Unholy Union geometry

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

  1. Dmaster

    Dmaster Been here awhile

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    Wow, thats interesting :D thanx!
    #41
  2. Prutser

    Prutser Long timer

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    #42
  3. Rucksta

    Rucksta SS Blowhard

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    Can't look at either end in isolation, one affects the other so thanks for this.
    I notice you have a long centre stand.
    Do you have a long front end already?

    Best all round geometry I've had is stock.
    Next best was 1" rise in the front and 1.5" rise rear.
    Bike became a slider with seamless transitions in and out of a slide.
    It used more gas as it sat higher on the road and created more drag.
    Top speed was down.
    Bike became a bit harder to flick through esses on road and became more lively pushing it down into a turn on dirt while maintaining an upright body position.

    Tyre selection is becoming more agressive as the thing becomes a dirt bike.

    Changes to swing arm
    - shorter better turning. twitchy under power. great wheels stands. reduced travel. didn't like it.
    - longer more progressive transition into slide and able to feed heaps more power through the slide to maintain it
    without the back wheel chasing the front. Less so returning to staight tracking about the same as stock.
    - Longer tavel.

    Longer also meant it was harder to break into a slide and the forward weight bias made the front stick better which was OK for 'wheel in line' dirt riding but made it a leap of faith to break the bike into a two wheel slide.

    On road behaviour is starting to fall off now as the lean angles required to turn the bike are becoming more than the knobby tyres are prepared to give.
    The bike is now a dirt road screamer with OK road manners and the potential to be an off road bike except for the front end It has to go.

    With a front end replacent and a long swing arm I have a great off road touring bike with heaps of clearance good travel great weight distribution when carring a load, stable in a straight line and hard to get a leg over worse with luggage on the rack.

    Fork offest is 20mm with Axle lead at 30mm travel is 270/240 and 'Im using 260/235.
    Bike is stable especially in a straight line even in sand but it is hard to dig the front end out of a rut using throttle.
    Weight transfer on the suspension needs to be controlled and some planning is required to throw the bike around.
    Dive under brakes is controlled by lever modulation.

    I want most of that but long distance offroad touring is not the the bikes primary or only use.
    I want to ride dirt roads, fire trails, farm tracks, and enough single track to join them together.
    I want to keep pace with the traffic on the motorway and have effective passing performance from legal speeds on two lane and single lane back roads.

    Changes were required.

    I now have a bike that sits on the standard centre stand with both wheels on the ground under compression (stable on the forecourt of a gas station but only just)
    Ride attitude is level as stock with about a 2" rise front & rear.
    Swingarm is std length with mods described elsewhere
    Wheelbase is near standard (shorter on suspension extension longer on compression)
    Travel used is 220/210
    Triple clamp offset is 25mm
    Axle lead is 30mm.
    Weight bias is towards the rear over the stock bike.

    I'd like some more axel lead.
    I think it will get me closer to the sweet characterisitcs of the original bike.
    I'd also like a little more rear wheel travel and / or a lot less unsprung weight in the rear.

    For me any comparison of the pros & cons has to be filtered by the bikes intended use and what compromises the owner is prepared to make to achieve what he preceives as a pro.
    #43
  4. Airhead Wrangler

    Airhead Wrangler Long timer

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    How do you take such good notes while you're riding? Do you put a dictaphone in your helmet? I come up with a lot of the same type of observations when I have my "alone time" in my helmet, but 98% of it is gone by the time I get to the next gas stop. Your descriptions of handling traits associated with the various geometry changes you've made are very good. Thanks.
    #44
  5. Rucksta

    Rucksta SS Blowhard

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    AW: Never thought about the how too much.
    When I make a change I test.

    Does it go better that it did before the change?
    Does the change prerform as expected?
    Are there any unexpected side effects?
    What else could I have done to achive the desired result?
    I get off the bike and make observations and even reride a section to validate the results.

    I also have ethe beneifit of a 30 year database of observations.
    If I forgot one before the fuel stop it would have got a second & third oportunity to present itself.
    #45
  6. Prutser

    Prutser Long timer

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    Thanks Rucksta. That a great report !!

    All the things you mention are the same as I notice on my bike(s).
    Using the bikes for the same sort of riding.

    I do have a long fork in my bike now.WP48mm with 300mm travel.
    The long straight center stand is changed for one with an angle in the middle. To be able to use some wheel travel before the thing is dragging through the sand.

    Next to my R80ST I do have a R65GS thats still original. The low center of gravity is the best for the handling IMO.
    But the wheel travel and ground clearance from the ST is great too.

    If I would have to choose between the 2 options it would be a real difficult choice.

