Author Topic: xj750 Seca Hossack Front (and mono rear, bodywork, etc)  (Read 12336 times)

Offline sebwiers

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xj750 Seca Hossack Front (and mono rear, bodywork, etc)
« on: September 17, 2014, 11:57:51 AM »
Figured I'd share some recent progress pic's of the Hossack front I'm putting on my project bike.  Pics don;t show the read pivot mounts because they aren't fabbed yet. In fact, I'm doing a whole new lower arm that will use actual swing arm pivots from a spare rear swing arm I picked up, and won't have any threaded parts loaded in flex.










Offline Brass_Machine

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Re: xj750 Seca Hossack Front (and mono rear, bodywork, etc)
« Reply #1 on: September 17, 2014, 11:37:05 PM »
How very awesome!!!!

Love the Hossack front end. I have a thing for the FFEs. You will keep us up to date on this? I would love to follow and see how it goes.

Eric
Science is fun.

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

Offline DavidA

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Re: xj750 Seca Hossack Front (and mono rear, bodywork, etc)
« Reply #2 on: September 18, 2014, 05:58:53 AM »
This looks interesting.

But my question is 'why'? I thought that this was done on trikes.
Clearly I'm missing something here.

I'm not being facetious ,  I simply don't know why you are doing it.  Obviously there is a good reason.

Dave :scratch:

Offline sebwiers

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Re: xj750 Seca Hossack Front (and mono rear, bodywork, etc)
« Reply #3 on: September 18, 2014, 10:18:26 AM »
To be honest, the 'why' in my case is mostly for its own sake (curiosity, looks, fun of building it).  I've always loved 'Mad Max' vehicles that mix junk yard aesthetics with high tech parts and 'futuristic' design concepts.

In general, the possible benefits of using such a Hossack front end on a motorcycle (or trike, or sidehack rig) are primarily:
1) changes (hopefully improvements) to handling (mostly by eliminating brake dive common to telescopic forks, but also eliminating striction)
2) reducing stress on the frame (shorter lever arm between wheel and frame attachment)

I'm mostly interested in 1.  Race bike builders (including Norman Hossack, the inventor of the system) take advantage of 2 to allow building a lighter motorcycle (both sprung and unsprung mass), but obviously that's not my goal (or really possible, given I'm adding it to an existing bike without otherwise changing the frame).  A good example of a motorcycle where the system is used mainly to improve handling is the BMW K series bikes, with the Duolever system.  My rear suspension setup is also similar to (older) BMW designs.  In some ways, its a ghetto K bike.

What you likely see on trikes is a girder fork- I've only ever seen one example with a Hossack setup.  A girder and a Hossack front look superficially similar (especially in my case, where the build isn't done yet) and both allow the same anti-dive braking benefits.  That's important to trike builders because trike is much heavier than a motorcycle. Being so heavy, trikes have much worse problems with front end brake dive if using a conventional motorcycle fork.  By building a Hossack system instead of a girder, I can also choose any rake angle I like without having to actually cut the frame and move the head tube, and tuned the wheel path / bump response so that rake and trail increase slightly with bump (using a telescopic fork, they decrease).  I'm going for a rake angle of 20 degrees (or less), and also reducing the wheelbase a bit.  Steeper rake angles are safe, as long as you maintain the trail at a reasonable figure (3-4"), and actually improves handling on rough surfaces.  The Hossack design lends itself very easily to steep rake angles, compared to a conventional headset steered fork.
« Last Edit: September 18, 2014, 12:30:30 PM by sebwiers »

Offline DavidA

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Re: xj750 Seca Hossack Front (and mono rear, bodywork, etc)
« Reply #4 on: September 18, 2014, 11:47:18 AM »
Thanks for the explanation.

Definitely an interesting project whatever you use it on.  I do remember the BMW front set up being similar.

Dave.

Offline bp

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Re: xj750 Seca Hossack Front (and mono rear, bodywork, etc)
« Reply #5 on: September 22, 2014, 07:32:52 PM »
Good project!!  I've always been interested in motorcycle suspension, in fact my final project at college was titled "Motorcycle Suspension".  So I'll be really interested to see how you get on....best of luck!
cheers
Bill

Offline sebwiers

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Re: xj750 Seca Hossack Front (and mono rear, bodywork, etc)
« Reply #6 on: January 06, 2015, 01:38:59 AM »
So, things have been rolling along pretty well.  Just don't get many hours in the shop, and was out all November with bronchitis, so its a slow burn of few hours over many weeks.

