'81 Yamaha XJ750 Seca w/ monolver rear and Hossack front
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'81 Yamaha XJ750 Seca w/ monolver rear and Hossack front

This is a discussion on '81 Yamaha XJ750 Seca w/ monolver rear and Hossack front within the NEW MEMBERS READ HERE! forums, part of the Caferacer.net Forums category; Folks from CustomFighters or XJBikes might recognize this build. Figured I'd post here because while not a traditional cafe bike (by a long stroke) the ...

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Thread: '81 Yamaha XJ750 Seca w/ monolver rear and Hossack front

  1. #1
    Member sebwiers's Avatar
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    '81 Yamaha XJ750 Seca w/ monolver rear and Hossack front

    Folks from CustomFighters or XJBikes might recognize this build. Figured I'd post here because while not a traditional cafe bike (by a long stroke) the cafe look was a pretty big influence on its inception and the concept of taking something not fit for racing and heavily improving it is very key here. Been wanting to do a custom bike for... maybe 20 years, but this is my first, and seems a neck-deep plunge! Engine and wheels are staying stock because of cost and because I want to get riding sometimes this year (hopefully early summer), but suspension and controls (and obviously body work) are changing radically. Front brakes might get an upgrade, not a lot of extra work when building a full front end. Not sure I could really better the engine anyhow, beyond the normal maintenance. Hopefully once I get past that and have some confidence in the build, I'll start improving other aspects (more horsepower, lighter wheels).

    An idea what the bike looked like stock- very '80s techno-UJM.


    Current project status and planned front end are summed up pretty well in this photo. Body work was done last winter, tank is from a Yahmaha Virago, was given to me by seller to replace rusted out stock tank (which now lives out back as the cowl / fender). Seat and tail light come from GSX600, rear shock from BMW Montauk. Am looking at doing some glass work to cover the space between seat and frame, and smooth the tank / seat / fender flow.


    Engine is in decent shape, want to do some external polishing and etching. Planning to etch this design (based on original Yamaha logo from 1812) into the clutch cover, using a laser cutter to burn away a plasti-dip mask and then acid etch & paint, then pull off the mask. Will also get a 4-1 system with under-body muffler and a custom air box (stock one won't fit due to rear shock location, hear bad things about pods and this bike's carbs, and I want the airbox for looks - clear plexi with a big automotive 'pod')



    Other accents in mind include some brass inverted levers I picked up. Yep, the old Seca has its got CABLE ACTUATED dual discs up front (or certain years do at least, via cable aactuated remote cylinder) -


    Combined with red anno aluminum and carbon fiber (rods will be used as steering linkages for the Hossack) -


    Here's a mockup of the upright design, and some steel I've bent to build the working model (iteration 1)






    Here's a screenshot of the software (form Tony Foale) I'm using to determine pivot configurations / link lengths.

    Y1 ans Y2 are the hieghts (above ground) of rear pivot. X = 0 is based on the location of the lower pivot, so X is only specified for the upper linkage. L upritght 0 is the distance along the upright between pivots, L upright 2 is the total upright length. Some of the bike specs (re COG and Wheel MoI) are estimates, but they don't have a huge impact on behavior (only affecting anti dive) and my build is adjustable (via threaded linkage lengths etc) to compensate; once constructed and assembled I can take actual measurements and fine tune.



    Design goals were to have zero wheel offset (minimal moment and wind effect) and move the front wheel back compared to stock (possible due to vertical wheel travel) with nearly flat dive / rake / trail figures (or, if not flat, increasing with compression), which graphs show this configuration achieved. 100% anti dive would mean braking has zero dive effect; this may vary depending on the actual COG and how I adjust certain linkages, but the shape of the curve will be fairly consistent (judging from trying various changes to those values in software). 50% anti-dive seems to be a popular figure to shoot for in racing due to rider preference for 'brake feel' (and habituation to feel from telescopic forks), but I'd like to run a bit higher for street use, and because I'm specifically looking to see what the absence of brake dive would be like in terms of ride feel.



    Running under 100mm trail seems quite short, but reportedly with such steep head angles, even less trail is still quite stable. I'll be running a steering damper just in case, and trail will be fairly easy to adjust without impact on other characteristics.


    The next big hurdle is to weld the mounting system for the A arms onto the frame. I've got a decent jig for doing that, but its intimidating because its requires precision both in location and the parts cutting (like locating a new swingarm mount using tube mitering onto an irregular surface) and I really only get one good crack at it. I suppose I can avoid doing more than tack welds until the whole system is in place and confirmed as running true / square, but still...
    Last edited by sebwiers; 01-05-2014 at 12:25 AM.

  2. #2
    Senior Member Pluto's Avatar
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    I've been interested in girders since I watched the movie "the Master" and got to watch the below bike cruise over desert flats.
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    But your version seems a bit more complicated than it needs to be. Maybe it's because you show little shot of bent rods instead. Also, it seems that the rear wheel is supported on one side only? Do you have more picts of the rear end?

