Monday, 11 November 2019

Julian Farnam's Dirtbag Rat - Yamaha Banshee-Powered Funny Front End Mutant

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350
Image courtesy Alan Lapp

"Hey Jason, I hope all is well. I thought I'd reach out and mention that I've just completed a new project that may (or may not) be of interest..."

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 Girder Fork FFE
Image courtesy Alan Lapp

The photos that followed in Julian's email made me lean back in my chair and giggle with glee. You are goddamned right this is "OF INTEREST". You should know my taste by now Julian, because every time you send me something you nail it.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 Front Wheel
Image courtesy Alan Lapp

If you've followed OddBike for any period of time you should be familiar with Julian Farnam and his designs. He has become a staple subject of the site for years, and for damned good reason: there are few backyard tinkerers as talented or as innovative as Julian is, and every time he puts Tig to metal he concocts something inspiring that speaks to the very heart of OddBike. Or at least something so thoroughly weird and wonderful that it will cleanse your palette of the dull Hondas and derivative Triumphs that populate your feeds for the remainder of the week.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 Rear Wheel
Image courtesy Alan Lapp


Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 Girder Fork FFE
Image courtesy Alan Lapp

Julian's work has run the gamut from OEM quality engineering to Frankenstein-ian monsters, with many permutations in between. This bike, which he has dubbed Dirty Rat, was Julian's entry into the Dirtbag Challenge for 2019.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 Funny Front End
Image courtesy Alan Lapp

The Dirtbag has been a favourite topic of mine and I've previously featured one of Julian's previous builds, as well as one built by his friend, photographer, and co-conspirator Alan Lapp. Traditionally the Dirtbag Challenge had three basic rules: you couldn't spend more than 1000 bucks (including the cost of the "donor" machine), you had 30 days to build it (with the start date kept a secret until the "go" announcement), and you had to complete a 150 mile journey without it blowing up and/or killing you.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350
Image courtesy Julian Farnam

Starting in 2016, Dirtbag rules have been altered slightly. Well, a lot. They doubled down on everything. Now the budget is 2000$ and the build time is 60 days. Damned slacking for veteran builders like Julian, but a way to make things a little more appealing (and little less aneurysm-inducing) to newbies. The caveat is that the shakedown ride is also doubled: now you need to survive more than 300 miles aboard your machination, over the course of three days and two nights, to successfully complete the Challenge.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 Rear End
Image courtesy Julian Farnam

Of course that didn't mean Julian had an easy time building the Rat. He managed to make things difficult for himself by breaking a few of his personal build rules.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350
Image courtesy Julian Farnam

Let's get the basics out of the way, so you can begin to understand the majesty of this monstrosity you currently behold: the engine is a Yamaha Banshee 350 two-stroke ATV engine, mounted in a RZ 350 frame, sprinkled with some Suzuki and Yamaha bits, a few snowmobile parts, finished off with a glorious girder front end and rear swingarm - both built from scratch by Julian.

Julian Farnam A-N-D Yamaha RZ350 Chassis
Image courtesy Julian Farnam

The project began, conceptually, as the desire to build a new chassis for a Yamaha RZ; if you are at all familiar with Julian's history, you'll know he has quite a bit of experience with Yamaha two-stroke platforms, both RZ and RD. Nevermind the half-dozen projects he has on the backburner that most of us have not seen. This concept developed as a response to a series of inquiries Julian has received to develop a modern chassis for the RZ, as well as his own desire to bring the Yamaha two-stroke twin into the 21st century. He has the experience and the know-how to execute the idea; his first big venture in motorcycling was AND, which specialized in building a top-quality chassis for Kawasaki EX500s before that platform got muscled out of middleweight club racing by the advent of the SV650.

Julian Farnam A-N-D Yamaha RZ350 Chassis
Image courtesy Julian Farnam

What ended up on Julian's sketchpad was a steel trellis frame linked to flat swingarm/suspension mounting plates, with a twin-shock girder fork at the front and a TZR-geometry steel-tube swingarm at the back.

