xj750 Seca Hossack Front (and mono rear, bodywork, etc)

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steampunkpete:
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).

sebwiers:

--- Quote from: micktoon 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

--- End quote ---

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.

--- Quote from: steampunkpete 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.
--- End quote ---

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

bp:
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

Gwil:
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.

sebwiers:
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.

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