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Project Bike SOTU: Bultakenstein

“Bultakenstein”, for the uninitiated, is my ongoing attempt to learn all the skills required to build a custom motorcycle by actually doing all the steps myself, from scratch. And I do mean from scratch. When I started, I had a frame, wheels, a swingarm and a fuel tank, all salvage parts not just from different models, but from different manufacturers and even different continents. Since then, nearly all the components I’ve added have been used parts purchased inexpensively off Ebay or Craigslist. I’ve been working on it on and off for almost six years, and while the rate of progress has been somewhere between glacial and tectonic—nay, nearly Wombatic—I manage to do a bit every few months. In the past year, I have made progress in three areas: the front suspension, the rear engine mounts, and the rear frame loop.

Front Forks

A lot of the parts I’ve bought for Bultakenstein where not selected for optimal functionality or compatibility, but simply because I found them on the cheap. The front forks were one example. They were leading-axle forks from a Suzuki GS650GL cruiser. Once I’d gotten the bike up on its wheels, I did some measuring and discovered a disconcertingly small trail measurement, something that could lead to dangerously unstable handling. The stretched-out cruiser forks simply didn’t suit the steeper steering angle of the motocross frame. Using online parts fiches, I found one Suzuki with non-leading-axle forks (for less offset and more trail) that I could positively determine would directly swap onto my bike: a 1982 GS750EZ. It used the same axle and had the same fork diameter as the forks I had, thus would be able to slot into my existing triple clamps. I needed to keep these, since the steering stem had been custom machined to mate to the Bultaco steering head. It took a while to find a good front end at an affordable price, but I did eventually score. Sure enough, once I got them, the replacement forks bolted up perfectly and gave me much more reasonable steering geometry. They also enable the option of running twin discs if I wish. The new forks are equipped with rather gimmicky, ineffective anti-dive modules, but those can be blanked off fairly easily.

Rear Engine Mounts

My earliest mistake was my choice of swingarm, an early Yamaha monoshock unit from a 1976 YZ125C motocrosser. I selected it solely because I happened to have one sitting on a shelf in my garage at the time. It has turned out to be very poorly suited for this project for a number of reasons. Not least among these, it made using the original Bultaco rear motor mount configuration impossible. Getting the engine and swingarm to coexist in harmony has been one of the most challenging and time consuming parts of this project.

After having already done a ton of work to get the swingarm correctly bolted in, I had to figure out how to simultaneously attach the engine to the frame. I hastily cut a set of templates, just to get the engine location correct…

…and set about making some real brackets out of 3/16″ aluminum, using my slow (but cheap!) drill–hacksaw–file method. At about the point this photo was taken, I decided I was unsatisfied with the potential strength of the skinny alloy arms I’d made. I created a better shape and my former False Neutral Podcast co-host Garrett Michael had them professionally laser cut from steel for me.


Two of the rubber engine mounts, sourced from Suzuki triples, are shown in the mock-up photos. I’m simultaneously designing a crossover shift linkage that will adapt a modern left-side shift pedal to work with the right-side-shift Bultaco engine.

Rear Frame Loop

When I last reported on Bultakenstein last year, I had made the bold and potentially disastrous decision to cut off most of the rear frame triangle. I mentioned in that article that I had just machined straight replacements for the original, curved vertical seat tubes. Last fall, I bought a handheld, ratcheting, die-and-roller style tubing bender for 3/4″ tubing off my local Craigslist for $120. It bends .065″ wall ERW tubing easily, with none of the kinking common to cheaper benders.

I was able to make a smooth 180° bend for the rear and two 30° bends for the front.

To put it all together, I headed over to my lathe. The ’70s Spanish frame tubes were thinner-walled than my ERW tubing, so I had to make up coupling lugs. The result is that everything goes together like Tinkertoy pieces, as shown in the lede photo. I decided to have my neighborhood welding shop do the final fabrication. I’m on the schedule for about two weeks from now. In the meantime, I need to cut and fish-mouth the vertical tubes and make up some extra gusset plates.

During my trial fit-up, I took about 20 minutes to quickly mock up a cardboard-and-tape seat cowl, just to visualize how the final result will look. If you blur your eyes and look at it sideways, it kinda looks sorta like it should.

As you can tell, this project won’t be finished and running anytime soon, and perhaps never at all. I continue to pick up random parts I think might be useful down the line when I find them going cheaply on Ebay. I just purchased a sport fairing from a Kawasaki AR80 for $70 shipped. It should arrive this weekend, and it will most likely still be sitting on a shelf, untouched, when PCSOTU 2018 rolls around. But that’s okay, I’m taking a very long view on this and telling myself that if I don’t give up, I’ve won.

  • Alff

    Glad you are making progress and having some fun in the process. Get on it – I want to see this pull up at Fritz’.

  • outback_ute

    Fascinating, arguably much more freedom for this in the bike world than with cars.

  • jeepjeff

    Glad you’re still plugging away at this one. I hope to see it run some day.

  • “…between glacial and tectonic—nay, nearly Wombatic—I manage to do a bit every few months.”

    Speaking as a geologist, when there’s no progress for over a decade it qualifies as Skylinerific.

    https://uploads.disquscdn.com/images/8a8098501a5c65ef31b25cc557be54e2a1cf98e910ebe7e24899e33cdcd368c0.jpg

    • Rover 1

      That must be one of those geologic ages?