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Hossack Motorcycle Front End

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sebwiers:
Nope, the guy with the new OR Hossack project came along a couple weeks after me.  I'm the guy doing the mod for a 1981 xj750 seca.  100% street bike (sport tourer).  The shock I have is from the rear of a BMW Montauk.  Given the load / travel on the rear of the Montauk, I figure its rear shock should be good for about 6" on the front of my bike, which is probably more than I need (but matches the stock fork nicely).

I downloaded Autocad Inventor (3 years free and legal if registered as for educational purposes) but really can't get past the basic interface far enough to do anything.  Honestly, I'm more comfortable just doing everything on paper, building a prototype, and testing it under actual physical loads (stationary, not on road). 

I'm pretty solid on the dive and other characteristics I want (75% anti dive, increasing to 100% with compression, ~15 deg head, ~4" trail, stock wheelbase or a bit less) and the design is set up to allow fairly easy adjustment / replacement of the parts that control those (and other) aspects.  That way if I don't like the way the end product rides, I can figure out why and change it, with minimal expense / labor.  In fact, seeing how different Hossack setups actually ride is one of the main points of doing this project.
The big trick is just making it strong enough while keeping the weight down, while also keeping fabrication easy (allowing decent precision in construction and simple welding) and achieving the look I want (decided to go with a "high tech wrought iron" look).  A lot will just depend on building a really sturdy clamping jig for my welding / bending. 

hopefuldave:
I'm also going to recommend Tony Foale's book - I have a copy and hope to apply some of the principles to my VMax when the opportunity arises (which may be in the spring if the impending house move turns out to be to the place with the huge garage with 3-phase power...) - the VMax frame is a definite refugee from the 70's!

Re the front fork construction, also take a look at the design of the "Girdraulics" fitted to Vincents, a compromise between a tele fork and the full-on Hossack design - and in the words of the Dalek engineer - "Triangulate! Triangulate!"

A Lattice / space-frame construction will probably be stiffer (weight for weight) than a simple large-section tube and less susceptible to fatigue fracture around the pivot points etc.

Working in steel, you could consider fillet brazing rather than (TIG?) welding - a lot less distortion and less of a stress-raiser at the joints. This was often called "lugless brazing" in the world of exotic bicycles (back in the day, 40's - 60's?) and gives a very clean look with less hassle in jigging - the joint fits have to be spot-on though! I've seen a few "tube notcher" set-ups online which use a milling cutter in the lathe chuck / a collet, the same size as the tube to be fitted to and clamp the tube-to-be-notched in a V-block "toolholder" so the topslide can be set over at the required angle (and height if the tubes aren't exactly co-planar), looks like a simple and useful setup? I intend to try it some time, given the necessary tuits.

Just my ha'pennorth,
Dave H. (the other one)

sebwiers:
Yup, I plan to do fillet brazing, at least for the smaller web-work; I'm actually much better versed in bike frames than motorcycle.  I've seen fillet brazes done using a tig welder as the heat source, results are VERY nice (and you don't need flux!)  Fit has to be pretty good, but not as mechanically snug as it would be with torch brazing.  All my cuts can be flat, so I don't need a notcher, just a band saw, vice, file, and patience.

For my prototype, I've settled on 4 3/8" hex rod legs (meaning all my web cuts can be flat) on each side, forming a girder that is 6 deep and 2 wide at the lower pivot.  Extensive triangulation with .25" hex rod in all 3 planes, no unsupported spans over 3".  I'll be tack welding the structural connection points to a heavy c-beam, then doing the basic construction.  It need to be jigged up TIGHT, because I plan on heating the hex rod to do black-smith style twists before welding in the webbing, then also adding twists to the webbing.  Fillet brazing is a good compliment to this, I figure it will look pretty sick when its sand blasted and clear coated... 

:drool:

hopefuldave:
Sounds pretty slick!

 If you're going with a clear coat, the braze will show up against the steel of the girder (may be part of the inherent charm?) and the twisted hex will add a lot of visual interest :)

I plan on using tube rather than rod for the stiffening on the VMax (it already has frame braces, but the swingarm tends to twist under "firm acceleration"), but there are Other Issues to sort first (ran-wot-I-brung with fairly disastrous results - accidentally changed down too many gears at the end of the run, bounced and bent a valve - engine rebuild/mods due, and while I'm at it there are lots of second-hand superchargers[1] popping up on Ebay...) - my lad has a daft plan to build an Akira bike[2] too, but I think that's a *long* term project!

Dave H. (the other one)

[1] As it'll need belts and pulleys to drive one, vari-speed drive from an automatic scooter transmission, Nice Big Lever marked "cruise-competition-combat"?
[2] http://www.geekologie.com/2012/04/the-only-officially-recognized-akira-bik.php ( - but with proper hub-centre steering?)

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