Author Topic: Hossack Motorcycle Front End  (Read 10024 times)

Offline sebwiers

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Hossack Motorcycle Front End
« on: November 15, 2012, 09:40:16 PM »
I'm looking at maybe building one of these for my bike.  I'm figuring to use a control arms design based off an oval track racer, so I'm not much worried about strength there (should be over done by a large margin).

However, the fork has me a bit worried.  I was planning to use 6061 t6 aluminum tube for the legs - maybe not as efficient as a true girder, but better able to resist twisting, and easy to build with (I want adjustable mounting).  I don't really know what numbers I should look at to figure out how big / thick my tubes should be, though.

My guess (based on some online beam deflection calculators) is that a 2.5 tube with .25 inch walls and 12" length (about as long as I can see needing to go from the ball joint to the axle) would (if loaded with 1000 lbs, at least double what I'd expect to ever see) deflect .05 inch and have a bending stress of  13,250 psi. 

The formulas used for this are:
MI for Solid Round Beams = (pi * (OD^4 - ID^4)) / 64
Deflection = (length^3 * force) / (3 * E * MI)
Bending Stress = (force * length) / (MI / (0.5 * height))

I don't know what those mean, so want to be sure they are the right ones for situation at hand.

The deflection doesn't bother me much (that actually seems pretty stiff) but the stress seems pretty high, close to the yield point.  On the other hand, I would have TWO of these tubes, and so the total load would need to be 2000 lbs (roughly 3g acceleration, which is MUCH harder than I could stop) to cause this strain.  I figure the load won't always be exactly even, so want some safety margin even with the whole load on one leg.  Bump loading doesn't worry me as much, as it would pretty much be entirely compression.

Would that tubing be sufficient, or do I need to step up the diameter (thicker walls are hard to find)?  Can I get away with thinner tubing (.125 inch) in a similar diameter, while still having a decent margin before yield?

Offline Brass_Machine

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Re: Hossack Motorcycle Front End
« Reply #1 on: November 16, 2012, 12:48:19 PM »
What kind of bike are you building this for?

Eric
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Offline sebwiers

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Re: Hossack Motorcycle Front End
« Reply #2 on: November 16, 2012, 02:29:54 PM »
It'll be going on my Yamaha XJ750 Seca (think its an '83).  Bike is a bit over 500lbs, I weigh about 200.  Target weight balance is 50/50.  Design of the system will allow fairly easy removal for use on a different bike / return to using the stock telescopic Seca fork, although the bike will end up with some extra mounting tabs, etc.

No really compelling reason to do it, by all accounts the Seca fork is a decent piece for its age, with a fairly competent anti dive setup.  Just having some fun, looking to make a rat bike / cafe racer mashup with an 80's influence.  The design will also let me experiment with various rake / trail combos pretty easily; I've seen some info indicating that bikes with MUCH steeper head angles, but still running 3-5" trail, are actually a lot nicer to ride on rough streets than the normal setup.

Offline Brass_Machine

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Re: Hossack Motorcycle Front End
« Reply #3 on: November 16, 2012, 03:04:09 PM »
When I was considering building a hossack style front end, I had taken some serious thought of adapting a BMW duo lever system. Then also the thoughts went to doing machined style girders instead.

Have you looked at Chris Cossentino's setup?

LINK



Eric
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Offline bp

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Re: Hossack Motorcycle Front End
« Reply #4 on: November 16, 2012, 07:47:34 PM »
As it happens over 20 years ago my final design project at college was entitled "Motorcycle Suspension".  The system that I ended up with was based on a Hossack.  From memory I made the decision to use steel for all structural bits subject to fluctuating loads.  The fork legs were 50mm x 25mm x (I think) 3mm RHS steel.  The design used a Honda 500cc XBR engine, because I had one.  So no powerhouse!!
Best of luck
cheers
Bill

