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Old Streetfighter Motorcycle Forum Post 10-13-2017, 02:42 PM   #2

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Default Re: October FOTM Entry Thread

the SECApocalypse

6 year long build started and finished here on Customfighters (bike was bought from another member). Started as a simple tank swap / repair, but pretty quickly progressed to a rear suspension mod and an experimental front end. The front end was inspired by The Britten V1000's use of Norman Hossack's design, which can also be seen on modern BMW K series bikes as a "duolever" system. It uses ball joints and a steering linkage to create a true 4 link suspension on the front end of a motorcycle, allowing the same anti-dive behavior as a girder fork. Unlike a girder fork, because the steering is on the ends of the links, there is little or no change in steering geometry (especially trail) during wheel travel.
The version I built has adjustable arm length and can be shimmed under the lower ball joint, allowing front end geometry and anti-dive to be fine tuned.

The bike now has very different weight distribution from stock (60-40 front-rear vs 40-60) and very different geometry (~20 degrees rake with slightly shorter wheel base and trail). Combined with a strong anti-dive and no tendency to create binding when braking due to slider flex, the hope is for a bike that handles nicely when running the front brake over bumpy surfaces. So far, this seems to be the case.

It still weighs 500 lbs, so it rides more or less like a rather heavy, stiffly sprung sport bike; very happy to be leaned over, with a lot of feedback, but taking some muscle to do anything quickly. There's no tendency to speed wobble, and you can as easily ride one handed or even no handed as on any other bike. I put about 300 miles on it in the week I had it running and it was a lot of fun to ride. It's currently waiting for an engine swap because I blew an oil line on the way to Ratfighter's house and seized the engine.

Voting picture - as first made ridable

4 other pictures -

In Ratfighter's garage after the oil line fail was fixed and tires were swapped (but then found engine dead)

At a show held the weekend I got it back from being dyno tuned (because carbs needed rebuild, no help for power but it ran NICE) - got the trophy for "Ugliest Bike".

Front View

Pic tasken during assembly to show front suspension elements

Fork bridge with lower ball joint.... or a big pair of knucks?

Build thread -

Frame - 1981 XJ750rh "SECA"
Engine (to replace currently seized engine) - XJ750rj "Maxim"
Title / registration - 1982 XJ750rh "SECA"

Modifications (in chronological order, done by self over past 6 years):

- Virago 750 gas tank w/ right side petcock mount welded over, full size battery mount under tank just in front of seat

- GSX 600 saddle (remove via screws, also holds down tank)

- Rear fender cut from original (rusted out) gas tank. Fender inside coated with 3M spraycan firewall sound deadening coat. Tank and fender scuffed, primed with Rustoleum red-brown, painted with black semi-gloss, top coat of black chalk board paint, then abuse like hell during frame modification / engine install / removal, etc.

- Brake / tail light is upside down GSX 600 light, plate mount surround is also brake / tail

- Rear monoshock conversion w/ BMW RC1000 shock, 3.5" travel, load tested @2000 lbs vertical axle force. Stock suspension mounts retained and usable, hidden by fender. Ride height 1" higher than stock. Shaft Drive & drum brake means there are no bearings on the right side of the wheel, so running shock on left arm only has no downside, may actually reduce axle flex.

- Hossack derived front steering & suspension, hardware self designed, software aided geometry, 90 degrees lock-to-lock, load tested @2000 lbs vertical axle force, 1500 lbs horizontal axle force. 3.5" travel, ride height 1" higher than stock, adjustable for ride height, rake, trail, and wheelbase.

- Secured to 3/4" 6061 aluminum plates spaced 8.0" appart via 7.5 deg tapered holes. Plates are secured to frame via 3/16" steel mounting rails using 4 x 1/2" bolts on each side. Mounting rails coped & welded to frame using wood jig to center and align relative to frame mid-plane. Stock fork can no longer be mounted (mounting rails block lower triple clamp).

- Fork: girder design with 6160 aluminum crown and bridge, 7/16" grade 9 bolts, 1x11g 1018 steel round tube legs and misc welded & machined bolt points (axle, brakes), 3/16" steel at crown clamp plates. Tubes bent on cheap pipe bender, welded on wooden jig.