    Cool! ---RIP

  3. #3
    Senior Member Geeto67's Avatar
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    A Hossak/duo lever isn't the same thing as a girder. Though they look similar, the Hossak removes suspension influence from steering action while a girder does not.

    this looks like an interesting project. I would be concerned about the forces being transferred to the neck. When you look at a BMW with a Hossak the neck is ten times beefier.
    Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.
    - Samuel Beckett
    A tool is just an opportunity with a handle
    - Kevin Kelly

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  5. #4
    Senior Member Pluto's Avatar
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    Ok, Another suspension for me to investigate. Thanks!

    Just went to youtube. That's cool!

    Last edited by Pluto; 01-05-2014 at 08:55 AM.

    Cool! ---RIP

  6. #5
    Senior Member XB33BSA's Avatar
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    good luck with the hossak maybe you can get it to work better than telescopic forks
    nice to see somebody cares enuff about their suspension to build their own !
    Last edited by XB33BSA; 01-05-2014 at 09:04 AM.

  7. #6
    Senior Member XB33BSA's Avatar
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    on the rear suspension,is that simply what it looks like, an extreme layed down shock ?there is a reason why it is never done like that... what caused you to do that?

  8. #7
    Member sebwiers's Avatar
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    Pluto - yes, its only supported on one side in the rear. That's why I call it a 'mono-lever' system; the BMW monolever worked the same way. My bike happens to have an arm on the other side of the wheel as well, but it is very spindly. Because the wheel using a drum brake and a shaft drive, all of the wheel bearings are on the drive (left) side of the wheel. There's literally nothing supporting the axle from the centerline out to where the axle runs through the right side of the swingarm, so I'm pretty comfortable not putting a shock on that side- in fact, I'm more comfortable doing so than trusting the spindly right side swing arm to transmit wheel motion to the right side shock, or expecting the unsupported axle to do that job. My guess is that in the late 70's, BMW took a look at the dual shock shaft drive bikes the had, came to a similar conclusion, and developed the monoshock.

    Geeto67 - not sure what forces you mean. If by 'the neck' you mean the headstock, it is taking almost zero force. The drawing shows the shock mounting to its bottom (handy because it can easily take the load) but that won't actually be the case (not quite enough clearance for that with the suspension compressed). I'll be running my handlebars on a bicycle headset held in reducer cups inside the original steering head, just because its a handy place to mount them ergonomics wise, and because cutting the head tube off would make it impossible to register the vehicle on its title (its where the VIN is engraved). Also, the BMW system is hugely over-built, partly for liability insurance reasons ('experimental suspension kills BMW owner' makes for a great lawsuit) and partly because its a compact system (short control arms and control arm spacing) which leads to magnified forces.

    Pluto - the setup in that video works differently (enough so that different patents would apply) but does suffice to get the idea across. A 'true' Hassoc system has two control arms fixed to the frame (both much like the lower one shown in the video) and the steering can then be handled via any variety of linkages, cables, whatever.

  9. #8
    Senior Member XB33BSA's Avatar
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    but the angle of the rear shock is too flat

  10. #9
    Member sebwiers's Avatar
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    XB33BSA - yep, its pretty much a simple lean forward rear end. I picked that configuration because it gives the (decreasing) leverage needed to get the desired (progressive) rate from the shock I'm using. I determined the mounting points simply by measuring a bunch of point-to-point lengths for various wheel positions and running the results through a spreadsheet. A central monoshock would be somewhat preferable, but harder to fabricate and the shock location might interfere with the carbs. A linkage setup would be a lot harder to design and fabricate. The original BMW monoshocks were also fairly 'lay down', though not to the extent mine is.

    What specific reason it is never done like that are you referring to? AFAIK the only drawback (assuming your setup isn't regressive) is increased stress on the swingarm mounts and pivots, which means the pivots and frame would need to be built heavier than otherwise. That's enough to make it pointless on a production or racing design, where you can just pick a different shock and mounting location, but doesn't mean it can't be done with good (if not ideal) results. I was stuck with a pretty limited selection of shocks, a custom shock not being in my budget, so I did what would work with the only shock I could find that had the force absorbing ability I needed. I do figure I'll have to live with increased pivot stress, and beef up the frame in the areas that take the rear shock forces. I have a spare parts bike AND a spare swing arm, so replacing bearings isn't much concern (though may encourage me to find a better solution, like a bearing upgrade). Its also possible to still run the original shocks, or to easily re-fit the upper shock mount (which attaches to a machined mounting block that bolts to the frame) to run with a less 'laid down' shock that has longer throw and a softer spring, if it proves a serious concern.

    Don't have any better pictures, will be getting some eventually as there is still work to do to (like a direct force transmitting link from the current lower shock mount to the rear trans housing where the original shock mounted.
    Last edited by sebwiers; 01-05-2014 at 06:10 PM.

  11. #10
    Senior Member XB33BSA's Avatar
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    your leverage ratio is extremely high is it not ? what is the rate? how much shock and wheel travel?

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