Unholy Union geometry

Discussion in 'Airheads' started by Ras Thurlo, Nov 3, 2012.

  1. Ras Thurlo

    Ras Thurlo Desert Lion

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    Picked up above quote from Ontic, from another thread on airhead off road suspension

    Am looking to tap into the ADV knowledge-pool on how to tailor a full-throttle unholy union conversion (full length fork upgrade and extended mono/R1100 driveshaft) so as to end up with the right handling characteristics.

    We get into these full-throttle conversions in order to max out on suspension travel to create a badass dirtbike- but in the process move in the opposite direction to a supermoto conversion.

    I am making the assumptions that in output there is a trade-off between "flickability" and "stability", while the inputs include wheelbase, ride height and trail variables such as fork rake, front offsets etc.

    For my part, I am looking to preserve as much flickability so as to mitigate the wheelbase and ride height that comes from monster forks and extended rear.

    I have tried to do my homework on trail variables- but have only succeeded in getting more confused, as on the interweb there are diametrically opposing statements of "fact" regarding the impact of the differing variables. So have now given up on the abstract theory and am appealing to you from a practical perspective......
    #1
  2. bereahorn

    bereahorn Long timer

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  3. ontic

    ontic

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    Haha, me too!:D It is a little bit scary quoting me in a thread like this.
    From my rough measurements, quite a while ago, and they were rough, relative to the fork tube centre, the WP50 axle centre seemed around 13 mm forward of the G/S fork axle centre.
    I don't know how that compares to a 4860 fork, from images I've seen it must be a bit different, so it might a be a good thing for us to actually accurately re-evaluate and compare these measurements.
    It would be interesting to look into the big DRZ forks (and others) as well in this regard.

    There really are so many variables. I'd love to hear from people that can actually predict the results of so many different changes let alone only one (which I have also read of and been told of diametrically opposite effects), but for me it will have to come down to the actual riding of the thing.

    I've done a bit of short test riding on my new front end, at a few different fork lengths, before I had to put the project on hold, and it feels pretty good- but really Roadsacalling, Solo Lobo and Hardwaregrrrl and any others who have done a good amount of real riding on this front end should comment.

    As you would have seen the WP50 forks are very easy to change the length of. For starters you can slide them up through the triples, but shortening them from within is also pretty simple. I found changes in the fork length made for some very significant changes in how the bike felt. Too long felt all choppered out... short feels a lot more 'flickable'.

    I don't know of any HPN mods for 'flickability'. I think they sometimes change the rake of the headtube (or at least there has been a lot of speculation on this?)... increasing rake, increases trail... with common theory saying that would increase stability, not 'flickability'...

    But again, most of these builds really do have a lot of different changes going on at once so it probably can't be that simple.

    Driveshaft- I don't know the length of the R1100 shaft, but it longer yes?

    I'm keeping mine 'short' (ie stock) and will be raising the rear a little by attempting to emulate a mod that Rucksta here on this forum has described a few times- by moving the the lower shock mount on the swing arm forward a little (and then a bunch of other stuff to make sure it works).

    With the shortened forks (to somewhere around G/S length) which I probably shortened a bit too much, I really do like how the bike rides now- it feels very 'flickable'. I am leary of creating a super stable long and tall beast that would be great for bashing through deserts but not so great for twistys and trails and touring and basically most of the riding I do. I'll sacrifice some stability for a nice turning bike that I can still touch the ground on.
    but I do want a bit more travel than I currently have, so I am going to raise the rear a little, and see how much I can raise the front and have it still feeling right like it does now.
    It is really just going to come down to that with mine- changing things more subtly from now on and seeing how it feels.

    What might help deal with 'instability', I've got a scotts stabiliser on the bike as well.

    Triple Clamps
    With the 50mm forks (and other smaller conventionals) and with some creativity, there are quite a few triples from big USD forks that could be made to work (sleeving down the tube holes which are usually bigger than 50mm) which allow for different offsets. I've got a set with 24mm offset on the shelf I want to try one day.
    HPMGuy told me, for an extra cost he could make the same kind of Rdubb triples but with a different offset, so that might be an option as well.

    Forks
    In the end, I probably would not let the geometry issue rule my fork choice- I'd choose the forks I wanted and then try to deal with geometry... and yes, the 4860's do seem to come in a quite a few different versions, not just in length but in internals... confusing to me.


    And that is me done. Sorry for the ramble.
    #3
  4. Ras Thurlo

    Ras Thurlo Desert Lion

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    I guess that ride height + wheel base will come down to a compromise between suspension travel and handling - influenced by subjective factors such as rider height and typical ride terrain

    This therefore leaves trail as a more objective set of trimming variables.

    So following this line of argument, the question then becomes (for my goals) what can be done about reducing trail - be it through reduction of rake or offsets.

    A recent conversation with Klaus @ HPN led me to believe that they do address rake angle on the headstock, but I am not quoting him on it as I did not dwell on this at the time, so may have misunderstood.

    Common sense would say that triple clamps would be an easier and less invasive means of getting to the same end. But dont know what is a reasonable bandwidth of t-clamp rake and offset variance.