    After riding Dmaster's bike a few times I think thats the way to go. (R65GS with original swing arm length using max travel with custom shock and a shortened DRZ fork.) (Now only some wing shaped custom triples with the proper off set)


    #46
  7. Ras Thurlo

    Ras Thurlo Desert Lion

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    Given that many are opting for DRZ forks as a cheap/easy way into upgrading the front but not changing the rear - there must be enough demand on ADV for a specific triple clamp........
    #47
  8. Prutser

    Prutser Long timer

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    First I will ask a friend to make some drawings. When me and Dmaster are happy with them we will ask if there is anybody interested in the 49mm triples.
    #48
  9. Dmaster

    Dmaster Been here awhile

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    Well i did :evil
    [​IMG]

    This is with my old set up (Kawa KX 250 front end whit stroke limited to 200mm with really strong springs)
    Bottomed out hard front and back.(I still wonder why my frame didn't break :huh)
    But still with my DRZ front end with 275mm travel (or 270mm sorry i forgot) it will bottom out if I do this kind of crap.
    #49
  10. Ras Thurlo

    Ras Thurlo Desert Lion

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    check out the rake differential between the two bikes.

    was the GS specifically designed with significant rake, or where all bikes like this once?

    [​IMG]
    #50
  11. Beater

    Beater The Bavarian Butcher

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    If they have the same spread (width) between the forks as the DRZ forks do .... count me in. Also as long as the offset is at least a cm greater than the DRZ forks ...

    I want to get that 10 degrees back on either side.

    Thanks!!
    #51
  12. Prutser

    Prutser Long timer

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    We have been working on the drawings this afternoon :evil

    They will have more spread than the DRZ triples. 210mm.

    When I know more I'll let you know Beater.
    #52
  13. Phreaky Phil

    Phreaky Phil Long timer

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    Hi Prutser, what is the reason for wider spread of the triples, More clearance to the tank for steering lock ?

    What suspension travel are you looking to run with your new project. I guess its always going to be a compromise between low COG and easy handling in tight going and enough travel to be able to soak up bumps and holes at higher speed.
    What are the negative aspects of your current handling / suspension.
    Cheers Phil
    #53
  14. Prutser

    Prutser Long timer

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    Hi Phil,

    I would like to fit the triples in my girlfriend's Xcountry too. And that bike needs the wider triples.
    The off set should give more clearance to the tank and the frame.

    On my bike i'm going to use the full travel of the DRZ fork (285mm)
    The negative things about my WP48 mm is the friction those forks have.
    I have tested loads of upsd forks, and even after a lot of time and effort they don't run as smooth as the drz forks.
    With the WP 48 I tend to back up most of the re-bound and compression to get more comfort.
    But than there is no controle left :baldy

    I've re-valved the fork with all the settings you can think of but the friction of the two tubes causes the discomfort.
    The only thing that helped was shortening the fork so the the fork tubes had more overlap.
    But than the DRZ fork was still better, even without re-valving which I will do anyway.
    #54
  15. AntonLargiader

    AntonLargiader Long timer

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    Of course, you have the deepest sump installed. Not representative of a GS.

    I can't see if the bike in back is flat on the ground or on a stand. Looks nose-down. I'll try to compare my XR400R to my GS.

    I also have concluded that 300mm front travel is not at all the holy grail of forks. Not for me, at least. Using that much travel would be far too uncomfortable for the riding I do which is more like rocky jeep trails than fast desert. And for rocky singletrack I ABSOLUTELY do not want the additional wheelbase. 225~250 travel is fine for me. Modern bikes are giving more rear suspension than front, which is very anti-GS. So I would like to improve my front forks without adding a lot of travel, but I'd like to add more travel at the rear.

    As for the appropriate amount of trail, I don't think the GS is perfect at all. Remember, trail is only accurate on flat ground. When you hit a rock with a glancing blow, the contact point is well forward of the point from which trail is measured and can easily give 'negative trail' for that condition. In my experience, the GS is not a very good performer in this way and has less trail than any similar bike that I have noted. Your basic KTM or HP2 has 20+ mm more trail, meaning that when your front wheel bounces off that rock you get that much less leverage fighting your handlebar. BTDT; the Airhead GS is a real workout in rocks. It's 25 year old geometry based on 40 year old street bikes.
    #55
  16. Prutser

    Prutser Long timer

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    My G/S and GS do have the same problem. With the ST its the back of the sump that can sometimes hits the ground.
    That part is not much lower than a GS sump.
    #56
  17. AntonLargiader

    AntonLargiader Long timer

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    It's just over an inch lower in the back; that's a lot in my book. Less, though, once you add in a skid plate.
    #57
  18. naginalf

    naginalf Handy Schtroumpf

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    I'm really sad to hear this. I thought perhaps that Slavens had this figured out with the skf fork seals and better bushings. But, with all of your work on getting the bushings right (have you tried the skf seals?), even with the overlap to get to the stock ride height, would you say that the WP48 forks would then be good, Prutser?

    Damn, every time I read about a solution to problems on these forks, I find another problem. Starting to question it again.
    #58
  19. Dmaster

    Dmaster Been here awhile

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    I do own a XR400R myself, the fork angel is steep, and the trial less then a stock GS don't know how much, can't remember.
    Even now with the proper front springs and a rebuild rear shock its unstable as f...
    It scares me in soft sand :eek1.
    Its only the weight that makes me go faster on the XR (not even much faster).
    #59
  20. Phreaky Phil

    Phreaky Phil Long timer

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    The XR250s are even steeper than the 400s and were great in tight going but a bit scary in the open fast stuff. Fun bike though.
    #60