Julian Farnam A-N-D Yamaha RZ350 Chassis
Image courtesy Julian Farnam

With the Dirtbag build looming and a desire to prototype some of his ideas, the RZ re-frame project became the basis for the Rat. Unfortunately time constraints would preclude the construction of the one-off frame. Instead an ex-race, de-tabbed RZ frame of dubious collector value that had been gifted to Julian would serve as the basis of the build. It would prove to be the only conservative component of the build.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350
Image courtesy Julian Farnam

To power the machine a Banshee 350 ATV engine of unknown quality was bought off Craigslist for 500$, breaking Julian's golden rule of starting the DBC with a running donor. To those not familiar with the infamous Banshee, Yamaha's batshit loony entry into the sport quad scene, the YFZ350 was more or less a re-tuned RZ350 engine dropped into a 400 pound, 80 mph, six-speed, two-wheel-drive ATV that was sold to unsuspecting victims from 1987-2006 (2008 here in Canada). Anyone who had an adventure with a Banshee had a near-death experience to share, whether they owned one or borrowed one. The most common injuries I was familiar with were the result of burnouts and donuts gone wrong, when the 'Shee caught showboating riders off guard by hooking up violently and tossing them over the highside. Usually while a camera was rolling and the maximum number of witnesses were present.

Julian Farnam Dirtbag Proto Rat
Image courtesy Julian Farnam


While the YFZ packed a mere 30 horsepower on paper, no one left them stock for long and their long production run means there are plenty of go-fast parts at your disposal. To make things more interesting, the Banshee lacks any sort of power valve system; the RZ featured YPVS to make things a bit more tractable, but the YFZ keeps things simple and violent with an unmitigated two-stroke torque spike when it comes on the pipe.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 Bushings
Image courtesy Julian Farnam

Julian's engine wouldn't benefit from too much nuttery given the limited budget and looming deadline, which caught most builders off guard by being announced earlier than expected. The go date was announced in June with the Challenge to be completed on August 25th, rather than the typical Fall -ish dates of previous years. In fact Julian had booked a five day vacation to go adventure riding in Vermont, which ended up cutting into his build time.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 Swingarm
Image courtesy Julian Farnam

It soon became apparent to Julian that he would need to build the bike twice, cobbled together once with parts from the scrap heap to verify the function of the engine and get an inspection done for registration, and a second time to complete the proper Dirtbag build and its accouterments. Hence the importance of starting with a running donor (preferably one with a title).

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 Swingarm
Image courtesy Julian Farnam

Adapting the YFZ engine to the RZ frame proved to be relatively straightforward, with only a few minor hurdles. The YFZ lacks a thermostat and has different coolant line routing to the cylinder head, which necessitated some cleverness at the parts store to find a solution (Gates 19760 coolant hose for anyone who might be trying to shove a Banshee mill into an RZ frame). The YFZ cylinders also lack flanges for pipe mounting so adapters had to be fabricated to install the John Lassack chambers capped with Toomey stingers. The Craigslist engine also lacked carburettors so a used set off eBay got slapped on with just a cursory inspection to make sure jets were installed (don't laugh, it would not be the first time I've heard of a jet rattling around in the bottom of a float bowl).

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350
Image courtesy Julian Farnam

Slapped together with enough parts to (theoretically) pass an inspection, Julian took the proto-Rat to the DMV... and was failed by an overzealous inspector for a spongy rear brake. An afternoon of scrounging better brake bits out of his spares pile fixed that issue and allowed him to proceed with registration, and starting the DBC build proper.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 Girder Fork
Image courtesy Julian Farnam

Inspiration for the Rat's distinctive front suspension came from a concept rendering from Bulgarian shop Galaxy Custom of a FFE BMW R1200, executed in true Dirtbag fashion: with snowmobile parts, a liberal application of Chinese hardware, and lots of fabrication.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 CNC Parts
Image courtesy Julian Farnam

In what is likely a first for a DBC build, Julian called upon some contacts he has made in Chinese manufacturing to CNC mill a box full of aluminum brackets and suspension components. Seven hundred dollars and a week and a half later he had one off front radial caliper brackets, chain adjuster eccentrics, suspension triples, and mounts for the gauge and handlebars.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350
Image courtesy Julian Farnam

When you witness how cheaply and quickly the Chinese can produce appealing results, it is little wonder that some very expensive boutique motorcycle manufacturers secretly use Chinese manufacturing to achieve their goals with bespoke components, despite their claims to the contrary. That tidbit is strictly off the record, of course.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 Laser Cut
Image courtesy Julian Farnam

The legs of the front girders are bent steel tubing with milled pivot bushings, reinforced with the laser cut gussets provided by Nevada shop SendCutSend. Suspension on both ends is via Yamaha snowmobile piggyback shocks: but why snowmobiles? Hot tip for budget builders: late 1990s and early 2000s Yamaha sleds are a cheap source of Ohlin's shocks, which were OEM fitment and are readily available on the used market for around 100 bucks a pop. Initial testing revealed the spring rates to be too stiff for the Rat's geometry and weight, so lighter Hyperco springs were installed on both ends.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 Laser Cut
Image courtesy Julian Farnam

