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Discussion Starter #1 (Edited)
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...
 

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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.
TheMasterMain.jpg
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?
 

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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.
 

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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 !
 

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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?
 

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Discussion Starter #7
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.
 

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Discussion Starter #9 (Edited)
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.
 

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Discussion Starter #11
Yeah, the leverage ratio is really high, running down from nearly 4-1 (to about 2-1 for the last inch). Don't know the exact rate, that's not how I figured things. I forget the exact spring spec, but its on the order of 1000lbs per inch. Travel is 4 inches, maybe a smidge over.

The bike the shock comes off is a BMW Montauk, which has very nearly the exact same weight and rear travel as the old Seca, so I aimed to preserve that. The Montauk runs a very high leverage ratio, via a very long swing arm with the shock mounted in conventional vertical fashion but quite close to the swing arm pivot rather than back by the wheel. The shock NEEDS a high leverage ratio to work right, hence the unusual setup. If I moved the front shock end up to reduce leverage (which I can do) the rear end would be so stiff I might as well have built a hard tail bobber.
 

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wow i dint think you could get a rising rate that steep without a linkage carry on good luck test carefully
 

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I'm dubious of the monolever. (pardon me for clicking right past it in the title.) I know you have all kinds of reports and spread sheets and all but it's not in me to trust it. I just don't see a reason to not have a full swing arm. I can't see how you are saving weight if you add a shock the length of a side of the swing arm and the diameter of one and a half to two shocks. Then you got to stick the thing out to the side to get around the tire and then get the frame around the outside of that. I don't see how it's an improvement.

I was hoping to be proven wrong in my assumptions by seeing other picts of your work on the rear end of the bike. I also assumed that you had pict of this since you have posted you progress on other sites. I guess all you have at this time is dramatic shots of handle bar parts, bent rods and graphic representations. I look forward to seeing more of you work. Maybe you have a link to the other threads? Already it the front end has caused me to consider alot more.

Since this is more about your bike and less about you I would recommend this thread be moved to the "Project Builds" category if you and the site admin agree.
 

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Discussion Starter #15 (Edited)
I don't think it ANY of the suspension mods I'm doing will save weight. In the back, at best it might improve weight distribution (shifting the shock weight down and forward for the rear) or suspension dynamic (making the rate highly progressive in the last inch, and simply running a better shock). The changes I am making are largely for the benefit of novelty and learning. If they come in at 'as good but different', I'll be happy, as I think that's a fair accomplishment for a garage fabrication first timer who had never picked up a mig welder before.

I do have other pictures of the rear, they are just equally crap so I didn't dig up the links. Buying a decent camera hasn't been a priority, and my workspace doesn't have the bright (or natural) light my phone camera would need.

Here's some different angles.
https://lh4.googleusercontent.com/-v_WmEQVMGHI/UpqLxg32vzI/AAAAAAAAA6M/Saz1ksUEgU4/w789-h592-no/CAM00156.jpg
https://lh3.googleusercontent.com/-U9UyuDU8rLc/UpqL7TaKbiI/AAAAAAAAA6Y/2CPu_mCJYTE/w789-h592-no/CAM00157.jpg

Here's some WIP shots with a different frame support that ended up getting cut away because it blocked full shock compression at the top, poked me in the ankle at the bottom, and looked like a dogs anus. Both versions of that support are vastly over-built given I'm not using the frames top shock mounts, but I figured it was smart to retain that option (allows me to directly compare stock ride quality vs modified, to see if it flexes etc).
https://lh5.googleusercontent.com/-kY5KCMMvmZ4/UaLC1CVFdYI/AAAAAAAAAYU/dJ5J-a_wygQ/w444-h592-no/CAM00032.jpg
https://lh3.googleusercontent.com/-3RayGvZOABY/UaLDF7qsPoI/AAAAAAAAAZI/BPA-zWrOYtY/w444-h592-no/CAM00039.jpg
https://lh6.googleusercontent.com/-s3bIyOWOaAo/UaLDFzAv1YI/AAAAAAAAAZI/lRnoyNxEbmE/w789-h592-no/CAM00041.jpg
 

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Those shots are exactly what I was hoping for. Thanks!!!! You are going to keep the right side of the swing arm for a while? Please say yes. While you are taking an engineer's approach to alot of the bike, I feel like you are counting alot on an unknown in the swing arm. Perhaps you have tested a swing arm by shearing it apart. (I don't mean that as sarcasm.)

