G/S swingarm + driveshaft extension 50mm

Discussion in 'Airheads' started by cenji, Feb 10, 2012.

  1. cenji

    cenji out4fun

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    I am in the process of upgrading my Front-end of a 1981G/S. The new KTM setup will raise the front by two inches. I might be on the wrong track here and ask those to chime in if indeed I am, but I thought too match that rise in the back as well. Bringing things back in balance. There are three options to my knowledge:
    1. a custom shock
    2. extending the driveshaft and swingarm
    3. or combination of shock and swingarm with driveshaft


    What I like to accomplish is based on, but might make no sense to some:

    A. change the ride characteristics of a stock G/S very little, means tight cornering/handling should be the least effected.
    B. I will not race the bike, so a long wheel base for stability in sand etc. is not necessary.
    C. easiest maintenance.
    D. I want the additional clearance = 2", so shortening the forks is not desired.

    My questions are as follows:

    Is there an ideal way how to proceed and if there is, please share your thoughts why it is ideal?
    Is there a non desired/false/wrong way that should be avoided?
    If you have done a similar conversion, what have you found out (does it work well or not, are your happy)?

    Does any one have the capability to do a schematic drawing or come up with a formula or might know where one exists, that will explain, what rise I will get in the back with a 50mm extension?

    I have a Wilbers shock that is currently on the way to the Beemershop/TED and he will look if a shaft extension can be done to extend the shock to reach a measured 38.75cm, measured from eyelid to eyelid of shock.

    I have found two values that explain the shaft-extension ratio to rise under the motorcycle. 1 to 1:6 or 1 to 2.0, that means, that if the shock is extended 2 cm, I will raise the bike either 3.2 or 4 cm.
    This shock extension will result in a steeper angle of the shock and require to change the frame cross member above the swingarm, the one that the battery case is mounted too. That can be easily done, by cutting out and re-welding it. No expense for me. It also requires to take some of the inner bell housing off, slight dremel work. I has been done by others and although the angle is steeper for driveshaft, the joints are still fine after 100k. No concern there.


    Your help is appreciated. Safe riding.
    #1
  2. Solo Lobo

    Solo Lobo airhead or nothing

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    I would think option two would require some re-work of your shock (re-valving and/or re-springing) as the ride characteristics would change due to the increased forces applied by the longer swing arm.

    Theoritically, to determine the effect on ground clearance with a 50mm extension you could measure from the center of the final drive to the ground, and then run a straight edge from the center of swing arm pivot point through the center of the final drive and measure 50mm beyond the final drive center the distance from the bottom of the straight edge to the ground.

    take the difference between the first and second measurements and that would be the increase in ground clearance.
    #2
  3. One Less Harley

    One Less Harley OH.THAT'S GONNA HURT

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    Be very careful in lengthening the rear shock as it may result in u-joint problems.

    My rear shock (YSS) was adjustable for length and when lengthened there was a noticeable vibration in the drive shaft, so take your chances. I can't tell you how much I lengthened it, but I suspect under 2", probably closer to 1".
    #3
  4. Airhead Wrangler

    Airhead Wrangler Adios Mexico

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    #4
  5. kbasa

    kbasa Roubaix! Super Moderator

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    Please read the Old's Cool rules when making a new thread.

    Thank you.

    Your mod,

    Kbasa
    who just had to move your thread.
    #5
  6. supershaft

    supershaft because I can

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    That dashed line isn't rake. Axle offset has nothing to do with rake. So that necessarily means that that diagram isn't accurately showing what trail is either. Who comes up with some of these diagrams? There's a Bing carb diagram that is floating around too that is all wrong.
    #6
  7. Airhead Wrangler

    Airhead Wrangler Adios Mexico

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    hmmm. I think you're reading it wrong. There are two frames shown here: stock G/S and longer travel front end with lengthened swingarm. The dashed yellow is the rake of the raised front end shown in the pinkish red. See the 43mm triple offset and 35mm axle offset at the bottom. The original rake of the stock front end (darker red) isn't shown.
    #7
  8. cenji

    cenji out4fun

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    ALL THE OTHER INPUT, VERY APPRECIATED.

    BECAUSE I AM LAZY TONIGHT AND HAVE NEVER USED THE FREE CAD PROGRAM, COULDN'T I JUST USE THE NUMBERS PROVIDED IN the DIAGRAM:

    100MM EXTENSION IS RAISING THE REAR ROUGHLY 75MM. MEANING THE 50MM EXTENSION WOULD RAISE IT HALF OF 75MM = 37.5MM? That would be close enough.

    SOLO, WHAT YOU SAID MAKES PLAIN SENSE. I will give that a go tomorrow if I can do it on my own. Finally with have some time to plunk down a whole day on it.