    I am trying to understand how to have max travel suspension and min trail - as my ride terrain lends itself to flickability rather than straightline desert blasting. However understanding how to get there helps both ways
    #4
  5. Dmaster

    Dmaster Been here awhile

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    Sins i have put in a DRZ fork I'm not completely happy with the geometry.
    Actually,.... before the DRZ fork I had a Showa KX250 USD fork with KTM triples, and that didn't work either.
    Suspension is pretty much ok now, but when I ride a stock R65GS it handles SO MUCH BETTER from trail riding to "high speed" soft sand. (high speed with a R65GS :rofl)
    Suspension sucks on the stock one but geometry is godlike in my opinion.

    So I'm thinking about making custom triple clamps matching stock geometry.
    But there is one thing 1 keep thinking about, my bike is higher than a stock one so what to do with the trial/ head angle to compensate it?
    Anyone knows what BMW is doing with the G/S and GS models? Somehow I can't find specs :(
    #5
  6. Clay Spinner

    Clay Spinner Been here awhile

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    dmmaster...There are a few things that may be useful... try reading THIS.... then add your name to THIS
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  7. Prutser

    Prutser Long timer

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    Those are not wing shaped like the bottom DRZ triple.
    That would mean the bike will be higher than necessary using the DRZ fork if you would like to use the full travel.
    #7
  8. Clay Spinner

    Clay Spinner Been here awhile

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    But it is something that may be useful should he wish to use another fork... that with the yokes takes the guessing game out of the equation. I believe the thread also gives some details... or at least links to... some numbers regarding trail and anglie thingies.
    #8
  9. Dmaster

    Dmaster Been here awhile

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    Its probably somewhere in all those threads but I can't find the data I would like to see.

    This is helpfull (found this on the ORGS build op thread)

    Code:
            R100GS             
    wheel base      1513 mm            
    rake            28.0 deg       
    trail           100 mm            
    triple offset   37.5 mm           
    fork lead       38.0 mm            
    total offset    75.5 mm          
    
    But i would like to compare it to the G/S and other similar bikes.

    And like Prutser mentions I need the "wing" shaped bottom triple clamp because I don't want much extra hight at the front and would like to use as much travel as possible.
    #9
  10. Dmaster

    Dmaster Been here awhile

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    The DRZ fork is the best I've had in my hands till now. (for this bike)
    And I've seen a lot of forks ;)
    #10
  11. Prutser

    Prutser Long timer

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    :evil

    The WP extreme wasn't to bad ! ? :D
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  12. LukasM

    LukasM Long timer

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    I might have to help out a friend with a WP swap on his Beemer so I'm in.

    It's pretty simple really, if you want to keep the stock handling but with better quality suspension then just copy the stock total offset (triple clamps + fork lugs), fork length, and stroke.

    KTM triples are available in a variety of offsets, although nothing below 11mm or above 35mm I believe. Fork bottoms on the WP43 and WP48 have the axle leading by 35mm if I remember correctly.

    So what are those measurements on a stock G/S?
    #12
  13. Prutser

    Prutser Long timer

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    Hi Lukas,

    What fork are you going to use ?
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  14. Dmaster

    Dmaster Been here awhile

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    But those had the gold blingbling on the inner legs. THAT one wasn't bad at all ;)
    But other WP extremes i felt :cry
    #14
  15. LukasM

    LukasM Long timer

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    Hi Bas,

    Probably the open cartridge 48s from a 690 or 950, have both here. Not sure if the wider or narrower spacing between the upper and lower clamping area is better?

    My friend is on a tight budget so we'll have to look for a cheap set of KTM OEM clamps that will work well. I'm thinking 950/990 SM or Duke will have the highest offset.
    #15
  16. Dmaster

    Dmaster Been here awhile

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    The thing is, i do add a bit extra height and especially stroke. (I'm after suspension travel)
    So what to do to make it steer like stock (or at least feel like as much as possible)
    Do I need more trial because of the height increase, and how to get that.
    Or stock trial and another steering head angle?
    #16
  17. LukasM

    LukasM Long timer

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    Unless you are going to have it professionally done I would forget about changing the actual steering head angle on the frame.

    To compensate for the longer forks you would want more offset to keep the same turning ability. Of course it's a balancing act to see when it's too much etc.

    Or just increase the rear travel and height the same so the bike stays balanced?

    Keep in mind that only a part of the additional travel will be actual height increase due to additional sag and the fact that the forks are at an angle.
    #17
  18. Prutser

    Prutser Long timer

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    Erik is going to make the clamps for me and Dmaster. But he is also going to do some in the WP size for the X with 210mm
    The OEM 190mm WP triple size does effect the turning circle a lot !
    #18
  19. Clay Spinner

    Clay Spinner Been here awhile

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    Anything useful in this....
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  20. LukasM

    LukasM Long timer

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    Interesting! He should advertise them on the forum, I bet there will be some interest. Erik really needs to get a helper to put all his stuff online.. :wink:

    I'm pretty sure that the big KTM street bikes (and even the LC4 SM) actually have 210mm spacing, so that should help.
    #20