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

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

  1. One Less Harley

    One Less Harley OH.THAT'S GONNA HURT

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    6-9/16"" from the bottom inner race to the bottom of the knurled adjuster nut just below the top clamp. which If I can tell wraps down past the top of the inner race. My best guess would be 6.75" allowing 1/16" for the metal thickness of the silver knurled nut.

    R100GS front end on a G/S frame.

    Does this give you what you need??

    [​IMG]

    Not sure how far the race goes into the silver ring, guessing the top of the ring is <1/16" thick...something like that.
    #21
  2. igormortis

    igormortis Cafe Reise

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    Forks were stated to be off a 2003 KTM 250SX. Fork lower clamps are stamped &#8217;02.

    I have an Airhead frame I may be able to measure from. *edit: should I just confirm OLH&#8217;s measurement above?
    #22
  3. Airhead Wrangler

    Airhead Wrangler Long timer

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    No, I need the measurement between the bottoms of the bearing seats inside the steering head. Those are the stepped shoulders that the bearing races rest on. Here's a drawing:

    [​IMG]

    139mm is just an estimate. I need the actual. Thanks.
    #23
  4. One Less Harley

    One Less Harley OH.THAT'S GONNA HURT

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    Don't think I'll pull my bike apart to get you those measurements, hope someone comes through for ya!

    BMWblake had a busted up GS frame, wonder if he still has it laying around???
    #24
  5. Airhead Wrangler

    Airhead Wrangler Long timer

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    Oh come on. You mean you have something more important to do? :rofl

    Doesn't need to be a GS frame. I'm pretty sure they're all the same.
    #25
  6. One Less Harley

    One Less Harley OH.THAT'S GONNA HURT

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    I'll guess about 142-145mm, by looking at the outer bands of the headset. See where it is straight then angles inwards at 6'1/4" mark on the tape. The race cannot go lower than that, but you don't want a guess huh. It'll be very close to that.

    I May have some old bearing races to help get me even closer if no one comes through for you.

    How much tolerance do you need??
    #26
  7. Airhead Wrangler

    Airhead Wrangler Long timer

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    Well, I just looked it over and it looks like we have a huge cushion with the measurements you gave me. The steering head would have to be a huge amount longer or shorter before we'd run into trouble. Here's what it looks like in the middle of your range with a race to race dimension of 143.5mm. We'd get a full 27mm of height adjustment sliding the legs through the triples. Everything looks healthy to me:

    [​IMG]
    #27
  8. One Less Harley

    One Less Harley OH.THAT'S GONNA HURT

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    I know 139 mm is way to short do to the angle of the bearing recesses at the head tube, 142- and no way over 150, closest to 143-145 trying to allow for the unknown .

    What year/model of KTM's have the 4860's or has this been discussed elsewhere? What about spring rates, would stockers be useable??
    #28
  9. AntonLargiader

    AntonLargiader Long timer

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    I finally got around to measuring the ones I have.

    66mm upper clamp area
    137mm gap
    91mm lower clamp area
    #29
  10. One Less Harley

    One Less Harley OH.THAT'S GONNA HURT

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    137mm really!!!! didn't think they could go down that far at least looking at it from the outside.
    #30
  11. bgoodsoil

    bgoodsoil Dare to be Stupid

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    I'm getting 134mm.
    #31
  12. One Less Harley

    One Less Harley OH.THAT'S GONNA HURT

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    OK I went outside and looked at my old G/S front end and looked at the steerer tube, for the inner race on the steerer tube spacing is 137mm. The outer race distance would be less. So my original guess is wrong.
    #32
  13. Renner

    Renner combustophile

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    136.8mm between outer race seats on an early /5 frame.

    sorry for the delay, had to tear it down.

    D'oh!
    205'd while wrenching :(:
    #33
  14. igormortis

    igormortis Cafe Reise

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    Just pulled the forks and steering stem.

    I get a measurement of 137mm without pulling the races.

    Hard to tell for sure without removing the races, since they have a taper, preventing a solid stop to measure against.

    Bike is a 1985 R80 Mono.
    #34
  15. igormortis

    igormortis Cafe Reise

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    Argh. Me too.

    #35
  16. AntonLargiader

    AntonLargiader Long timer

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    My notes (for an R65 frame I think) say 131mm between bearing seats.

    EDIT: full notes:

    The BMW head tube is 131mm between bearing seats, putting the inner races of the steering head bearings that same distance apart. The EXC stem allows the inner races of the KTM bearings to be 151mm apart, but the 7mm shorter stem (59401033000) from the smaller bikes should reduce that to 144. Machining the stem from 29mm down to 28mm lets the bearing drop down another 10mm, to 134. A spacer can take care of the rest, although that would raise the front of the bike. Furthermore, if the tubes are raised fully in the lower triple, the upper triple will not go down the extra 7mm, so we'd be gaining 7mm of height.
    #36
  17. Airhead Wrangler

    Airhead Wrangler Long timer

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    Hot damn. A lot can happen during the ride home from work. I'll update the drawing tomorrow. None of this changes much, everything still works fine. Woo.
    #37
  18. bgoodsoil

    bgoodsoil Dare to be Stupid

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    thanks! :lurk
    #38
  19. Airhead Wrangler

    Airhead Wrangler Long timer

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    OK, 137mm seems to be the consensus, but Anton's measurement of 131 is a bit troubling - maybe chalk that up to being an R65 frame. 137mm seems to jive well with R-Dubb's design, specifically the length of the steering stem.

    [​IMG]

    This measurement is more in line with what Anton was talking about - the bottom clamp just barely staying within the bottom clamping area. Only 3mm to spare here.
    #39
  20. Airhead Wrangler

    Airhead Wrangler Long timer

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    Let's change gears and talk about brakes:

    I'm not sure what happened since the last time I bought a brake rotor, but apparently somebody decided that all brake rotors from now on should look like goofy ass ninja throwing stars, something a 17 year old might buy at autozone for his juiced up honda hatchback with a huge muffler. :huh

    [​IMG][​IMG][​IMG]

    Anybody know of a good quality regular old ROUND brake rotor for KTM dirt models? Ideally, something larger than OEM KTM and floating.

    Does anyone know about compatibility of dirt model, LC4, and LC8 rotors? They look like the same bolt pattern and diameter. They don't seem to have any lateral offset. Anyone tried switching them between bikes?? I plan to use the stock caliper and MC of a 2006 250SX-F.
    #40