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 Long timer

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    Here's your homework for you. RFS (dirtbike) model forks shown on the left, adventure forks shown on the right. Assuming you use triples that are level straight across from the bearing seats to the clamps and of similar thickness to R-dubb's design, here's what it'll look like. With a 6mm spacer under the top triple (shown in pink), you're triples will be fully on the claming sections of the fork legs. The legs will protrude 29mm through the top of your top triple. You could make a lower triple where the clamps are lower than the bearing seat - sort of boomerang shaped - if you don't want the legs sticking so far out the top. Not ideal, but definitely workable.


    [​IMG]
    #81
  2. bgoodsoil

    bgoodsoil Dare to be Stupid

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    I guess the question is: where does the taper on the fork tube stop/start? Is the flat area equal where it meets the top triple and the lower triple so that they can both slide up and not run into the tapered part?

    I wouldn't be worried about that axle at all. The weight difference between the dirtbike and the BMW doesn't matter. The fact that the KTMs are made to fly 30 feet in the air and crash down on that axle probably more than makes up for it.


    mmmmmm popcorn :lurk
    #82
  3. Airhead Wrangler

    Airhead Wrangler Long timer

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    Also, the 950 Super Enduro used the smaller 26mm dirt bike axle. The claimed dry weight of the Super Enduro is 407 lbs which means in reality on earth it weighs 430-440. That bike is made to do things that nobody but Stagehand would attempt on a BMW, so I feel pretty safe using the 26mm axle.

    EDIT: Woops I just noticed that I already said all of this above.
    #83
  4. Chadleys1

    Chadleys1 working on it

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    Airhead Wrangler, THANKS for "doing my homework," posting the diagram a couple of posts back, and for addressing my issue directly.

    Maybe a 6mm "boomerang" in the bottom triple could eliminate the spacer up top?
    Maybe I can get someone affiliated with WP to tell me it is OK to clamp a little outside the lines too ?

    I am grateful for your helping me get this in order, I'll keep working on it and post if I come up with anything good.

    Most importantly, the situation seems WORKABLE.

    About the axle, I would feel safe on it too, with no worries whatsoever.

    Bgoodsoil, you brought up a good point. I'll definitely take that into consideration.

    Thanks fellas.
    #84
  5. igormortis

    igormortis Cafe Reise

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    Just be aware that the 950 sliders are longer than the dirtbike forks - see linked pic on 1st page. So, for the same amount of given travel, you end up with a taller front end.
    #85
  6. Airhead Wrangler

    Airhead Wrangler Long timer

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    Also, check out posts #68 and 71. That guy used forks off a 950 Supermoto so he had to make triples with a monster offset, but it looks like he had the same distance between clamping sections to deal with as well.
    #86
  7. AliBaba

    AliBaba Been here awhile

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    I wrote the comment because I've bent the axle on my EXC400 - but it might have been a freak-accident.
    If your bikes starts to eat wheel-bearings then check the axle :evil
    #87
  8. Chadleys1

    Chadleys1 working on it

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    Or in my case, with an unmodified 950 front end compared to the SX, the same overall length fork with a little less travel.
    The SE fork in that photo igormortis mentioned is modified, having 310mm travel, so it is considerably longer overall than mine.

    An early measurement on mine showed my 950 forks around 1-1/2" longer from top tube to axle center in comparison to stock Marzocchi GSPD fork.
    With this considered, the 29mm of fork protruding out the top triple seems just about right for me. Ideally, I'd only like one inch of extra height in the front end.

    Thanks for the heads up, to you and Airhead Wrangler.
    This is all valuable information for me.
    #88
  9. aerone

    aerone Apex Addict

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    I had my machinist put the whole fork leg including the lower brake mount in the lathe. I taped the lower to the upper so it would not sling around during machining. He used a steady rest and did not use a center on the lower end. We took off about 3 inches taper to get the proper rake/trail. The lower clamping area is no problem, just the upper. It's working great BTW.
    #89
  10. Chadleys1

    Chadleys1 working on it

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    aerone,
    I looked at your pics from earlier in the thread... COOL BIKE !
    I especially like the excellent brakes, you MAY have the most sophisticated front brakes of any Airhead GS. ?

    I wonder if your SM 4860 forks had the same spacing on the machined-for-clamping-areas as the 4860 forks I have from a 950 ADV ?

