USD forks on GS's

Discussion in 'Old's Cool' started by Padmei, Feb 20, 2011.

  1. Padmei

    Padmei enamoured

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    For those that have done USD conversions on their GS's, could you please tell what mods had to be done on the steering heads, cups etc for the different types of forks.

    I have read many different threads where it has been done however if the different mods are listed on one thread it would make it a lot easier to choose which type of fork to use.

    I have a set of YZ's in the shed which I'm tossing up putting on my project R80GS, however just reading Udos R65 thread it seems a CR set maybe easier.

    So YZ
    CR
    KTM

    What are the Pitfalls?
    #1
  2. Udo

    Udo Been here awhile

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    Finding and using the CR500 forks and wheel was a totally random act. I wanted thick, strong tubes, disc brake, and 21" wheel. The CR500 I saw on Ebay, being sold by a local bike wrecking yard, fit that bill. The fact that the head bearings were the same was just a bonus. It would have been fairly easy to mix and match various sized bearings. Knowing then what I know now, I would have looked much harder at the the fork angle or rake. The CR500 forks have a larger steering angle than the BMW R65 did. This makes for a large turning radius, not what you want in a dirt bike. '
    Hope these thoughts help you decide.
    #2
  3. Rob Farmer

    Rob Farmer Long timer

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    Pete Keys has posted some good info here on fitting DRZ forks into a bike he's rebuilding.

    Link

    Looks good?

    [​IMG]
    #3
  4. Padmei

    Padmei enamoured

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    Thanks guys.
    I have been following Petes build Rob & it's looking good.

    Have any guys sleeved their usd forks to reduce the amount of travel & stuck with then para rear end.

    My rear shock is rooted so need to kind of work out whaty i'm gonna do with the rear end (ie do a mono conversion) before I spend money on a good shock
    #4
  5. Steve in NZ

    Steve in NZ scared/cheap

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    #5
  6. bgoodsoil

    bgoodsoil Dare to be Stupid

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    The problem with fitting most forks is the offset--the distance between the center of the stem and the center of the fork tubes. BMW used around 35mm and almost everybody else used 20mm. All the KTM triples will have the tubes banging into your tank. Also, it'll be twitchier on the highway.

    Ducati uses 36mm on many of their triples. I've thought about trying to use one of their triples and some KTM tubes/wheel/brakes. I'd need to find a center to center measurement between the tubes for the Ducati and the KTM though. If they weren't the same the axle wouldn't fit right and the calipers wouldn't line up with the rotors. I don't have a good way to find those numbers.

    As far as the stem goes you can probably find a bearing that'll have the same ID as the stem and OD as the BMW. Worst case scenario you have a few thousandths machined off the stem.

    Most USD forks will be several inches longer than stock but that's not a big deal. Any suspension shop can cut the tube and rethread it. Then you find a spring from Racetech or Eibach.

    I've got a set of cheap DR650 forks on my G/S that are worlds better than stock. They're not USD and they don't have the correct offset. I'm mostly happy with them so I'm in no rush to do all the research to make the Ducati triples work.

    Also, you could get custom triples from a company like this.

    http://www.bankeperformance.com/flattrack/tripleclamps.html
    #6
  7. Oz Nutter

    Oz Nutter Long timer

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    Interesting article about a Yankee bloke , Chris King, he uses RM 125/250 forks (USD Showa) on his G/S, makes his own steel steering stem.
    #7
  8. Houseoffubar

    Houseoffubar fine beer sampler

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    Unless I have a fundamental misunderstanding of steering geometry, by going with KTM triples (or any with LESS offset) this will increase trail, in theory making the bike more stable, though heavier steering. It will, of course, as you say, decrease steering lock.
    #8
  9. bgoodsoil

    bgoodsoil Dare to be Stupid

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    I probably got it backwards. My front end twitchiness is probably related to my bent frame :D
    #9
  10. Houseoffubar

    Houseoffubar fine beer sampler

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    Ouch!:cry

    Excessive trail can be an issue as well. Faint rider inputs can cause a weave, and the combination of longer forks, and less offset, will increase trail even more, unless the rear is raised the same amount as the rear.
    #10
  11. bgoodsoil

    bgoodsoil Dare to be Stupid

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    I found this while looking at Ducati triples. This is from a company that makes aftermarket parts for Ducatis.

    #11
  12. Houseoffubar

    Houseoffubar fine beer sampler

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    Wow, you mean THEY got it backwards too?:lol3

    I ain't proud, I could be wrong, but to my thinking, the more trail (in theory) the more stable. The more offset you have the less trail, so.......????

