Hossack front post-apoc Seca

Discussion in 'Some Assembly Required' started by sebwiers, Aug 2, 2016.

  1. sebwiers

    sebwiers  

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    Sorry if I'm supposed to post an intro of some other social fluff... I'm not an "ADVrider" as such, just done a few long trips on my sport tourer (a 1985 FJ1100 with classic plates). Google research in the past has shown me folks here post some interesting suspension mods and such, figured I'd share mine.

    Bike's been torn down a couple years while I build the suspension / steering, am about at the point of putting engine back in, building wiring & controls, etc. Hoping to make the end result dirt-road usable, mostly by way of slightly higher ride & seat, DP tires tires, skid plate, and high pipes. Travel is 3.5" (stiff, and very strongly progressive) in back, 5.5" (straight rate) in front. These pics are all very much WIP.

    Frame / engine / drivetrain / wheels are 1981 Yamaha XJ750rh.

    Tank is Virago 750, fits pretty well on the original frame mount bits after raising rear 4 inches and re-positioning front stops. Has a MASSIVE tunnel, my battery fits under there.

    Rear shock is from BMW Montauk, which is same weight and travel as XJ750. I played around measuring shock length vs wheel position for a lot of mounting points until I found something that seemed to give the right travel and wasn't regressive. Later testing showed is was off a bit (less travel / stiffer than expected & strongly progressive) but not enough I feel the need to re-do.

    Front is based on Hossack design, but after a lot of shop noodling and head scratching, ends up more like Fior's B'ol Dor bike. Shock is GSXR1000, brakes are Yamaha blue dots, keeping the stock cable actuated MC (will fab cable pull ratio adjuster if needed) so I can run reverse levers. Steering link design was suggested via MC Chassis mailing list, functionally similar to the one Alan Lapp used on his DBC bike. Handlbars mount to a BMX stem and bicycle headset bearings are mounted in reducer cups inside motorcycle head tube. BMX stem is adapted to support swing link for the steering linkage.

    Target geometry is 17.5% rake and 3.5" trail, with 50% anti-dive - the steep rake inspired by Tony Foale's experiments with rough surface stability. Rake, trail, wheelbase, and dive are all adjustable to various extent (with some obviously affecting others). Steering lock is 45 deg or more each way - uncommon for Hossack fronts, and something I made a priority of because I figure I'll be duck-walking the bike in tight quarters a lot (in the shop, garage, at events, etc). Wheelbase is slightly shorter than stock (wanted more front wheel weighting, the XJ's are a bit ass heavy), made possible by the vertical wheel path. Dive adjusts via shims under the lower ball joint, just as in automotive designs, with 1mm of shim giving a roughly 5% decrease in anti-dive. With no shims, it should be roughly 100%. As assembled currently it has a half inch (12.5mm) shim. I'm not sure what a "good" amount of anti-dive for daily / casual use is, and apparently neither is anybody else (I asked on the MC Chassis mailing list), which is one reason I'm building this - I'd like to see what its like to ride with various amounts of kinematic anti-dive.

    [​IMG]

    [​IMG]
    #1
  2. k-moe

    k-moe Long timer

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    Go ahead and link to your build thread so people can see how it started.
    #2
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  3. Tim_Tom

    Tim_Tom Long timer

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    Whoa! That is a wild build! More pictures and details PLEASE!!!!
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  4. Lomax

    Lomax Nanu-Nanu Adventurer

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    Interesting piece of art.
    #4
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  5. sebwiers

    sebwiers  

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    Build thread? How you know I have one? Ah, who am I kidding, of course I do.
    http://www.customfighters.com/forums/showthread.php?t=61895

    Bought the bike from a (since ghosted) member on Customfighters.com, build thread there collects posts from 3 (or maybe 4?) winter buildoffs I manged to DNF my way through making it to this point. Total hours work are... well, a lot, but I only work on it on Saturdays, at a local makerspace, and had to learn machining (and brush up my welding) along the way.
    #5
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  6. sebwiers

    sebwiers  

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    Some shots of load testing to ensure the frame / hardware / welds could take reasonable loads (strain equal to 2g brake force pushing back on front wheel, and 6g bump loading upwards on both axles). Actual vibratory cycling at lower loads would be better demonstration of fatigue resistance, but these tests at least gives me confidence it won't fail right off the bat if I nail the front brakes / hit a big pot hole. I did manage to bend one bolt (the one that holds the bottom of the shock link turnbuckles) so will need to fabricate some better hardware for that point.

    [​IMG]

    [​IMG]
    #6
  7. k-moe

    k-moe Long timer

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    #7
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  8. sebwiers

    sebwiers  

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    I'm guessing that's meant as some sort of off hand dig, but yeah, the art bit is there because hey, if you are gonna build something, why not make it artful? I learned welding many years ago in art school / making sculpture & furniture, and got back into it recently. I'm not trying to conceal a lack of high end fabrication tools and skills, and would rather make something that honestly shows how it is made and works. If somebody sees it and thinks "what a piece of crap, I could do better".... well, that's exactly what I hope they do. Better. Or at least do something. I like it when people do interesting things, better or not.