Offline sebwiers

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Re: Hossack Motorcycle Front End
« Reply #5 on: November 17, 2012, 09:29:55 AM »
BP- yeah, I looked at steel, and could easily make it work for just a bit more weight (and a bit less money, and maybe easier fabrication).  However, I think aluminum is a good choice IF you are comfortable working with it and paying for it (which I am - I'm not planning anything fancy, and I'm looking at well under $100 in tubing, which isn't out of line with other costs; carbon fiber was WAY to spendy for the dims I needed).  My impression is you can get a stiffer, stronger structure from the same mass of aluminum IF you are able to use fatter, larger tubes (or deeper girders, etc).  If I end up needing tubes over 3" od, I'll probably go with steel, but I'd like to keep the fork weight at a minimum to reduce unsprung mas.
For equal lengths and loads, it looks (from those same online calculators) like 2x1x.125 rectangular steel is pushed closer to its limits than a 3(od)x.25 aluminum, and they weigh about the same, so it sounds like I'm on track.  I'm not sure what bits would NOT be subject to fluctuating loads - I assume you mean the frame itself, maybe the bushings & pivots of the control arms?  For me those may go either way, since their weight doesn't influence the suspension behavior as much.  Steel looks appealing for the control arms just because it is easier to weld, and costs less (so I can afford to build multiple generations to improve the design).  The idea is to have a nice, light, strong fork that is adjustable enough (ball joints go into "triple clamp" type setup) to allow me to set the geometry after the fact.

Brass Machine - The duo lever design is hard to replicate (compact construction w/ monocoque and cast parts) and doesn't lend itself well to adjustable geometry, but otherwise is a Hossack setup (according to both BMW and Hossack).  Cosstinos setup looks quite similar to what I have in mind, and is much like other prototypes (including Hossack's own racing bikes and BMW adaptation) I've seen photos of.  His geometry looks a LOT like what I had in mind- steep steering angle, a few inches of trail. 
The brake mount in his later designs is interesting- it goes at the bottom of the fork, under the axle.  Not sure why he did that- I'm guessing it moves the brake as close as possible to the center of the steering axis (which is actually why its in back on normal forks).  Looks trick, might be worth a shot.

The main differences between all those designs is the length / mounting locations of the control arms, and the mounting location / actuation method for the shock.  Cosstion looks to have some sort of rising rate linkage going, which I had considered.  I've got the full rear shock and linkage from a GSXR to play with, so the details would be a bit different, but much else looks similar (same rough control arm length, rod end pivots for control arms, ball joints sandwiching fork bridge).  My setup is just gonna have a hell of a lot beefier fork, since I'm working on a bigger bike.  That, and it will be bolting to an existing frame with an existing headset, so the steering will work differently.

Offline bp

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Re: Hossack Motorcycle Front End
« Reply #6 on: November 17, 2012, 07:41:57 PM »
All fair enough so far.  Are you looking at a street bike or a competition/road race bike??
My point about fluctuating loads is that steel has a far longer fatigue life than al. alloy.  So steel bits, as long as they are stopped from rusting, will have a much longer life. 
I designed mine from first principles....established what the likely loads would be, applied a factor of safety, then calculated all the bits.  At the early stages of any design exercise its far better to not make any assumptions but try and identify and quantify some reasonable extremes.
From memory the braking load is the largest normal load  and works out to about 6.5g.  The shock loads (hitting a kerb for instance) can be much higher.
Keep going!!
cheers
bp

Offline sebwiers

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Re: Hossack Motorcycle Front End
« Reply #7 on: November 17, 2012, 09:05:49 PM »
Would be a street bike.  Normal daily rider with sporting pretensions.  I could get fine use from the stock fork, but... meh.  Some folks build raked out choppers, this is what I want to ride.

6.5g seems crazy high for brake forces on a fork - how does that happen?  Do you mean the stress to the brake mounts (which has some leverage) or the load pushing on the fork?

