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

sebwiers

New Member
Folks from CustomFighters or XJBikes might recognize this build. Figured I'd post here because while not a traditional cafe bike (hence the 'Special' category, hats off for having that) the cafe look was a pretty big influence on its inception and I might do for some advice in keeping that flavor in the mix. Been wanting to do a custom bike for... maybe 20 years, but this is my first, and seems a neck-deep plunge! Engine, wheels, and brakes 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. Hopefully once I get past that and have some confidence in the build, I'll start improving other aspects (moar horsepower, lighter wheels, better brakes).

An idea what the bike looked like stock- very '80s techno-UJM.
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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.
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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')
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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) -
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Combined with red anno aluminum and carbon fiber (rods will be used as steering linkages for the Hossack) -
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Here's a mockup of the upright design, and some steel I've bent to build the working model (iteration 1)
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FFE analysis software screencaps

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.

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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.

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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.
 
Thanks. 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...
 
Do the control arms incorporate some adjustment to accommodate any variance in final assembly?
 
Yeah, they do. The A arms mount to the frame with Hiem joints threaded into tube nuts, so get maybe 3/4" fine adjustment for length just by taking the arms off and turning the Hiem ends in / out (sort of like dialing a tie rod in). Larger gross adjustments are possible via other mechanisms (parts that can shift via multiple bolt holes, be shimmed with washer stacks, or are so simple to fab on a drill press from cheap stock steel that they can easily be swapped out to allow dimensional changes). The locations where the control arms mount to the frame can't be adjusted, but can be changed if needed because the control arms mount to (3/4" aluminum) plates which bolt to (adapters welded to) the frame. There's no method for fine adjustment in that cse, but the location of the pivot can be changed by replacing (or if spacing allows, just drilling new holes in) those plates.

I'll be paying a penalty in weight (both sprung and unsprung) to allow such adjustments, but can get that down on later iterations once the exact dims I like are dialed in. Unsprung weight doesn't bother me much, but it will be worth reducing sprung mass where possible.
 
Actually made some progress on this recently. Got the main Hossack components mostly done - they still need the rear ends cut to length and rod ends mounted back there, and some additional decorative & functional machnining.

So, I bolted it all up and put it in the stand to get an idea of what it will look like rolling. Pretty damn happy with what I got. :)

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Ooo, Tony Foal! Subscribed! Is the rear suspension finished, or just mocked-up? Were you able to ride the Seca before the suspension mods to get a "before" idea of performance?
 
> Is the rear suspension finished, or just mocked-up

Needs some bolts, but its good to go. If I find a more suitable shock (one that is longer, but still stiff enough) the shock top mount can be moved without any frame mods. It felt OK bouncing up and down on it, but may prove to regressive on the road, or overly stress the swing arm pivots.

> Were you able to ride the Seca before the suspension mods to get a "before" idea of performance?

No, but I've bought two of these (an 81 and an 82). The spare one needs a bit of work (I think it has a stuck valve in the leftmost cylinder- there's no compression) and is slated to be a parts donor if needed, but I'd like to get it in ride-able condition eventually to see what stock form (plus ~30 years) is like. The Maxims are common as dirt around here, so if I do need engine parts for the second bike, they are fairly easy to get.
 
sebwiers said:
Needs some bolts, but its good to go. If I find a more suitable shock (one that is longer, but still stiff enough) the shock top mount can be moved without any frame mods. It felt OK bouncing up and down on it, but may prove to regressive on the road, or overly stress the swing arm pivots.
The original had 80mm of rear suspension travel so if you stay within those limits (assuming a centered shaft to start) you should be good as far as not over-taxing the shaft U-joint.

Is it all custom-made with a Montauk shock or did you modify pieces from elsewhere?
 
The travel is the very close to same, and I can mount both the mono shock and the old shocks at the same time (is how I 'jigged' it for welding) so starting position is the same. Its a montauk shock, top shackle bolt runs through urethane bushings (with a brass sleeve between bushing and bolt). I bought a used montauk shock cheap, figured it would work OK (using high effective leverage) because the montauk and the seca have very similar weight and rear travel.
 
Update - Finally got enough of the bits machined and welded to frame that I could mount the front wheel in place (using a mock-up wood part for lower arm, looking at figuring out shock mount / arm design).

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Got some of the bugs sorted in a second mock up. Namely, the wheelbase is now 4 inches shorter. Which changes the dynamics some (not in a bad way) and makes locating the shock as previously shown impossible, but that's OK because I had something better in mind. Gonna be a pull-link actuated set up, sitting behind the front wheel. Heat may be a concern, but that's what pipe wrap is MADE for. Might even route the pipes above the engine.

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Steering lock is also vastly improved with this upper arm configuration, something I should have planned from the start.

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Finally got it rolling on two wheels. Am pleasantly suprised with the 'stance'. Is very lumpy though, gonna tweak the flow a bit by angling the tops of the side plates and otherwise sharpening some of the blunt bits up front. Still intentionally breaks every rule about graceful lines, but the fact it's crazy low (bars will only come up maybe 1 inch from head tube) may help it work.

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Had it all apart for a while so I could redesign the arms to be much shorter and without any threaded parts getting flexed, and do some cutting to lighten parts like the fork bridge. Got the lower shock mount done, bolted it back up, and started fitting the shock link turn buckles.

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Its on its own two wheels and able to bounce up and down on the shocks now. Album of pics is http://imgur.com/a/3Vu0s

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