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SECApocalypse

Yamaha XJ750 "SECA"
with adjustable Hossack front end
and Mad Max style

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Motocycle suspension design has come a long way in the last 50 and even 20 years, but a lot of interesting design ideas have never really been explored, let alone available to the average rider to get first hand experience with. The "Hossack" front end is one such design

Such a system can be configured to create an anti-dive geometry. This is impossible with a normal telescopic fork, which is inherently pro-dive. Another idea that matches well with such a setup is to have a very steep rake (retaining conventional trail). Tests by Tony Foale indicate that motorcycles with very steep rake are more stable on bumpy surfaces.

To get an "ass in the saddle" feel for the effects of anti-dive geometry and various rake / trail combinations, I am grafting a highly adjustable Hossack front end on a 1981 XJ750 SECA. The motorcycle also has a custom rear suspension and body work, and post apocalyptic styling, hence - the SECApocalypse!

Front end design is similar to that used on current BMW K series bikes, called the "Duolever". It was originally invented by Norman Hossack. Claude Fior independently designed a similar system; both inventors had separate patents.

Frame is originally a 1981 Yamaha XJ750 Seca that I picked up for $100. The seller threw in a nice big Harbor Freight drill press and a Yamaha Virago gas tank to replace the rusted out stock tank. Engine was running, needed some carb / valve adjustment and the battery needed replacing. Things got a bit out of hand once I started on these mods, and reached a fever pitch when I discovered a local maker space.

I've done 99.5% of the work myself. I had done some welding previous to starting this, but only arc and gas. All this is MIG welded. I'd never done any machining, but am now pretty good with a mill (still get help with lathe on the occasions I need it). All work done at Twin Cities Maker / Hack Factory

seca_dyno.mp4

SECApocalypse dyno test after tune.

MPEG-4 Video - 17.84 MB - 10/13/2017 at 01:44

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  • Lower shock Mount Redesign, Paint, Engine In

    sebwiers01/27/2017 at 17:28 0 comments

    I discovered that the lower shock mount I'd built prevented the engine from fitting in the frame. I could have trimmed the aluminum plate down enough to make it fit, but felt it would give up to much strength. Instead I decided to redesign the lower shock mount. This required a bit of frame modification. During my strain testing, I'd also cracked the frame where there was a pre-existing nick, so I needed to repair that crack and decided to run some strengthening rails across the frame rails since they were not designed with supporting a loaded shock in mind.

    This is the ugly little crack



    This is the start of the new shock mount. The 3/8" engine mount bolts (bumped up from 10mm because I have better selection on threaded lengths and strength for SAE bolts) run into threads in the aluminum block, and half inch bolts run through the shock mount into threads in the aluminum block. This is structural but probably not strong enough for road use. Instead it serves to hold everything in place while I do frame modifications.


    The finished shock mount adds a beefy rectangular box beam expanding the lower engine mount, which the shock mount bolts directly to (the box beam has a threaded weld insert). Speed holes in the box beam allow paint in, the ones on the shock mount are just there to look nice.


    I then pulled everything apart and painted it. Rustoleum "rusty red" primer holds very well on the bare metal, not nearly as well over the existing powder coat, but will be the final paint color. One of the shop managers calls the color "hot rod red" or "prototype red" and that's exactly why I like it. As a kid I saw the same color covering the fenders and hoods on a lot of a lot of sweet 1960's muscle cars rolling around Detroit. You can also see the 3/4" x 1/8" bar welded to the front & bottom of the frame rails, adding some strength to the repair in the area that cracked.
    The bare steel suspension components are coated with linseed oil, which is commonly used to protect metal sculptures and as an inexpensive underbody coating. I really wanted to preserve the raw metal look of the parts I had fabricated. The linseed oil takes a long time to harden when applied to metal but forms a nice solid film. Its not as durable as powder coat or properly applied paint, but it's very easy to touch up and (unlike paint) can be applied over oily & rusty surfaces.