I understand your point about fatigue; I've been a bicycle mechanic and have seen (and created) a few frame fatigue failures.  From what I've seen, both fail from fatigue when pushed to similarly low weight, and do fine (fatigue wise) when built stiff and strong.  I suppose that's a compelling argument for steel, since its cheaper...

Honestly, I don't really know the "first principles" to go from - that's why I was asking here.  I just looked at some numbers I'd seen recommended (for steel) and figured out what that mass would let me do in aluminum, then ball parked the resulting stiffness / strength.  Seemed OK... but that's hardly a ringing endorsement!  I figured to do the build the Russian space program way; build the best thing I can with the materials and design I have, then test it destructively!  In this case, that would mean building a test rig to load the fork with the simulated loads equal to double the highest braking / bump loading forces I'd expect to see.   Doesn't let me know if I could go lighter, but it would tell me if I went to light...

Hmm, seems the logical thing to do is to prototype in steel, then shoot for similar strength in aluminum once I'm happy w/ the function.  Would give me time to get my fabrication skills up, etc as well.  4x1.5x11ga steel rectangle tubing is maybe 50% heavier than I was hoping for, but strong as hell and easy to work with.  Cheap, too- enough so that I can build another if I change brakes / wheels / etc rather than bothering to make it adaptable.
« Last Edit: November 18, 2012, 01:20:30 AM by sebwiers »

Offline Brass_Machine

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Re: Hossack Motorcycle Front End
« Reply #8 on: November 17, 2012, 10:56:13 PM »
I don't know if you have read Tony Foale's book, but you probably should. It has a lot of information you might find handy. He also has some analyses software for FFEs. A lot of information that man provides on motorcycle suspension.

Eric
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We're all mad here. I'm mad. You're mad.

Offline sebwiers

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Re: Hossack Motorcycle Front End
« Reply #9 on: November 18, 2012, 01:19:03 AM »
I've looked at getting those, yes.  Seems spendy (especially if I want the paper book- I'm in the USA), but probably worth buying BEFORE I purchase materials / do fabrication, rather than after.

Offline bp

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Re: Hossack Motorcycle Front End
« Reply #10 on: November 18, 2012, 01:53:58 AM »
I calculated the braking load by doing a series of timed stops from various speeds, 30kph, 60kph, 100kph etc on a quiet, smooth, well surfaced (grippy) bit of road.  The tests were done front brake only, rear brake only, both brakes together.  They established that the old wives tale about the front brake doing 75% of the work is about right (mine came out at 81%).  The tests also pretty much wore out the front disc and the front tyre, I also learn't how to do "stoppies"!!
I've just had a look at the paper and yes 6.5g is crazy, I measured the maximum deceleration as -6.85m/s2, somehow in my head, the "m/s2" became "g" over the last twenty years!!  Sorry about that.
I can second the recommendation for Tony Foales book, really well worth having a hunt around for.  I've just had a look at his web site, good stuff there too!!
cheers
bp

Offline sebwiers

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Re: Hossack Motorcycle Front End
« Reply #11 on: November 18, 2012, 09:41:26 AM »
No need to hunt - amazon has it, as does his website.  The CD PDF version is about half the cost (for me) and I have a couple tablets, so I'll go that route.

Brake loading calculation sounds pretty basic - I should have considered something so simple.  I think I'll just use the "60-0 deceleration" figures from the service manual, rather than burn out my brakes and tire.  :)  Probably better than I could mange anyhow - they've got pro test pilots for that stuff.  I'll assume 100% work by front brakes, for margin of safety and because the bike has MUCH better front than rear brakes (dual disk front w/ anti dive circuit, rear drum w/ shaft drive).

Claim is 60-0 mph in 150 feet when "lightly loaded".  Curb weight is ~530 lbs, and most test pilots are smaller dudes, so I think 700lbs is a fair call.

