MNSBR: Xj750 Seca, Adjustable Hossack Front / Mono Rear / Mad Max Style - MNSBR

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Xj750 Seca, Adjustable Hossack Front / Mono Rear / Mad Max Style

#1 User is offline   seb Icon

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Posted 31 January 2017 - 01:18 PM

Current state -
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More pictures and project explanation / details - http://imgur.com/a/VR3WH
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#2 User is offline   Kent Larson Icon

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Posted 31 January 2017 - 02:48 PM

:popcorn:
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Posted 31 January 2017 - 03:07 PM

What's going on with that front end?? Never seen that before...
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#4 User is offline   re tired Icon

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Posted 31 January 2017 - 04:05 PM

What Kent said.
:thumbup:
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#5 User is offline   seb Icon

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Posted 31 January 2017 - 05:41 PM

Pretty much what it says in the album - http://imgur.com/a/VR3WH

BMW K series bikes use essentially the same design (based on the same expired patents) but obviously differ in all the details of construction (and shock location, and steering link setup). The closest I've seen is Claude Fior's bikes & patent (shown in album). I designed it to be easy to adjust / modify / build / rebuild, and used thick metal to ensure my welds would be easy to run and plenty strong, tossed elegance out the window.

Has nearly 45 degree turn angle to each side. Travel is 3.5" rear, 4.5" front, and sprung quite stiff (probably to stiff for comfortable long term riding). Total dry weight .... unknown, really should weigh my pile of parts. Should be about 500 lbs; donor bike was not light, but I pulled a lot off (and added a lot back as well).

Currently working on making a custom fiberglass airbox, and improving the front steering link design, maybe mounting an oil cooler. After that, controls, then and wiring and fuel. Engine ran when I started, needs one valve shim replaced. Carbs were emptied, cleaned, and stored dry, but probably need some tuning.

Current winter's build thread (and previous years) is on Customfighters.com - http://www.customfig...ad.php?t=126849
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#6 User is offline   Jim Hubert Icon

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Posted 01 February 2017 - 07:47 AM

Quote

Kinematics program sold by Tony Foale, designed for the specific purpose of modeling various pivot based front ends (girders, Earles, trailing link, Hossack, DiFazio, etc). As tools for this project goes, it was the best $50 I spent.

For me, the above is the most interesting part of your imgur posts. Apparently, Tony has dropped the price of his software quite a bit since I looked at it. I think his software would be useful to me, but precisely taking all the required measurements is a lot of work. What I wanted was a database of stock bikes that were already measured. I suggested to Tony that he create an "exchange" where users of his software could post files. I wanted to be able to compare the geometry of various bikes and correlate that to how they behaved.

As Gordon Blair once said, "...until you can put numbers on your problem, you are not yet at the beginnings of a science...."

Seb, may I ask what your background/day job is?


P.S. Bobillier was a French mathematician noted for his work in geometry. I'm sure if you Google enough, you'll find a description of "Bobillier line".

This post has been edited by Jim Hubert: 01 February 2017 - 08:05 AM

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#7 User is offline   seb Icon

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Posted 01 February 2017 - 08:56 AM

It sounds like you were using Tony's suspension setup software, which calculates rear end wheel rates and I don't know what else. That program costs something like $150 and (afaik) is targeted at racing teams who use data capture on the track to fine tune race setups. I don't have that program, but I get the impression it is by far the more commonly sold / used of the two.

The funny front end software is a different program, and probably much simpler. By its nature, a database of stock bikes would not help much. Some sample designs sure would though, because figuring out numbers that work in the program (is, result in setups it can actually draw) can be hard. Even worse, some easily conceived setups (ones with parallel links) will cause math errors and crash the program!

I'm a web developer / computer programmer, have been for about 6 years now. Currently I work for a market research company doing a javascript based data processing and visualization. Went back to school (community college) at age 40 and picked up an AAS in software development to fill out my self education (also my son was born the same month I graduated). Before that I was a bike mechanic for a few years, before that took care of lab animals at the U of M, before that worked as a home health aide....

I've considered making my web app / phone app version of the funny front end software (have seen similar web apps for cars, to calculate roll etc) but obviously my free time is pretty well spoken for. My math is also not really up to the task.

I've seen information regarding Bobillier and 4 bar links, but its all a bit over my head, and doesn't seem useful to the problem of finding a (unique) fixed link that could be introduced into an existing 4 bar linkage without causing it to bind. I suspect that is in fact impossible, and Fior was either tossing in a red herring to keep people from duplicating some aspect of the patent or making a fairly trivial assertion in very fancy language.
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Posted 05 March 2017 - 12:12 AM

Mounted up an oil cooler (there's an adapter on the filter holder that sends / returns oil). Also stuck on the seat / tank so I could work out the airbox design (will be fiberglass). Filter goes on a 4" pipe which is capped, stock snorkle penetrates the cap, airbox volume is same as stock, should avoid any tuning issues.

