An Unholy Union III: WP 4860 USD Forks on an Airhead

Discussion in 'Airheads' started by Airhead Wrangler, Oct 21, 2011.

  1. Airhead Wrangler

    Airhead Wrangler Adios Mexico

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    [​IMG]

    Since I happen to know there will be (at least) 6 people fitting WP 4860 forks to various airheads in the VERY near future and since I know Solo doesn't want us cluttering up his thread with our BS, I thought it'd be a good idea to have all the USD folks solve our problems in the same spot.

    I'm using "open chamber" 4860s off of a 2006 KTM 250SXF. They'll be going on a reinforced ST frame by way of HPMGuy's triple clamps. I plan to build my own front wheel using a 1.85"x21" rim on an aftermarket KTM hub and use the stock KTM master cylinder and caliper.

    Here's a good writeup of the differences between various years and models of 4860 (and 4357) forks as well as servicing data and some relevant info about fitting them to other bikes.

    http://crossroadz.com.au/F650GS/Modifications/Forks/ForkNotesWP.pdf

    A good service manual available free from KTM:
    http://slorider.com/KTM/manuals/WP4860MXMA2003.pdf

    Two videos detailing all the considerations of fitting 4860s to the F650 (but plenty of relevant info):
    http://www.garagenight.tv/ep-11-ktm-forks-on-a-bmw/
    http://blip.tv/garage-night/garage-night-episode-12-ktm-forks-on-a-bmw-part-2-2375953

    Anyone else?
    #1
  2. bgoodsoil

    bgoodsoil Dare to be Stupid

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    Thanks for doing all the research on this, man. I'm hoping to do this next year.

    For anyone that didn't get in on the group buy you can get triples for these forks from EMIG racing. They make triples that have 30mm of offset. I'm sure they can machine the stem down to BMW specs.

    http://emigracing.com/
    #2
  3. Solo Lobo

    Solo Lobo airhead or nothing

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    Ouch :lol3

    Lookin' forward to seeing this thread move ahead!
    #3
  4. Renner

    Renner rockin' the toaster

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    subscribed :lurk

    thanks for initiating this AW.
    #4
  5. igormortis

    igormortis Long timer

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    Yeah, I'm in.
    #5
  6. Houseoffubar

    Houseoffubar fine beer sampler

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    BMW engineered this bike to be PERFECT in every way, if there seems to be a flaw in your machine, it is because YOU are not worthy!

    This thread can now be closed.............











    :rofl
    #6
  7. Airhead Wrangler

    Airhead Wrangler Adios Mexico

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    ...yeah, exactly how the fine bavarian craftsmen built it 35 years ago:

    [​IMG]

    :lol3
    #7
  8. Houseoffubar

    Houseoffubar fine beer sampler

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    Nobody likes a smartass!
    :shog

    I like to think, how they intended, you know, budgetary, and marketing concerns aside.........Oh, alright, I'm full of shit as usual!
    #8
  9. AntonLargiader

    AntonLargiader Long timer

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    I looked at this a year or two ago. I think the forks I have are from a 450EXC.

    The biggest issue was that the clamping distance on the forks was barely short enough to cover the BMW head tube length. The KTM has a longer head tube, and in order to mount these they would have been too far down in the clamps. I'm not looking for a tall bike, nor 300mm travel.

    The stem was also too long; it had a taper in the middle and the 29mm diameter parts (for the KTM bearings) weren't close enough together. Even machining them down to 28mm (for the BMW bearings) barely brought them close enough together. But a new stem is not so hard.

    I think different, shorter forks (like on the 950SE) would have a much easier fitment for my purposes.
    #9
  10. Airhead Wrangler

    Airhead Wrangler Adios Mexico

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    Here's what we did about the stem:

    [​IMG]

    The 950 uses the same forks, but with shorter springs and a spacer under the topout spring to make it top out earlier. Other than that and the clamps for the larger axle and dual discs, they're the same.
    #10
  11. Airhead Wrangler

    Airhead Wrangler Adios Mexico

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    can anyone with a pair of 4860 forks measure both the length of and distance between the two clamping areas on their fork legs? I would measure mine, but my forks are in Seattle and I'm in Mexico.