    The engine went back in pretty easily, the only tight spot being there's just 1/8" clearance from the bulge of the cam chain to the upper shock mount. As a big bonus, the new lower shock mount design allows the original exhaust to fit without any modification. The headers do run very close to the shock, and will be wrapped with insulation to keep heat away from the shock. They actually make exhaust pipe wrap specifically to keep heat away from other under-hood components, although on motorcycles its more often used for looks and safety (to prevent burns). I get a two-for-one deal, because I both need to keep the shock from getting hot, and get to hide my ugly old pipes.



    I'm really exited to have the major fabrication work done and engine back in, but there's still a LONG way to go before I can ride it. :-P

  • Trimming Pivot Plates

    sebwiers07/01/2016 at 02:13 0 comments

    I decided to cut down the side plates that hold the front arm pivots. Pictures tell the story.





  • Steering Link Re-work

    sebwiers06/06/2016 at 17:04 0 comments

    In an on-going effort to chase slop out of my steering link setup, I re-made the top drag link holder. The original design had two separate parts held to the swing link with a bolt; if that bolt came loose (which it easily did with any rotational force on the link holder) the forces on the drag links were able to wiggle the parts, and it would quickly get even looser.

    The new design is a single part that sandwiches the swing link such that even if the bolt gets loose, the bolt is only a pivot and its tightness does not affect positional integrity. Its also made from thicker metal, so flexes less and had enough stock that I could machine the contact faces flat, eliminating another source of slop.

    The new design is also beefy enough to serve as a mounting point for my steering damper (which is laid in the location it will be mounted for this photo).










  • Brake Caliper Mounts

    sebwiers05/29/2016 at 05:58 0 comments

    I knew eventually I'd have to get around to mounting front brakes, and with most of the other fabrication work done, it was time to take on this task. Since I was doing new mounts, I figured I'd pick up some improved brakes, and took the gamble of buying to cheap Yamaha "Blue Dot" R6 brakes on Ebay. The pads don't cover my whole rotor (the old mono-pot was a wider pad,) but that's OK, and the otherwise fit my rotors pretty well.

    Most importantly, they don't hit the spokes, although the wheel clearance is VERY thin when spaced for my rotors! The photo shows the very narrow space the spokes pass through. It looks scary, but that's how cast / machined aluminum wheels tend to be - simple thin disks with cut-outs.



    To get the positioning right, I took four key measurements with the brake caliper in place:

    • radial axle center to the center of the lower bolt
    • direct bolt center to bolt center
    • perpendicular fork leg to lower bolt hole
    • perpendicular fork leg to upper bolt hole

    This gave me the dimensions I needed to lay out a pair of tabs that would put the brake pads in proper relation to the swept portion of rotor. Below is the result of that layout work, tacked in place. My rotor is made for a single pot brake, so the brake track is only 80% covered by the new brake, but the new brakes are correctly aligned on the outer part of the rotor, and the pads do not go past the edge (or even within 1/8" of it) at any point.


    To ensure the mounts were in the right location and (mostly) aligned, I used a fairly crude jig. The task of the jig was to get the lower bolt hole the right distance from the axle, and keep the mounts as close to aligned and centered as possible. The tabs themselves provided alignment for the upper bolt hole.

    Once they were tacked down, I fully welded the tabs. I "stitch welded" (using multiple short spots of weld) rather doing this all in one run, to minimize distortion.


    Despite these efforts, I had no expectation the brake tabs would be exactly perpendicular to the axle. Welding also left weld bead in areas I needed to be flat and smooth for the brake / bolt heads to sit properly. The solution was to clamp the halves of the fork in the mill and square up the caliper mount brake faces, get rid of weld bead where needed, and cut away the "bridge" that aligned the two tabs during weld assembly. Machining after welding is often necessary where parts joined with welds need precision alignments / spacings. I also took this opportunity to add some (more) decorative cutting on the fork tips and bridge (also removing a tiny bit of unsprung mass).

    And here's what the bike looks like (from the front) now:





  • Geometry Adjustment & Setup

    sebwiers05/22/2016 at 04:30 0 comments

    TCMaker was having an open house exhibit today, so I bolted the bike up for display and took the opportunity to measure the actual component dimensions and make adjustments to get a geometry I hope will feel good for test rides. This also gave me a chance to nail down the actual "as built" dimensions to enter into the motorcycle geometry program I'm using.