    I sent you a PM.
    #90
  11. igormortis

    igormortis Cafe Reise

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    OK, what’s everyone using to work on their forks? Is there a reasonable source for the special tools or are people making their own?
    #91
  12. Airhead Wrangler

    Airhead Wrangler Long timer

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    They can all be improvised with random stuff you have laying around, but this stuff would probably make life a lot easier:
    http://slavensracing.com/products/suspension/suspension-tools1
    #92
  13. igormortis

    igormortis Cafe Reise

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    FYI, here are the bits I used to make the GS wheel fit the KTM forks with HPM/R-dubb clamps.

    <a href="http://www.flickr.com/photos/58374333@N07/6837204074/" title="IMG_0009 by c00300875c487ce28b95948225d005e0, on Flickr"><img src="http://farm8.staticflickr.com/7058/6837204074_07b6f667f9_b.jpg" width="1024" height="768" alt="IMG_0009"></a>

    From left to right:

    - Monolever road bike axle. 30mm, 25mm (mine was from an ’85 R80)

    - standard axle spacer. 11mm
    - standard axle spacer, machined down to approx 17mm (spacers and axle came from the same road Mono bike)

    - steel sleeve, 0.5mm thick to bring the BMW axle up to the KTM’s 26mm. Split to allow tightening. Approx 58mm in length, with about a 4mm “hat” to keep it located. This is against the inside of the axle clamp and acts as an additional spacer.

    - steel shim. This is what the hat part of the sleeve tightens against when the axle is pulled into position by the end bolt.

    Not pictured: Axle end shim and bolt. These are standard items from the above Monolever bike.
    #93
  14. aerone

    aerone Apex Addict

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    That looks much the same as what I did to use the BMW axle and wheel with the 950 sm front end. I have not measured the length, it does have to be split so that it will clamp.

    As far as machining the top, I guess it depends on which application you are using, sm, adventure, sx or what. I believe there is enough metal in the tube not to worry about getting too thin, no matter how long a surface you cut the taper out of.
    #94
  15. Solo Lobo

    Solo Lobo airhead or nothing

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    HPMGuy is making a new run of his triples for this application:

    http://www.advrider.com/forums/showthread.php?t=778520

    #95
  16. Airhead Wrangler

    Airhead Wrangler Long timer

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    Another question. Has anyone actually installed their 4860 triples and forks? I know 6 people bought them, but I haven't seen any questions or photos posted here. What happened? I'd be all over this myself if I weren't frameless and my forks and triples weren't 3000 miles away.
    #96
  17. AntonLargiader

    AntonLargiader Long timer

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    I wonder, also. I'd really like to ride a bike with this installation to see how the geometry works. The impression I got was that the offset was selected to create the right trail on a Monolever when the front is higher but the rear isn't.

    I don't want to raise the front much at all. Looking at the forks I have,it seems that I'll end up a bit higher just due to the position of the clamping areas, which is OK, but I want to limit that as much as possible.

    Interestingly enough, the KTM triples (from the EXC at least) have very little offset, meaning more trail. In fact, the R100GS has less trail (100mm) than any comparable bike, which I suspect is why there is so much kickback when you hit rocks sideways. I'm not in love with the BMW geometry so I see no need to duplicate it for handling's sake. OTOH, of course, I do appreciate the greater steering lock that comes with more offset, but you have to compromise somewhere.

    So, the new group buy is tempting, but I'd like some sort of indication that the offset is appropriate for my particular needs. I'll have to sift through the details more, but in the meantime I'd like to see some feedback from the existing user base.
    #97
  18. Airhead Wrangler

    Airhead Wrangler Long timer

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    I don't think that's right. R-dubb designed these for his own bike which had an extended monolever swingarm - 100mm longer than stock, not sure on the shock length. Also, they were originally designed for the leg-to-axle offset of WP 4354 USD forks I believe. It's all in his design thread (which I'm too lazy to go back and check myself right now)
    #98
  19. AntonLargiader

    AntonLargiader Long timer

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    OK. Dunno if the 4354 has a different axle offset (I sort of doubt it's significant) but the last diagrams I see in his design thread show 35mm offset and the clamps are made to 38mm, so clearly there was some more work done.

    Anyway, you sort of confirm my suspicion that this clamp may not be optimized for my wants. I'll just have to do a bunch of figuring and see where the trail ends up on an R100GS.
    #99
  20. Airhead Wrangler

    Airhead Wrangler Long timer

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    Try talking to Mex on here. His bike uses a stock monolever and 4860 forks. He used triples from a german company that have 39mm of offset and said the handling was much improved over the stock KTM triples that he tried. I'm not sure how much if any he shortened the forks.

    [​IMG]