As tends to happen with these projects, some of the work was premature.  The pivot mountings I ended up building were not really compatible with the arm's I made.  So now I have the fork bolted on with some new mock-up arms made from wood.  :)






Offline steampunkpete

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Re: xj750 Seca Hossack Front (and mono rear, bodywork, etc)
« Reply #7 on: January 06, 2015, 02:30:47 AM »
I haven't seen a set up like this before, and don't really understand the how it works.

Weight transference is unavoidable, so how does the Hossack arrangement prevent or reduce dive under braking?

How does the un-sprung weight compare with that of a telescopic fork?

Answers on the back of a post-card please ....

The hub centre steering (HCS) concept seemed to have a lot going for it, with the possible exception of the difficulty of achieving the same lock-to-lock angle as telescopic forks. The Earles forks as used by BMW also appealed as in both increased rigidity and reduced un-sprung weight. A particular problem with teles is their tendency to twist when braking (except when twin discs are fitted - I think this is why twin-disc conversion kits for Bonnevilles were so popular).

Offline sebwiers

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Re: xj750 Seca Hossack Front (and mono rear, bodywork, etc)
« Reply #8 on: January 06, 2015, 02:50:58 AM »
I haven't seen a set up like this before, and don't really understand the how it works.

It works pretty much like trailing parallel arms on a car would, only with the arms located above the wheel.  Or, as Hossack describes it, like a pair of conventional A arms rotated 90 degrees.

Quote from: steampunkpete
Weight transference is unavoidable, so how does the Hossack arrangement prevent or reduce dive under braking?

The dive that it cancels it is the portion of brake force that normally goes into compressing front suspension.  If you do a vector diagram overlayed on a conventional fork, its obvious that if you push back on the wheel (braking force), you also push it up (suspension compression).  Because the Hossack (and other linkage designs) allow the wheel path to be nearly vertical (and the wheelbase constant), the resulting upwards vector component from braking force can be reduced to nearly 0.  If the wheel path is such that the wheelbase actually gets longer, the resulting vector component actually opposes weight transfer.  Hence, dive (even from weight transfer) is minimized.

Quote from: steampunkpete
How does the un-sprung weight compare with that of a telescopic fork?

For equal design & fabrication skill and cost, the Hossack is claimed to weight less.  Mine probably has more unsprung mass than the fork the bike came with, but if I built a telescopic fork, it wouldn't be very light!  Norman Hossack has built a Ducati that he claims has less upsprung mass less than stock, but I think the larger weight saving was in sprung mass; the design places less stress on the frame, because the lever arm is shorter.  Another advantage is the it doesn't have issues with the sliders binding, so there isn't that variable force resisting the wheel motion.

Quote from: steampunkpete
The Earles forks as used by BMW also appealed as in both increased rigidity and reduced un-sprung weight.

Earles also offers the same potential for anti-dive.  The early BMW's that ran sidecars had the fork set up so that you could move the pivot between two locations.  One provided stronger anti-dive for when stopping the greater weight of a sidecar.  Pivot based front suspensions are pretty much standard for sidehack rigs; tele's just dive way to much with all that weight.

BMW also currently sells a Hossack design very much like the one I'm building, as the Duolever on thier K series bikes.  Apparently the upright is on the heavy side, but they also use the same parts on all K models, including the behemoth 6 cylinders.
« Last Edit: January 06, 2015, 03:33:36 AM by sebwiers »

Offline micktoon

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Re: xj750 Seca Hossack Front (and mono rear, bodywork, etc)
« Reply #9 on: January 06, 2015, 04:08:31 AM »
Hi sebwiers, I am not really into bikes that much but interesting projectcwell explained and nice fab plus welding. I look forward to seeing it develop.
  In response to the question you had of why .................... I think as lots of modders look at it ..............I have the gear to give it a go so its more a case of Why Not  lol.
 Good luck with the build.
Cheers Mick

Offline steampunkpete

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Re: xj750 Seca Hossack Front (and mono rear, bodywork, etc)
« Reply #10 on: January 06, 2015, 09:39:10 AM »
Got it! Its like the Honda 90 step-throughs the dive under braking is countered by the change in geometry in the suspension that effectively forces the chassis back up - a dynamic cancellation. (The Honda 90 was slightly different in that,  if there was only a light rider , the resultant force from the braking acted below the linkage pivot point, with the disconcerting result that the front end actually rose under braking, rather than dipping. Weird).

