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Building Custom Seat Question

This is a discussion on Building Custom Seat Question within the Technical forums, part of the Caferacer.net Forums category; This whole project is still a bit of a work in progress. Motor is a GS500, frame started out as a steel dirtbike frame that ...

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  1. #11
    Junior Member ams14's Avatar
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    This whole project is still a bit of a work in progress. Motor is a GS500, frame started out as a steel dirtbike frame that I widened and lengthened, front end and swingarm are from a ninja zx6.

    Here is some of the intermediate modeling steps
    Name:  model.jpg
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    here is the suspension linkage/rotation scheme
    Name:  suspension.JPG
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  2. #12
    Junior Member ams14's Avatar
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    Here are some pics showing the monoshock details. I still have some finish welding to do and to remake a few linkage parts (currently have some placeholders ones in there).

    The mount at the top is pretty close to where it was originally, I moved it back about an inch.
    Attached Images Attached Images

  3. #13
    Ews
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    Senior Member Ews's Avatar
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    AutoCAD? Damn I need to learn me how to use that. My shit is generally scratched up on some scrap of graph paper with crayons LOL

    It resembles the rendering so good on you for that
    Dream as if you live forever, Live as if you die today.~ James Dean

    I feel the need, the need for speed.~ Mavrick (Tom Cruise before he was a big ol' bag of crazy)

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  5. #14
    Senior Member Geeto67's Avatar
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    well I am impressed - a lot more thought went into it than the average trash heap monoshock conversion.

    That being said, I get the feeling while you grasp complex engineering concepts, some basic motorcycle practices are unfamiliar. I think you would really benefit from tony foale's book: https://motochassis.com/

    dirtbikes are designed for rough terrain and often aren't required to lean over all that far to turn. If you ever watch arena cross you'll see all the turns are banked. To that end generally what works for them in frame design doesn't work for road racing motorcycles all that well. even still most follow the tenant that you seem to have deviated from - the best handling motorcycles are the ones with the strongest connection between the headstock and the swingarm pivot. This is why most modern sport bikes have "wishbone frames" be it a trellis design like ducati or cast aluminum like a zx10. the stronger and more direct you can make that connection the better your bike will ride.

    here are some notes:

    - I don't think your swingarm pivot is robust enough, I think when cornering hard the front wheel and the rear wheel will try to come out of alignment.
    - I don't think there is enough reinforcement between the rear frame upright and the headstock and again I think the wheels will try to move out of alignment under hard cornering.
    - I don't think that front down tube is strong enough, and if you made the engine a stressed member (more than it already is) it might be unnecessary, but that requires fixing the head to the backbone.
    Last edited by Geeto67; 04-23-2019 at 02:02 PM.
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  6. #15
    Senior Member Cyorg's Avatar
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    I wouldnít be inclined to ride it. With that front end, decent tires and assuming that engine puts out what it is supposed to, itíll be a wild ride if you push it. That frame will flex a fair amount with the stock engine in it. The bracing for the top shock mount looks thick, but the question now becomes what is the wall thickness of the tube it is welded to? That doesnít necessarily pose a problem because a good welder can deal with different thickness in material. However... how good are your welding skills assuming you did the welding. Judging by the extent of the grinding, I would say you need more practice and maybe a better welder. Judging welds in a photo is sometimes redundant, but it sure looks like you took a lot of material off that tube in the last photo..just under the rear of the tank. I know with welding tubing in situations like that you can overlap a weld and end up with material you want to remove so itíll look better, but shouldnít be a need to go to town like that. Good you changed the countershaft location from the drawing. How is that shock mount tied into the backbone of the frame? Did you re-drill that hole in the linkage to alter the ride height? Any idea what the trail is when everything is sitting where itís supposed to?
    Last edited by Cyorg; 04-23-2019 at 03:24 PM.
    "Non urinat in ventum"

  7. #16
    Senior Member woodsman's Avatar
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    I too think you'll have problems with flex

  8. #17
    Junior Member ams14's Avatar
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    Geeto67
    ebook bought and downloaded! I will start break into it tonight, thanks for the recommendation, looks like a classic! Good comments and notes which I will take under consideration. I think I could beef up the swingarm pivot. The other two would be more of an undertaking with this starting point. I will start with looking through the book.

    Cyorg
    I am defiantly not an expert welder, and yes, I did a LOT of grinding, which tends to look suspicious. I did multiple passes on a few of the sections and then ground them down to look smooth. The top shock mount is welded directly to the main upper tube, not sure on thickness for that tube. I believe the drilled hole you are referring to it a temp piece I made to adjust the ride height when I was working out the linkage (it is a place holder). I don't know what the trail (and rake) are, I will measure/estimate.

    Frame rigidity/flex concerns seem to be a recurring theme here so that is something for me to think about.

  9. #18
    Senior Member irk_miller's Avatar
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    You guys need to read up a little more about dirt bikes. A GS500 and a DR650 produce virtually the same horsepower and the DR is geared for more torque. If you guys don't think dirt bikes, or even more so, dual sports don't do much leaning than please let me never get behind you on trails or twisties. Dirt bikes lean, especially in supermoto gear, you're just in a different riding position.


  10. #19
    Senior Member Cyorg's Avatar
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    Quote Originally Posted by irk_miller View Post
    You guys need to read up a little more about dirt bikes. A GS500 and a DR650 produce virtually the same horsepower and the DR is geared for more torque. If you guys don't think dirt bikes, or even more so, dual sports don't do much leaning than please let me never get behind you on trails or twisties. Dirt bikes lean, especially in supermoto gear, you're just in a different riding position.]
    Not sure who ďyou guysĒ are and not sure how you know itís a DR650 frame unless my reading comprehension has gotten worse, which is a possibility or he mentioned it on another thread. Donít most dirt bikes have all kinds of interesting flex built in? You tell me, I donít really have any desire to read up on them. Letís assume yes. Combine that with a (presumably) first attempt to make major modifications to a frame. USD forks, a swingarm that isnít likely go to flex behind that engine, fat tires etc etc. and my guess would be that for a street bike... or whatever he uses it for, it will flex more and not in a desirable way.

    OP, did you plot out the rake and trail before you started.? If not, you may be in for a surprise. Even if you did plot it out, the change in ride height will have changed that. You may want to make further changes or swap out the triple, to get more trail. Might want to add a steering damper, but better to have them set up so they donít shake to start with.
    "Non urinat in ventum"

  11. #20
    Senior Member Cyorg's Avatar
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    OP. Iím operating with a really sketchy internet connection. I type shit and it disappears and punts me out. I have some suggestions for measuring rake and trail, so you can see what your changes do to them in real time. Iíll post it when I get back to civilization.
    "Non urinat in ventum"

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