    This has always been my understanding, perhaps I need to be taken out, and educated with a big stick!
    #12
  13. Lornce

    Lornce Lost In Place

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    Sounds to me like you're one of the few folks posting here who actually understands a motorcycle's steering geometry. :augie

    You're absolutely correct, Fubar. Less offset isn't as big an issue when putting these front ends on airheads, as making sure the rear is raised the same amount to maintain workable geometry.

    Less offset increases trail

    Increased trail increases dynamic stability. ie: it's more solid on the highway.

    The reason many of these more modern front ends have less off-set is because they've been sourced from bikes with steeper steering head angles than the airheads we're putting them on.

    That'll probably only make sense to you if you understand the relationship between rake and trail and their effects on a motorcycle...

    :brow

    :augie
    #13
  14. Ron Seida

    Ron Seida Adventman

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    [​IMG]
    I built this front end for my PD back in '05. Due to clearance issued describer earlier and the expense of an HPN tripple clamp, i built my own and replicated the stock geometry.
    [​IMG]
    The forks are WP 4860 from a KTM dirtbike, with the large 26mm (i think :scratch) axel. The forks were shortened to 8.5 inches and re-sprung. The tubes do not need cutting and re-threading, only spacers placed inside to prevent them from retracting the full stroke. Brake fitment was complicated as i failed to replicate the stock KTM spacing, a lesson learned. If i was to do it again, as i have done with a DR650, i would replace the entire front end including the wheel and axel, solves a lot of fitment problems. Also allows you to use common bolt-on brakes. WP 4860 are an excellent option due to their huge availability on the used market for a fair price and the excellent bolt-on brake products from KTM racing. Properly tuned they are excellent forks.
    #14
  15. Stagehand

    Stagehand Imperfectionist

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    I love that bike, Ron. It totally makes sense to use a complete front end. How was getting the steering head bearings and stem to work?
    #15
  16. Country Doc

    Country Doc Wanderer

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    You made your triples, just from scratch???

    :bow.

    Care to make a few more sets??

    dc
    #16
  17. Solo Lobo

    Solo Lobo airhead or nothing

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    You can buy a version from http://bmwboxersupplies.com/ or

    Member HPMGuy also made a run of sets using plans drawnup by R-Dubb and successfully used by Roadsacallin to ride around the world... expect to pay north of $500 for a set.
    #17
  18. Country Doc

    Country Doc Wanderer

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  19. Solo Lobo

    Solo Lobo airhead or nothing

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    Those are the one, don't forget to deduct 19% for VAT we don't pay.

    Matt, the owner has told me these work for 50mm conventionals, but I would confirm. The word from Belgium based ADVR's is that they are fine, but are a compromise. I don't know any more that that, and don't know if R-Dubb's design solves these compromises or not.


    #19
  20. Ron Seida

    Ron Seida Adventman

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    Yes, the clamps and risers were milled from billet aluminum. It took me a whole weekend and a box of beer, but if i were to do it again, i would draw them up in CAD and e-mail it to a water cutter. For the cost, well worth it. Those clamps mentioned are stupid expensive, and the main reason i built my own, but at the time i was blessed with full access to a machine shop. I believe you may be able to use a 22mm offset KTM clamp with the WP 4860 forks with the non-PD tank with no clearance issues, but i have not confirmed this. The major benefit to the 4860's are multiple. They are found on every KTM build in the last 10 years or so, so there's lots around. So is the know-how for re-building them, which is pretty much mandatory if you want to bolt them onto the "Rubber Cow", unless your lucky enough to get a set from a 950 or 990 non-S model. They are stiff enough for the weight of the beast and cheaper to rebuild. Working the stock BMW wheel into the mix is a nightmare unless your ready to do a lot of trial and error machining. The front wheel is worth enough on the used market to almost pay for a brand new one from Woody's. You will need to shorten the forks, otherwise you rake out the bike and will be left with really bad handeling. The stock master brake cylinder has the advantage of replaceable cylinders, so you can play with the different sizes, mine ended up being a 13mm for the Berringer 4-piston caliper, resulting in VERY strong front brakes. Send your forks to a pro-shop to get the internal work done, you will be astounded by the result if they are done right :deal. Bolt on some wicked brakes and off to the races!
    :freaky
    ...oh, to answer Stagehands question. I milled the stem from a piece of solid 6061-T6 aluminum, there is no steering lock. I felt this was more than adequate as KTM uses a hollow aluminum facsimile of said stem. I machined it to fit the stock BMW bearings but made a point not to interference-fit the bottom bearing for ease of replacement without a puller or a press, for travel purposes. Bearing adjustment is also the same as a dirtbike, you need to loosen the top clamp first, undo the pinch bolt, adjust and then tighten everything up. I milled the adjusting nut so that you can use a screwdriver from your kit instead of some rediculously huge wrench you don't want to carry with you :huh.
    #20