Fork cartridge for 40mm tube

Discussion in 'Thumpers' started by AntonLargiader, Jun 3, 2010.

  1. AntonLargiader

    AntonLargiader Long timer

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    I'm looking into adding cartridges to a BMW R100GS fork. It's a 40mm tube with a 34mm (I think) ID so I'm looking for forks with cartridges that will fit. I assume the cartridge from nearly any 40mm fork would work but possibly if it's a USD fork the wall would be thinner, meaning greater tube ID and possibly a cartridge that's too big.

    Obviously I'll be doing some machining.

    So, if anyone has any relevant dimensions for this sort of stuff it would be helpful. I'm not looking to extend the travel so I don't need more than 225mm stroke, although I might run into overall length problems if they come from a bike with 300mm travel.

    So for instance, what is the cartridge OD and tube ID for a 4357 fork? How about for a Magnum 45? What other forks might have a good cartridge that will fit into the R100GS fork tubes?

    I'm thinking about a similar conversion for street forks also, but of course I'd be looking for even less travel.
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  2. Phreaky Phil

    Phreaky Phil Long timer

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    Hi Anton, Fancy meeting you here, XR 250 Hondas have a 41mm cartridge fork. Unlike the GS fork that just has a bolt up through the bottom of the fork, the Honda, and most others, have the compression stack bolt up from the bottom so a much bigger hole is required through the bottom fork leg.
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  3. AntonLargiader

    AntonLargiader Long timer

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    Thanks! Looks like that started in '96. About 270mm travel, but that shouldn't be too hard to shorten.

    Think the Kayaba cartridge is any good, and could be tuned for the R100GS? Any idea what the fork tube ID is, or what the cartridge diameter is? I could probably get this from Thumpertalk.
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  4. Phreaky Phil

    Phreaky Phil Long timer

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    Could tell you the sizes off hand. I had a set on my GasGas Pampera, but I sold that a while back. Stock they work ok and you can do good things with them with some RaceTech bits so ive been told. I have a set of XR 600 forks lying around in the shed. Should have a measure of the cartridge size of those
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  5. Phreaky Phil

    Phreaky Phil Long timer

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    Looking at the XR6 forks it looks as though you would need to fit the chrome sliders as well as the cartridge tubes as it looks like they are designed to work together. I guess the GS fork legs could be bored to 41mm to accept the XR250 chrome sliders ??
    #5
  6. AntonLargiader

    AntonLargiader Long timer

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    The GS forks have slider bushings so changing tubes to 41mm would be remanufacturing the whole thing. I just want to add a cartridge. Why do you think those cartridges need the XR fork tubes?

    I've asked over at Thumpertalk.
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  7. rz35027

    rz35027 Been here awhile

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    How about some older 40mm White Power forks off a pre 95 KTM...?
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  8. Phreaky Phil

    Phreaky Phil Long timer

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    I Doubt they would be a cartridge fork, more than likely just a damper rod setup.
    Anton, I will take some pics of they XR6 cartridges later so you can see what I was talking about.
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  9. AntonLargiader

    AntonLargiader Long timer

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    Thanks for the suggestion. I don't know the KTM line that well but the fiche at cyclehuttktm goes back to 1994. It shows 1994 models with a USD fork; was that 40mm? Maybe that cartridge would fit. Can you give me a specific 1994 model that you know has a 40mm fork? Then I can chase down the cartridge.
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  10. AntonLargiader

    AntonLargiader Long timer

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    OK, a bit of searching shows that the WP 4054 USD fork was used on KTMs up to '95 at which time the Marzocchi conventional 45mm fork was used. Not on all models, it seems. And they were introduced in the early '80s!

    So possibly I am looking for a set of those. It would be nice to find cartridges that were built for less than 300mm stroke. And it would be nice to find a set of those cartridge that I could measure.
    #10
  11. col klink

    col klink the str8n'r

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    The 40 and 41mm forks will easily reveive 20mm (of which I have many) and 25mm cartridges (of which I have a few).
    FYI, a 20mm cartridge will not be real good in offroad/dualsport/gravel washboards situations, except on mini bikes. 25mm only marginally better. All your current offroad bikes have 28 to 36 mm cartridges to acheive a firm plushness.
    Most current street bikes have 20mm carts, most roadracers upgrade to 25mm at great expense. A 25mm cartridge kit from Ohlins or Racetech will cost about $1500 plus oil and install. These kits are best installed by pros.

    You have to have a cartridge with enough stroke, caps that fit the cartridge rods and the fork tubes, and able to bolt the cartridge base into the fork lower. then any cartridges you fit to your adv bike will greatly benefit from goldvalve kits to tailor them to your application, weight etc.

    I have adapted many carts into frx that were originally damper rod frx.
    In the long run, the Racetech Cartridge Emulators do a damn fine job of emulating cartridges. For a fraction of the cost and much less fussing around.
    We have installed many emulator kits into bmw's of various types including the gs.

    Some of the comments above suggested swapping to the mx type lowers to your uppers, now that's a bucket of worms.

    What drives your desire to tackle this? If your goal is simply good suspension, the best, simplest route is emulators. they work very well, good bang for buck value, lick and stick.