    #8
  9. Airhead Wrangler

    Airhead Wrangler Adios Mexico

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    No. There's a lot more going on there than just lengthening the swingarm. The relative shock length has been increased as well. Notice that the two swingarm configurations shown are not parallel to each other.
    #9
  10. cenji

    cenji out4fun

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    seems like you have collected some practical and theoretical knowledge that is needed to make this G/S project successful.
    Compared to the stock shock, how much would I need to lengthen the stock shock
    A) with the 50mm extension?
    B) without the 50mm extension?

    to match the raise on the front of 2"?

    Thanks for your feedback and what formula are you using to calculate the missing numbers.

    #10
  11. supershaft

    supershaft because I can

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    Thanks AW!

    Now I see what the diagram is up to but effective rake and trail all depends on how much sag a setup has. It isn't just about ride height.
    #11
  12. Airhead Wrangler

    Airhead Wrangler Adios Mexico

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    Correcto. R-dubb figured in some rough sag numbers in the fork lengths and swingarm angles. I can't remember what exactly, but it's all in that thread in the link.
    #12
  13. supershaft

    supershaft because I can

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    Thanks again for straightening me out on this one AW!
    #13
  14. Airhead Wrangler

    Airhead Wrangler Adios Mexico

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    I haven't. R-dubb did all this. I don't have access to his CAD file and I don't have my frame around to take measurements from. Sorry. You could always experiment with different lengths of pipe to simulate a longer rear shock with some shims under the front wheel to simulate a longer front end. ...but then a piece of pipe won't give you any sag.
    #14
  15. ontic

    ontic

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    Just on a quick break from the shed (from fitting my WP50 front end:evil)-
    To address Cenji's question about how much the rear end would be raised from a 50mm swing arm extension-

    my guesstimate is around 13-14mm- give or take. No way near 37.5mm.
    I got this by simply looking at the geometry of the bike, particularly the angle of the swing arm- and translating that with a 50mm extension-
    ie. angle of swing arm to me looks around 20-25 degrees- extend that out by 50mm and it goes up around 13-14mm.
    This is assuming that rear suspension sag percentage doesn't change.
    If my 20-25 degree eyeballing of the swing arm is off, then obviously the height would have to be adjusted, but this is just very rough ball park figuring.

    Does that sound about right, nothing to engineer from, but just an OK guesstimate of increase in height?

    Back to the shed now,
    #15
  16. lewisjohn

    lewisjohn Adventurer

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    My project bike has a pair of DR forks and a 75 mm swingarm and driveshaft extension,will use a standard length shock.The bike sits level,i extended the centre stand by 100mm
    #16
  17. ontic

    ontic

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    Lewisjohn, To me a 75mm extension of the swing arm using the same rear shock that required a 100mm center stand extension means that your forks are a lot longer and the geometry has changed-
    but then I'm far from an expert at this, don't know how to use CAD, and am just using high school geometry:D

    Show us a picture of your bike:wink:
    #17
  18. Rucksta

    Rucksta SS Blowhard

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    Ontics is on the money with his estimates.
    50mm extension = 12- 15mm increase in ride hight.
    The final figure depends on the amount of sag you run.

    If you use the same spring and lower shock mounting point that you used on the stock setup your ride hight will remian unchanged.

    Longer shocks or moving the lower mount forward require removal of metal inside the swingarm to avoid the uni fouling.

    The joint itself seems good for the extra angle.
    I haven't replaced a uni for 90,000km or ever since I jacked the rear end 38mm on a standard length swing arm.
    #18
  19. cenji

    cenji out4fun

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    ...as it confirms that I don't need the extension to make my Geometry fly. If yours lasted 60K miles and is still going, and the other one that I know off lasted over 100K miles.....it makes sense to me to not spend the $400 on the 50mm extension, not lengthen the wheelbase,keep more off the G/S characteristics and have a blast riding it. The 400 buckeronis can go towards a new u-joint if necessary.

    Even thought the 50mm extension will lessen the sharper angle at the U-joint, it would be "ONTICS" geometry or trigonometry skills that could determine the change of angle for the curious, but he has his plate full with his remodel.
    My guess, it is not going to be much. I am getting curious actually, so I will measure the length of the swingarm and will come up with some number later.

    Thanks so much to everyone for the feedback, it was quite helpful. I am always amazed about the willingness of the forum to share knowledge. I should spend more time on here, but for whatever reason, I am not. Beats me!

    Down the line I will post some pics with someone's help, as I don't have any of those accounts. Nothing fancy, just my old bumbler with a KTM fork ready to go more off-road...yipppeehhhhhh.

    #19
  20. Solo Lobo

    Solo Lobo airhead or nothing

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    FYI, I had HMPGuy replace my u-joint with a cir-clipped replaceable one for less than $300
    #20