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Posted 25 November 2017 - 12:30 AM

Heh, got busy and forgot about this forum...

Bike was ridable in September, I took off for a trip and blew an oil line because the routing put it against the header. Lasted 250 miles or so because the contact was in a spot down low where it doesn't get hot (or at least not enough to burn rubber through the header wrap) unless the engine is heavily loaded... which a load of gear and climbing hills in the Dells managed to do.

Recently got don outfitting it with a Maxim engine that has only 15k miles on it. Much better shifting and runs stronger (the one I blew was kinda knackered anyhow).



Also got some new tires and a "glove compartment". Theres an 18 watt LED flood on the other side under the oil cooler to balance the weight. Not shown is shock upgrade - new front shock is from a Hyabusa, 850 lbs/in instead of 650 because I was needing to run outrageous preload to avoid excessive sag, loosing my small bump sensitivity. New shock rides great with almost no preload, and fit right in.

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Posted 26 November 2017 - 10:35 PM

Thanks for the update! That's definitely a unique project. Does that front end feel any different than a standard setup?
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#11 User is offline   seb Icon

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Posted 27 December 2017 - 02:15 AM

Since the last post, I bought a new engine and put it in. Got that done putting that in around Thanksgiving, did some riding before the snow hit. I decided the front spring was too soft so got a shock that is 850 lbs / in (vs 650 lbs / in), and it is a big improvement - I don't need to run ANY preload now, so small bump response is much better.

The front end really doesn't feel much different from a normal bike, though I'll admit I've only ridden maybe a half dozen different bikes, all of them on the older side. I also didn't do much riding over the summer, spare time went to the build, so my memory is fuzzy. It probably would feel weird to anybody else for a few miles, maybe longer depending on how different out taste in setups & ergo is.

Once I got over the oddity of seeing the front wheel turning out ahead of the handlbars, I just forgot about it on focussed on what adjustment it needed. That's partly because I intentionally set it up to ride like a normal bike - minimal anti-dive, normal trail figures. I recently cranked trail down (to 60mm) and anti-dive up (to maybe half the dive a normal bike has). ^0mm of trail might be a bit short, but it feels really good (the steering was heavy and "floppy" with more trail) and I can easily ride no-handed. However, I never had a chance to get up over 50 mph or ride a long distance - it may be too twitchy. Really hammering the front brake results in the front end compressing 20mm or so. The next step would be to ramp the anti-dive WAY up - its possible to set it so that the front end actually stays dead level no matter how hard you haul on the front brake, which is supposedly where things start to feel strange. If there is a strangeness, its maybe that I'm probably using more front brake than I think I am, though I haven't noticed crazy short stopping distances or anything (but have noticed that hissing noise you get from a tire pulling max brake effort).

The springing is a bit stiff, probably because the wheel rate is regressive (softer at the end of the stroke) on the front (and strongly progressive in the rear). I've worked out a custom linkage design that will fix that, making the front slightly progressive and giving me an extra 20mm or so of travel to boot. With the new shock, it certainly was not bad to ride by any stretch - I ripped it down Phalen Boulevard a few times, which is a park road with a lot of pothole strewn sweepers, and felt very comfortable intentionally steering it at bumps while leaned over and doing 35+ mph. It also gobbles up speedbumps without complaint. I think the only time I managed to get the rear to feel loose was when I leaned it hard enough to touch the muffler to the ground and skipped the back out a couple inches.

Currently I'm working on crash bars and ice tires so I can go ice riding. :)
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Posted 27 December 2017 - 09:02 AM

View Postseb, on 27 December 2017 - 02:15 AM, said:

Currently I'm working on crash bars and ice tires so I can go ice riding. :)


Awesome!

I'm getting my Grom ready for ice riding. Just waiting on my shipment of ice screws right now.
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#13 User is offline   seb Icon

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Posted 27 December 2017 - 01:57 PM

I won't be racing, but I'm meeting up with a crew of other Mad Max freaks to do a 'half time show' at the indoor flat track at Mama Tried. figure as long as I'm there, might as'well hit the show and ride the ice.
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#14 User is offline   ZZR Joel Icon

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Posted 27 December 2017 - 03:41 PM

What a beautiful beast!
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#15 User is offline   seb Icon

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Posted 29 December 2017 - 01:21 AM

Thanks! Here's a pic with the new engine and shock in, out in the snow when I finally took it over the shop for winter fabrications bits.

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