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Author Topic: '81 Yamaha XJ750 Seca w/ monolver rear and Hossack front  (Read 9117 times)

Offline sebwiers

  • Posts: 23
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.


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)



« Last Edit: Jan 02, 2014, 12:36:54 by sebwiers »

Offline sebwiers

  • Posts: 23
FFE analysis software screencaps
« Reply #1 on: Jan 02, 2014, 13:12:17 »
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.
« Last Edit: Jan 02, 2014, 17:42:44 by sebwiers »

Online canyoncarver

  • DTT SUPPORTER
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  • Posts: 3218
  • 'hacking is learning'
Re: '81 Yamaha XJ750 Seca w/ monolver rear and Hossack front
« Reply #2 on: Jan 02, 2014, 13:27:34 »
Pretty cool front end mods planned.  Looking forward to seeing it.

--
YZF750/1000R The Fly
KZ 750 Twin
ZRX 1100
The Shovelhead springer chopper
A bunch of KZ's...some Suzukis', and a disassembled CB550
--

Offline sebwiers

  • Posts: 23
Re: '81 Yamaha XJ750 Seca w/ monolver rear and Hossack front
« Reply #3 on: Jan 02, 2014, 13:51:10 »
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...
« Last Edit: Jan 02, 2014, 14:14:25 by sebwiers »

Offline Sav0r

  • Posts: 459
    • ChrisLivengood.net
Re: '81 Yamaha XJ750 Seca w/ monolver rear and Hossack front
« Reply #4 on: Jan 03, 2014, 09:49:52 »
Do the control arms incorporate some adjustment to accommodate any variance in final assembly?
Sav0r the adventure.

Visit www.chrislivengood.net to get more information on my RD350 dubbed Mia Wallace as well as my other projects and snafus.

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Offline sebwiers

  • Posts: 23
Re: '81 Yamaha XJ750 Seca w/ monolver rear and Hossack front
« Reply #5 on: Jan 03, 2014, 12:04:54 »
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.
« Last Edit: Jan 03, 2014, 12:07:58 by sebwiers »

Offline sebwiers

  • Posts: 23
Re: '81 Yamaha XJ750 Seca w/ monolver rear and Hossack front
« Reply #6 on: Jul 20, 2014, 00:20:15 »
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.  :)






Offline zap2504

  • Posts: 346
Re: '81 Yamaha XJ750 Seca w/ monolver rear and Hossack front
« Reply #7 on: Jul 21, 2014, 10:45:06 »
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?

Offline sebwiers

  • Posts: 23
Re: '81 Yamaha XJ750 Seca w/ monolver rear and Hossack front
« Reply #8 on: Jul 22, 2014, 03:22:55 »
> 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.
« Last Edit: Jul 22, 2014, 03:26:17 by sebwiers »

Offline zap2504

  • Posts: 346
Re: '81 Yamaha XJ750 Seca w/ monolver rear and Hossack front
« Reply #9 on: Jul 22, 2014, 18:10:16 »
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?