    It turns out that as I had it set up at Maker Fair last week the trail was only about 2". I probably didn't notice that pushing it around / coasting down a hill because of the low speeds. This is actually a good thing - the design makes it very easy to increase the trail, but to reduce it I'd have to make some clearance cuts. Once I'd made the needed bolt adjustments, the wheelbase came in exactly what it was on the stock bike, and the front tire fit nicely snug against the shock.


    The program I'm using was written by Tony Foale. It predicts various handling characteristics of a wide variety of "Funny Front End" designs based on user inputs for relevant dimensions and bike weighting. Using the software in the design phase was vary helpful, as it allowed me to determine what dimensions (roughly) I needed to shoot for, and showed me which ones would need to be carefully controlled to get the behavior I wanted, and which ones had little impact on suspension behavior (and so could be tweaked to meet packaging / structural requirements). Here's the setup screen and predicted kinematics for the geometry I have dialed in in the picture above. In a standard motorcycle magazine, the relevant stats would be as follows.

    • Rake: 21 degrees
    • Trail: 3.5 inches
    • Wheelbase: 57 inches







  • Maker Fair, Test Coast

    sebwiers05/15/2016 at 00:46 0 comments

    I went took the bike to the Minneapolis Maker fair today, which was a pretty awesome experience. Had a few great conversations with fellow motorcyclists / makers, including a mechanical engineer / cad expert looking for fabrication advice, and a guy who builds electric bikes.

    This is the first time I've had the handlbars and wheels on at the same time, so I took the opportunity to ride down a hill. I hadn't carefully tuned the handling, but was still pretty happy with it.

  • Safety First-ish!

    sebwiers05/14/2016 at 04:24 0 comments

    Since I am not a trained engineer or fabricator, I built using components that I calculated had a high safety margin for the loads they would see. But, there's hand cut threads here that need to support pretty high loads, and a lot of welds that might not be as good as they look (though a couple professional welders have given it thumbs up).

    My solution was to actually load the assembled structure to twice the max load that it could be expected to encounter. According to most motorcycle builders, this means 1g of braking force, and a 3g 'bump' force coming through the suspension to the frame. I figured I would do this testing before bothering with any steering mechanism, because if it failed the tests, steering wouldn't matter. However, I needed to have my load bearing structures built to pretty much the form they would be in for actual use, which means I worked on the bike for nearly two years (though only a few days a month) before doing these potentially destructive tests!

    To simulate the braking force, I built a long lever who's fulcrum was the axle. It applied its force to the fork in the same fashion brake caliper mounts would (via machined blocks) and was long enough that the load on the end was multiplied by 4 as a simulated brake force. I then suspended 375 lbs from the end of the lever arm, creating a simulated braking force of 1,500 lbs - double a 1g braking force for a bike with a gross weight of 750 lbs. I also used some dial indicators and mag mounts to measure the amount the fork was bent back, and the amount the frame itself was bent back, allowing me to get an idea how stiff the structure I added is. Turns out is it very stiff indeed, total deflection during this test was under 0.25 inch.


    Constructing a lever setup to do the bump load tests was a serious design challenge, so I bought a 1 ton crane scale and used a turnbuckle to apply the load. Each axle got 1950 lbs pulling up on it, with the shock replaced by a ridged strut and the motorcycle frame held in the jig via its engine mounts (as for the above test). Both resulted in a fair bit of frame flex (no surprising given the diamond shaped hole in the middle of the frame), but the load remained stable, indicating the loads never got past yield strength. On a racing bicycle this would be considered desirable "vertical compliance".





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Alain Mauer wrote 05/18/2016 at 18:55 point

Hi, I see in your title "Hackadayprize2016", but you have to submit it. As long as you have no logo for the Hackaday 2016 you are not in. After all cool project

  Are you sure? yes | no

sebwiers wrote 05/21/2016 at 22:43 point

Eh?  How do you do the logo / submission?  I just went to the rules page and clicked the "start your entry" button.  I'm not sure what "submit the Project to 2016 Hackaday Prize" means.  (Hmm, think I figured it out.)

  Are you sure? yes | no

Alain Mauer wrote 05/22/2016 at 06:56 point

Hi, You found it. Now you are in :)

  Are you sure? yes | no

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