You have never welded before this project? Do you have someone to closely supervise your work? Please, PLEASE say yes!!! I've been back yard welding for 3 years on tig and a few more on mig. And because of the important nature of the welds you are doing, this project would give me more than a little pause. Putting a tab on the frame for a regulator/rectifier is easy. Fabricating a structure that can flex, pivot, and suspend somewhere around 7-800 pounds hitting a bump at 70+mph is a massively technical undertaking for a hack welder. Good welds should strengthen a structure. Bad welds will weaken the base metal or sit on top of the base metal with out penetrating the base. To the untrained eye they can all look the same.

I'm routing for you here. But I don't know you well enough to be more than a little nervous about your endeavor.
 

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Discussion Starter #17 (Edited)
I have no plans to remove the right side of the swing arm. For a true single sided arm, you need a much thicker axle than this bike has. The right side may not be needed to support any weight, but it does a bang up job of keeping the axle aligned and transmitting brake force, can't see how I'd do that with it cut away.

I'd welded before starting this but only torch and stick, and never structurally. Did a fair bit of other work on the bike (including a whole rear swinger that I scrapped) before doing that work. I've attempted to make sure everything has more weld than needed without to much HAZ, and tried to get proper penetration. Will certainly be doing a lot of crack inspection as part of testing and normal operation. For initial testing before any riding, I at least plan to load the suspension to full compression using ratchet straps or some such. If that pans out, I'll move on to running it down some hills without power (or even an engine- just mount some dead weight in there) intentionally hitting some obstacles along the way. After that I'll maybe have to confidence to mount the engine and do some careful cruising, working my way up to using it as a daily ride that I don't push to hard. After a season of that and a winter for full dis-assembly & inspection, I might take it to a track and see if I can push the limits a bit in a controlled environment.

BTW, dug up some pictures from testing that shock, to give you an idea of the spring rate. I was wrong - its not quite 1000lbs/ inch... its 2000lbs/inch. Suffice to say I'm respectful of the forces involved, and just as concerned for my safety as anybody else.

My initial test jig design wasn't up to the job.


Once I beefed it up, I manage to load 325lbs on the arm, resulting in 1500lbs force on the shock... compressing it all of half an inch (500lbs pre-load, 2000lbs per inch). Bouncing up and down on the test rig was a lot of fun though- nice way to experience the effects of rebound damping. Also how I broke the rig shown above. Because of the damping, it was a very gradual, gentle failure, but I knew it was gone when I heard the first crack.
 

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Discussion Starter #18
Pluto - here's a CAD drawing of a system that is VERY similar to the one I'm building, even down to the shape of the upright that holds the wheel. The main difference is that my handlebars / steering head are not above the upright (and are also at a much different rake) so I need to use a different steering linkage design.

 

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I have no plans to remove the right side of the swing arm. For a true single sided arm, you need a much thicker axle than this bike has. The right side may not be needed to support any weight, but it does a bang up job of keeping the axle aligned and transmitting brake force, can't see how I'd do that with it cut away.
Some people do. Some of those people succeed. Most of the others have some really nice yard art.
 

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Discussion Starter #20
Some people do. Some of those people succeed. Most of the others have some really nice yard art.
I'd love to see that. Not the yard art, the successes. I assume the successes were swingarm & wheel swaps? I've seen lots of single-side-swingarm swaps with chain drive, but none that I recall with shaft.
 
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