The subframe of the RZ frame was chopped and replaced with a cleaner unit that incorporates twin shock mounts for the new swingarm. This probably marks one of the few cases of someone ditching a rising-rate monoshock for a pair of straight-rate piggybacks, but I can forgive Julian given how utterly sexy his resulting swingarm design is for something he slapped together in his garage over a few days. Plus the old RZ suspension isn't exactly cutting edge, regardless of the number of shocks it employs.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 Funny Front End
Image courtesy Julian Farnam

The more mundane bits came from a variety of sources befitting the junkyard dog modus operandi. Rear wheel is a Suzuki SV650 cast off, the front a cheap eBay score from some year of GSXR 750. Front brakes are Yamaha's corporate Aisin ADVICS radial calipers. The fuel tank is first generation Yamaha R6 item, while the electrical system and wiring is cobbled together from spare RZ parts Julian has accumulated over the years.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 FFE
Image courtesy Julian Farnam

The build was completed just under the wire with almost no time to perform a shakedown run before the Friday, August 23rd beginning of the three-day DBC run. Loaded with camping gear, the gang departed from San Francisco and headed towards Jenner along the Pacific Coast Highway (Highway 1) for an overnight stay at the Willow Creek Environmental Campground.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 Dirtbag Challenge 2019
Image courtesy Julian Farnam

Along the way Julian quickly realized that the Rat's eBay carburettors were set up extraordinarily rich and it was drinking fuel at 15 miles to the gallon, enough to exhaust his supply of two-stroke oil long before the end of the ride. This ain't 1975; you aren't going to find premix oil at every small town mom-and-pop gas station - especially in California.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 Dirtbag Challenge 2019
Image courtesy Julian Farnam

Once settled in at the campground Julian dropped the needles to their lowest position. The following day he averaged a far more reasonable 25 MPG, enough to stretch his oil supply to the finish. A small crisis averted, and one I note for one simple reason: this proved to be the only mechanical hiccup the Rat experienced all weekend. I've had more dramatic shakedowns after an oil change, let alone after cobbling an entire machine together from disparate parts under a strict deadline.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 Dirtbag Challenge 2019
Image courtesy Julian Farnam

The second day saw the group continuing along the PCH to Point Arena, with a stop at Zen House to check out their high-quality European restoration work; an intentional foil to the Dirtbag Challenge's quick and dirty values perhaps? The ride continued East to Eel River for the group's second night of camping.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 Zen House California
Image courtesy Julian Farnam

Julian notes:

"Having been caught off guard with the early build time this year, I rushed through the design of the the suspension and didn't have time for fine tuning of the geometry... Although not perfect, the front and rear behave very well. The front is a bit pro-dive under braking and could use a little more travel, but for a system that was essentially designed in a week, I'm very happy with the results. The rear works very well and has no negative issues. Overall the bike rides very well. It's super fun in the twisties as it is very easy to flick over and holds a line very well and doesn't do anything funny."

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 Dirtbag Challenge 2019
Image courtesy Alan Lapp

The final day, Sunday, saw the group turn south through Clear Lake, Napa Valley, and Vallejo en route to Oakland for the final celebration, where the gang would gather for the signature DBC debauchery of music, food, fun, and lots of noisy burnouts after completing their 370 mile journey. And that is where Julian would receive (another!) "Cleverest" award from his fellow builders.

Julian Farnam Dirtbag Rat Yamaha Banshee RZ350 Dirtbag Challenge 2019
Image courtesy Alan Lapp

"By the way, I should mention that this bike is a kind of milestone as it is 30 years since I built the Yamaha Recon while I was student at ArtCenter Pasadena in 1989. And a HUGE thanks to my wife Laura for putting up with two months of complete chaos!"


Julian Farnam Yamaha Recon 1989 ArtCenter Pasadena California
Image courtesy Julian Farnam

Julian's Dirtbag Rat is just the latest of a long line of innovative and fascinating machines that have come out of the Farnam garage, and is certainly not the last. With the 2019 Dirtbag Challenge in the... uh, bag... Julian is returning to his RZ 350 chassis project, an endeavor that is sure to produce something spectacular if the Rat is the quick-and-dirty junk pile prototype. As always I highly anticipate Mr. Farnam's next project; he never ceases to amaze with his innovation, no matter how humble the project.



Interesting Links
Dirtbag Rat Photo Gallery

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