    So yeah, its a piece of art. Hopefully interesting. Functional, as as far as I can manage. Seems to function OK so far.



    The geometry should be sound and is very easy to adjust / reconfigure (it was actually set up rather poorly in the above video, as I'd just bolted it up after transport in pieces). Dimensions and dive behavior were worked out in Tony Foale's funny front end program - a good investment for anybody building such a thing, smartest $50 spent on the project so far. It might have a bit much unsprung mass because I used heavy tube section to ensure I could do very solid welds. I'm not an engineer, but its obvious now I could use smaller / thinner tubes, as well as one piece construction. The bolt up design did save me from having to re-fabricate the whole thing a couple times when some bit of machining went sideways, so served its purpose. If I rebuild the fork, I could easily shave a few lbs, but for a first go I was cautious given its my neck on the line.
    #8
  9. FR700

    FR700 Heckler ™©®℗

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    Your imagination.


    :thumb


    Chill, dude. We like your work.


    .
    #9
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  10. sebwiers

    sebwiers  

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    Ah, well, that's good, because I stopped by the shop on the way home from work and got artsy fartsy with it.

    Had to re-build the front lower shock mount. Old one held up in the test but looked crap, and made it very hard to get the engine in / out. New one should make that much easier, also looks much better. Still thinking on if / how to lighten the aluminum block (which was the first part made, and served as a key to align the rest).

    [​IMG]
    #10
  11. JagLite

    JagLite Long timer

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    Fascinating!
    I like unusual builds with performance and handling as the goal instead of "edgy style" like knobbies on a cafe' racer.
    Carry on! :thumb
    #11
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  12. sebwiers

    sebwiers  

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    Thanks! Not sure I'll actually achieve either performance or handling improvements, "enjoyably ridable" would be enough to make me happy. I'm more interested in the learning process of making it, and getting an "ass in the saddle" impression of what effect various levels of anti-dive have on riding. And obviously I'm not shying away from "edgy style", as long as it doesn't get in the way of usability / learning. Unusual, it has in spades for sure.

    I'm not sweating the performance - will be happy if I just get it running decently, and anything over 50 horsepower seems fine for such a project. Supposedly the bike made 74 hp stock, but is probably a bit knackered (though ran OK when removed) and hopefully will just get a valve adjust (needs a new shim under the leftmost exhaust) and carb clean (done before I put it up for storage) and adjust. Because of the frame mods the stock airbox and exhaust won't fit, but I plan to replace them with custom parts that have similar behavior (I don't like overly loud bikes) to keep tuning easy.
    #12
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  13. sebwiers

    sebwiers  

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    So, my wife gave me the last minute OK to go to Wasteland Weekend this year. Which I was hoping to have the bike done for, but... ah well. Looks like I'll be taking the FJ1100 out there instead. Seeing as it is a 4000 mile round trip, maybe its better that way.

    So, I'll probably be storing this project way now, until after the end of September.
    #13
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  14. sebwiers

    sebwiers  

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    Some work since September, among other things painted it and put the engine in.

    [​IMG]

    Also did an imgur album as a sort of progress summary. http://imgur.com/a/VR3WH

    Thinking og putting dual purpose / scrambler tires on it. Shinko 705's look good to me and the price is right. Any opinions on that / other tires? I know, with the under-body pipe, scrambler tires are a bit moot... I'd like to convert to high pipes, but would prefer to get it running first.
    #14
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  15. sebwiers

    sebwiers  

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    Bit of update.... lol, more than bit!

    Got it running in 9/2017, blew the engine up because the oil line melted on the highway. Rode pretty good.
    [​IMG]

    Replaced engine. Replaced front shock with Hybausa unit (850 lbs/in vs 650 lbs/in). Rides much better.
    [​IMG]

    Built crash bars. Still plan a few things. A custom front shock linkage will improve the wheel rate curve to make it slightly progressive, and give me a bit of extra travel.
    [​IMG]
    #15
  16. JCool

    JCool Long timer

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    :fpalm
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  17. sebwiers

    sebwiers  

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    Bike is back on the road this spring as a daily rider. As designed for, its a fun traffic carver even with lane splitting being illegal - the reduced brake dive makes constant brake / throttle cycling less obnoxious, and it flicks through lane changes at a thought and goes over potholes at a 45 degree lean quite happily.

    #17
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  18. k-moe

    k-moe Long timer

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    EEK! That idle is all wrong. It should sound like a well-oiled sewing machine.
    #18
  19. sebwiers

    sebwiers  

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    It idles fine, the carbs were done professionally on a dyno by a crusty old race mechanic who took the job on to show up a rival shop that said it couldn't be done. It just doesn't have any silencer inside that muffler body; its effectively a big straight pipe. The old baffle had so much back pressure it got launched from the pipe like a potato when I goosed it one day, so I drilled it to a metal lace. I could probably use something to cut the noise, but I've been having fun pissing off the local HD riders.
    #19
  20. k-moe

    k-moe Long timer

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    I can hear a missfire. Even with your exhaust setup it should not sound like it currently does. As a quick check, pull the plugs and give them a read. Even odds say you're lean on at least one cylinder.
    #20