open chamber conventionals as being the "ultimate" woods bike fork swap for someone wanting conventionals. They are notoriously plush and awesome in their valving, and about as rigid in terms of twist/flex as one would want in the woods without going to what some consider harshly rigid and twist/flex resistant in the fatter tubed inverted forks...
(*bike photos above with the 49mm RM Showa forks are borrowed from elsewhere on this forum! NOT MINE! I'll add photos of mine in 2023 after some winter shop work getting the triples machined for the stem swap, and rebuilding/revalving a set of these for woods)