    Also, a bottom of bearing seat to bottom of bearing seat in the steering head would be a useful measurement as well. I'd like to do up a quicky CAD drawing to see how much height adjustment we can get sliding the fork legs up and down.
    #11
  12. igormortis

    igormortis Long timer

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    I was led to believe that the 950 outers (legs? sliders?) were longer than ‘standard’ 4860s. See here.
    #12
  13. Airhead Wrangler

    Airhead Wrangler Adios Mexico

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    That's a new one on me. Everyone so far has told me they're the same. I guess they definitely aren't. Thanks for pointing that one out. One more consideration to add to the list when choosing your forks.


    [​IMG]

    Being a bit caught off guard by that, and the ground clearance concerns in the thread you linked to I ran a few numbers:

    R100GS forks:
    Top of leg to axle center (fully extended): 831mm
    travel: 225mm
    Top of leg to axle center fully compressed: 606mm

    WP 4860 (SX model):
    Top of leg to axle center (fully extended): 935mm
    travel: 300mm
    Top of leg to axle center fully compressed: 635mm

    To sum it up, using dirt model KTM forks as opposed to 950/990 (S model spec, long travel, not short) forks will not hurt your fully compressed ground clearance as compared to stock R100GS forks. You'll still gain 29mm of fully compressed fork length.

    I'd still like to get measurements on the machined clamping sections of the fork legs though as they appear to be dramatically different between the 950/990 and dirt model forks pictured above.
    #13
  14. espressodrinker

    espressodrinker Cape Town Wanderer

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    Small hijack!
    I upgraded my R80G/S forks to the R100GS, and put in a Gold Valve. Easy painless conversion. Would it be possible to take the cartridge out of one of these modern forks and then fit them inside the standard R100 forks? Similar to the hugely expensive HPN cartridge that isn't available any more?
    Seems to me to be an easy upgrade without the pain of all the extra machining mods - although you don't gain any extra suspension stroke.
    #14
  15. Airhead Wrangler

    Airhead Wrangler Adios Mexico

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    Anything's possible, but there would be a lot of machining involved probably. Also, the stock r100gs forks are 40 or 41mm or thereabouts if I recall correctly. Most modern USD forks are significantly fatter, so fitting one of their cartridges inside might be tough. It'd probably be easier to fit a new front end rather than going to so much trouble machining the cartridge/forks/damper rod
    #15
  16. One Less Harley

    One Less Harley OH.THAT'S GONNA HURT

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    You'll still have forks that weigh a metric ton. Plus I doubt changing internals can be easily done. There might be some machining involved. The WP forks are so much better and lighter than the stock ones. Not sure that chaning over to WP would have cost that much more than the change over to GS forks, well at least for me. I found GS forks and wheel in Germany, a complete front end, $700-800. Wheel was unu$able so I changed the tubliss rim out to an Excel tube rim, More money. Then switched the brake MC for a better brake response w/ a Brembo 4 pot, plus a gold valve and heavier springs. The suspension is adequate for fire roads, gravel and such, but has limitations on the rough stuff.

    Of course the GS forks are BMW if that matters to you and it's an easy swap.
    #16
  17. Airhead Wrangler

    Airhead Wrangler Adios Mexico

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    The forks themselves aren't much heavier than 4860s at all. It's the front wheel, brake rotor and caliper that are much heavier than the KTM ones.


    #17
  18. Airhead Wrangler

    Airhead Wrangler Adios Mexico

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    Alright. So with a mix of real measurements and derived measurements (scaled photos) I sketched up the mounting situation with the R-Dub triples. It looks like the clamping sections fall right where they need to, but I STILL NEED TO CONFIRM THIS WITH REAL MEASUREMENTS.

    [​IMG]

    If anyone has a set o 4860s and/or a bare airhead frame to measure I need the lengths of and distance between the clamping sections of the fork legs as well as the length of the BMW steering head between the two bearing races.
    #18
  19. igormortis

    igormortis Long timer

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    [​IMG]
    [​IMG]

    Here are some approximate measurements. Tape is illustrative - not what I measured from.

    - top of fork, to bottom of upper clamping area: 63mm
    - lower scribed line on fork, to bottom of upper clamping area: 56mm
    - bottom of upper clamping area, to top of lower clamping area: 139mm
    - length of lowerclamping area: 93mm
    #19
  20. Airhead Wrangler

    Airhead Wrangler Adios Mexico

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    Excellent. For the record, what year and model of bike did these come from??

    Also, you don't have a bare airhead frame to measure the distance between races, do you?
    #20