Converting to metric so the math is sane, that's gonna take the same (or less) force needed to haul 320 kg (round up) down from 26.6 m/s (round down) in a distance of 43m (round down).

Looks like it took 3.3s for the test pilot to stop from 60 mph (26.6m/s) - a reasonable figure.  That's an acceleration of 8.1m/ss

Given 320kg, how much force would that take? 2600 newtons, or ~585 lbs.  I'd been guessing a "typical" brake loading of 500lbs, and shooting for each leg to be able to repeatedly take 1000lbs without issue (at less than half yield strength).  I think given the horrors I've seen when cargo bikes with disk brakes fold their forks (and the fact I have no real way of knowing where local stress will shoot above norm) that the safety margin of 8 this would provide is easily justified.   

Offline j_e_f_f_williams

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Re: Hossack Motorcycle Front End
« Reply #12 on: December 18, 2012, 08:54:56 AM »
Hello,

Coming a tad late but thought I would pass along.  There is an e-mail list that both Chris C (it is a hossack) and Tony belong to as well as many other great resources for motorcycle knowledge (Ian Drysdale, Marty & 2 wheel drive dirt bikes, David Sanchez & his Moto2 bike etc)

MC-Chassis list - details @ http://eurospares.com/maillist.htm

I seem to remember some discussions on loads for suspension and braking a while back and it will be in the archives of the list

Jeff

Offline sebwiers

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Re: Hossack Motorcycle Front End
« Reply #13 on: December 18, 2012, 01:16:24 PM »
Yup - fella on another forum pointed me the same way.  It's been moderately useful, probably will be more so once I have photos to post.  Which means they can tell me what I've done wrong, but nobody is really saying what to do to get it right (other than buy Tony's software, which is probably a good move, just not in the budget quite yet).

Offline j_e_f_f_williams

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Re: Hossack Motorcycle Front End
« Reply #14 on: December 20, 2012, 04:55:25 PM »
So you are the 7" travel Hossack? (

I used to use SolidWorks and actually found it was just as good as Tony's SW (I also have) if you set things up correctly.  I used it to create drawings for my Foale/Parker type front end.  I stopped using Solid Works and moved to Alibre as then I was legal :)  Alibre I didn't find as good for making dynamic changes to my 3D and seeing the results in the 2D drawing where my % anti-dive etc are.  This is when I started to work with Tony's SW as well.

I think you got the answers you got because your general question was "Is that OK?" and the answer generally on MC-Chassis list is "yes, it's OK if that's what you were trying for" :)  I assume 7" travel = off-road so I would listen to Marty's comment that large bumps don't like the lengthening wheelbase.  Also I don't know enough about off-road to comment on anti-dive.  I had on-road have 50% to 75% anti-dive through the range of travel.

The hard part, as Tony pointed out, is to figure out the dive and trail you want.  I noodled over it for hours and at the end of the day you really won't know until you try it.  I probably noodled too long and should have spent those extra hours welding vs in Solid Works :)

My FFE never got built as my project time quickly disappeared and changed as I had kids :)  Keep thinking I should return to it some time soon...

TTYL, Jeff

Offline sebwiers

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Re: Hossack Motorcycle Front End
« Reply #15 on: December 21, 2012, 10:00:15 AM »
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. 

Offline hopefuldave

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Re: Hossack Motorcycle Front End
« Reply #16 on: December 23, 2012, 12:02:44 PM »
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)
Rules are for the obedience of fools, and the guidance of wise men.

Offline sebwiers

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Re: Hossack Motorcycle Front End
« Reply #17 on: December 24, 2012, 12:51:12 AM »
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:
« Last Edit: December 24, 2012, 10:10:50 AM by sebwiers »

Offline hopefuldave

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Re: Hossack Motorcycle Front End
« Reply #18 on: December 24, 2012, 06:28:12 AM »
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?)
Rules are for the obedience of fools, and the guidance of wise men.