Offline sebwiers

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Re: xj750 Seca Hossack Front (and mono rear, bodywork, etc)
« Reply #11 on: January 06, 2015, 10:24:02 AM »
Hi sebwiers, I am not really into bikes that much but interesting projectcwell explained and nice fab plus welding. I look forward to seeing it develop.
  In response to the question you had of why .................... I think as lots of modders look at it ..............I have the gear to give it a go so its more a case of Why Not  lol.
 Good luck with the build.
Cheers Mick

Thanks.  Access to the equipment was a big part, I got lucky and found a local maker-space that had the tools I wanted to use.  But its something I;ve wanted to do for a long, long rime, ever since seeing all those articles back in the 1990's about new bike suspension designs (Bimota Tesi, Britten, Yamaha GTS, etc).  As to 'Why Not' there's a lot of obvious reasons... aside from the enjoyment of doing it, its probably an awful idea.  Potentially dangerous to ride, somewhat expensive to build, insanely time consuming, generally less effective than proven commercial products... pretty much all the standard home build obsession drawbacks.

Got it! Its like the Honda 90 step-throughs the dive under braking is countered by the change in geometry in the suspension that effectively forces the chassis back up - a dynamic cancellation.

Correct.  Any system that uses pivots & one or more leading / trailing arm(s) to provide wheel travel will have some dynamics related to dive, for good or ill.  In fact, a telescopic fork can just be looked at as a special case of a pivot based system where the arm length is infinite.

Its not really very intuitive, and obviously figuring out linkage motion that gives the desired dynamics is tricky.  Fortunately there's a respected motorcycle designer who wrote & sells (for very reasonable cost) a program designed just for that purpose.  He also (literally) wrote the book on the dynamics of such systems.  Just to show how easy having the program makes this, here's a gallery showing what the program looks like for the configuration I have mocked up in wood.  I already had my pivot locations (fabricated for my previously planned arms) and upright lengths (as part of the fork fabrication, although the lower balljoint can be moved by mounting above / below the plate) so I just plugged those numbers in and played around with arm lengths until I found some that gave suitable dynamics.

http://imgur.com/a/OyThn
« Last Edit: January 06, 2015, 12:24:21 PM by sebwiers »

Offline bp

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Re: xj750 Seca Hossack Front (and mono rear, bodywork, etc)
« Reply #12 on: January 07, 2015, 04:44:10 PM »
Another aspect with telescopic forks is that under braking the steering geometry changes significantly.  Braking hard places the maximum requirements on overall stability, yet with Telescopics under heavy braking the stability actually reduces (steering angle steepens, trail reduces).
The Hossack design provides more consistent steering geometry.
Yes I know that MotoGP bikes use Teles, but there are other reasons for that.
Best of luck!!, keep going.
cheers
Bill

Offline Gwil

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Re: xj750 Seca Hossack Front (and mono rear, bodywork, etc)
« Reply #13 on: January 12, 2015, 03:23:57 PM »
Absolutely keep with it! I'm building a slightly Mad Max outfit from a Dnepr sidecar and an old Suzuki 1500 v-twin.

Been thinking of leading links or some girders to reduce trail and increase strength- and, like you, just for the look of it and the excercise! Currently shopping for an old/cheap lathe- not easy here in Portugal. Be very interested in your final setup.

Offline sebwiers

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Re: xj750 Seca Hossack Front (and mono rear, bodywork, etc)
« Reply #14 on: January 15, 2015, 01:57:31 PM »
Anybody got any suggestions on how I should configure my steering linkages?  I'm kind of in a corner given a bad combination of pivot locations, headset location, and packaging requirements.

Offline sebwiers

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Re: xj750 Seca Hossack Front (and mono rear, bodywork, etc)
« Reply #15 on: May 22, 2015, 11:20:43 PM »
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.






Offline sebwiers

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Re: xj750 Seca Hossack Front (and mono rear, bodywork, etc)
« Reply #16 on: November 18, 2015, 08:10:30 PM »
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.






Offline sebwiers

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Re: xj750 Seca Hossack Front (and mono rear, bodywork, etc)
« Reply #17 on: November 29, 2015, 06:39:38 PM »
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