- Upper arm: rear pivots tapered pins through rod ends, high miss-align rod end as ball joint threads into fork crown for trail adjustment without changing rake, arm length micro-adjustable for rake, 7075 aluminum machined girder bridge

- Lower arm: rear pivots spherical bearings mounted in 6061 machined aluminum bearing pockets on round bar with tapered ends, MOOG ball joint front for Hyundai Sonata mounted in machined 7075 receiver, main body 2"x4"x11g machine steel shaped to allow fork clearance when turning bars, bolt holes at 3/4" intervals allowing length adjustment for larger variations in rake / wheelbase by moving bearing pockets.

- Stock head tube is intact and has reducing cups; bicycle head-set and steering tube is mounted. Steering is via 2x drag links with high-misalign rod ends and rocker link to compensate for distance change. Bump steer is minimal and connection is very solid (no slop and slight flex, similar or less vs rubber mounted bars).

- Front shock & link from 2000 era GSXR 1000 with custom length adjustable dog bones. Lower shock bolts to modified lower frame cross and plate secured with engine mount bolts, upper shock mount is tube clamp bolted to tube welded into frame between front end mounting rails.

- Entire stock frame / welded modifications & swing arm re-painted with Rustoleum red-brown primer. All front end metal components covered with multiple coats of boiled linseed oil to slow corrosion but allow limited rust "patina".

- MAC 4-1 headers w/ basalt pipe wrap to keep heat away from front shock that is mounted right between headers (and from oil cooler lines). Muffler hanger made from U bolt, 12" nail, threaded rod, and scrap aluminum.

- BMX stem (with machined aluminum adapter plate for steering link) and handlebar, handlebar has welded extensions for "clubman" style ergonomics.

- Front "blue dot" brake calipers from Yamaha R6 using stock SECA rotors, brake lever, master cylinder, splitter, and hoses. Master cylinder is cable actuated and mounts on fork, potentially allowing hard lines from MC to calipers. Brake cable is Honda clutch cable; stock cable was to short. Brakes are strong and well modulated, but require strong hand to get full effort.

- Custom fiberglass airbox with aluminum plate for carb boots (frame modification means original will not fit) with 4" diameter black PVC pipe for air filter mount for washable automotive "cold air intake" style filter. Replicates volume of original, and stock "snorkle tube" mounts on plate that caps the air filter mount, under the air filter.

- Dominator headlights and small cheap scooter speedo & tach. Speedo indicates double actual speed (designed to indicate KPH) and goes to 160 (80mph actual). Both are mounted to cut & folded aluminum plate that serves as light bracket / gauge cluster and bolts to BMX cross bar with U bolts.

- Vintage aftermarket oil cooler on left side, with cheap rubber oil cooler lines that pop and ruin engine when exposed to header heat.

- Battery relocation uses new 8g 24" wires for hot (from pos to cut-out to starter solenoing / fuse block) and ground (from neg to engine / starter case bolt), 2 wires in parallel for each to handle max 80 amps.

- Wiring harness made by striping old harness. Original gauges / idiot lights / safety relays all removed and bypassed by forcing together mis-matched connectors, smooshing them with pliers, and wrapping with tape. Harness secured to frame with zip ties and slack bundled under wrapping of inner tubes. New fuse cluster is screwed to bottom of seat, regulator and ICU are zip tied to frame behind seat. No electronics box. All circuits are properly fused (starter is not fused but see below re cut off), bar switches all work per original purposes (but there is no turn signal wiring).

- "Keyed" battery cut off switch in front of seat; when battery is live, engine, horn, and starter are live, when switch "key" is removed, batter is entirely disconnected. Ignition key switches on headlight, tail / brake light, gauge lights. Done for simplicity when hacking old wiring, but also safety when working on system and eliminates any chance of battery drain / keep load off battery when starting. Happens to also allow running bike entirely dark...

- 16 watt LED floodlight on left side below oil cooler, wired to bar "aux light" switch for stock fog lamp. Ammo can on opposite side for tool / glove / misc storage. Cargo rack on rear fender.

If I didn't mention something, assume its stock for a 1981 or 82 SECA. The entire drive train and engine is stock; being a shaft drive, that's hard to change. In fact, I blew the engine on my first long trip, and am buying a stock replacement (from a Maxim, but its the exact same engine & drive).

EDIT - wow goddamn what a text dump. Let this serve as an example of what "to much to list" means when it comes to mods.
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