    To qualify my comments: I build m/c suspension everyday for almost eighteen years. My company does motorcycle frame alignment, geometry optimizations, and custom suspension solutions for riders from many genre of motorcycling.

    I can go on and on but I have to GET BACK TO WORK! lol
    Hit me back if you want more.
    All the best,
    Kent
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  12. Phreaky Phil

    Phreaky Phil Long timer

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    Here are some pics of the internals of the XR6 fork.

    [​IMG]
    The top of the cartridge has a slider bushing on it although its a loose fit on the inside of the forkleg. The bottom of the forlkleg is slightly recessed with holes, assumably for oil flow[​IMG]
    [​IMG]
    This is the bottom of the cartridge where the compression stack bolts in through the bottom of the lower fork housing
    [​IMG]
    #12
  13. AntonLargiader

    AntonLargiader Long timer

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    Hi Kent, didn't know you had a /5. And I still mean to bring you my R11RS for suspension retuning. Way too hard springs, etc. We talked about that a few years ago when I saw you more often at tracks. I haven't been doing that so much recently.

    Anyway, what do I hope to achieve? Well, a better suspension. Maybe better than with GVs. I have a spare set of forks so I can play.

    As you surely know, the old damper-rod Marzocchis with rebound in one side and compression in the other are OK but not great. The technology was not exactly cutting-edge when BMW started using those forks 23 years ago.

    The GV works in the right leg because of the damper rod design. Might work in the left with enough modifications. I have a pair of GVs and could use them in two right legs on my GS. Probably get a vast improvement.

    But a real cartridge with a shim stack (rather than a single plate and spring for compression, and nonadjustable rebound) might be better. Now I realize that some cartridges are junk, like several that BMW uses. But a real cartridge, with tunable compression and rebound taps, might be nice. And if there are forks that can be had on eBay for $75 with decent cartridges, I'm up for trying the machining. Plus I'd get on-the-fly adjustment to some extent.

    For a 34mm ID fork tube, what size cartridge is best? I am guessing that you don't necessarily want to fill all of the space with it if you need some oil flow around the outside. I have a set of 4860MXMA forks here that I am using for reference, as far as how much room is around the cartridge and how the top and bottom caps need to work. Unfortunately it is not that simple to just use these forks on a BMW. I probably will, on a different bike, but the current project is seeing what kind of technology I can pack into the OEM forks.
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  14. Phreaky Phil

    Phreaky Phil Long timer

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    +1 on all your thoughts Anton. The stock forks work ok with some adjustments to oils but they dont like square edge hits, and the "whack" back through the bars can jarr your wrists and forearms. The stock design just restricts the oil flow more and more as the fork is compressed. I dont believe fitting a gold valve emulator to just the rebound leg will accomplish much other than give some compression damping to the initial part of the travel, which stock, feels like there is none. It will probably make the harsh hits worse. I have contemplated machining up another damper rod the same as the rebound leg and fitting 2 emulators. ( i priced the parts and they were over NZ$400)
    I wish I had shelled out for a pair of the cartridge inserts while they were available
    #14
  15. AntonLargiader

    AntonLargiader Long timer

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    Looking at the pictures, they weren't exactly cutting-edge either. But they seem to be a huge improvement over stock.
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  16. AntonLargiader

    AntonLargiader Long timer

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    I don't know enough about this to judge where the harshness comes from, but disabling the damping in the left leg and adding it to the right leg (with a GV) would be a worthwhile test.

    Actually I will probably install two right-side dampers, disable the rebound in them (so there's not much going on at all in there) and put GVs on top. That way the high-volume flow across the piston (I think of this as primary flow but there's probably an official name for it) has less effect and the damper-rod flow does all of the work. Should reduce the hydrolocking and therefore the harshness.
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  17. Phreaky Phil

    Phreaky Phil Long timer

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    I assume you mean just disable the rebound in the second leg, the other one will still need to control the rebound. I think this would work well. Previous to owning my GS I had a DR650. I changed fork springs and fitted emulators and the worked pretty well. Only pain in the arse is having to remove them to make a change. I found a gadget that is used for picking up things that youve dropped in awkward places. it has 3 claws on the end and I could just remove the spring and reach down with it.
    #17
  18. rz35027

    rz35027 Been here awhile

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    The wp USD 40mm 4054 go way back (mid/late 80s?)... super adjusters (basically same fork) 93-95(?)
    They are cartridge forks.... Try looking at a 95 620 RXC on a micro fiche.

    I've seen complete sets sell for $20 on Ebay recently...


    #18
  19. Phreaky Phil

    Phreaky Phil Long timer

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    I dont have one to look at but thought you may, on the GS left side tapered damper rod, would it have enough meat to be able to machine it parallel, then rill holes in the bottom to flow oil. Then machine a piece of ali to replace the part that forces the oil through with one matching the damper rod. then an emulator could be fitted on top ?
    #19
  20. AntonLargiader

    AntonLargiader Long timer

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    I'd really like it if someone had these cartridges lying around and could measure them for me!

    Basically turn it into a right leg? There are more differences. The top piston doesn't actually seal against the fork tube. You'd be much better off just buying a right side damper rod.

    I have pics of all of these parts on my website:

    http://www.largiader.com/tech/gsforks/
    #20