Project Car SOTU 2016: Bultakenstein

There’s a point in nearly every project where moving forward means holding your breath and jumping in over your head. That could mean expanding the scope of a project to require greater expenditures of time and money than you expected to devote to it, and perhaps well beyond what you can comfortably afford. Or, it might mean taking on a fabrication task that is way beyond your level of skill, at the risk of ruining the whole thing. Or perhaps both. And at that decision point, you have to ether fold up, walk away, and write off your losses, or double-down and say I’m all in. And if you choose the later, what happens next is going to be pretty scary.

It is hard to believe that I started building Bultakenstein five years ago this fall. It’s harder still to believe that it has been almost two years since I finished the rear suspension and got it up on its wheels. After a flurry of activity once I’d gotten my lathe, the next logical step was frame modifications. And that’s when things largely ground to a halt.
For those who’ve just tuned in, the way the Yamaha swingarm mated to the frame had left me unable to use the stock rear engine mounting bracket. The alternative was to fabricate and weld in two frame crossbars to accept rear engine plates. I dragged my heels for three reasons: 1) moving forward on frame modifications would require complete disassembly, which I knew would feel like huge step backward. Being able to go downstairs and look at a rolling chassis felt as though I had something to show for my efforts, not just a collection of parts. 2) I am a neophyte when it comes to welding. It takes advanced skills I don’t have and is really important to get right. And finally, 3) If I screwed up the frame beyond repair, I really would be left with a worthless pile of parts. So I dabbled around with other components or spent my time working on the Honda.
There was one other issue with the frame. It’s from a 1977 Bultaco Pursang motocrosser. It was built right around the time  motocross bikes were going to long-travel suspension. In order to increase the rear shock length and still keep the seat height reasonable, Bultaco modified the existing frame design by bending the seat stays forward and arching the tubes up higher than the front half of the seating area. The whole arrangement made fitting anything other than the stock seat rather wonky, but one of my parameters at the inception of this project was we’re not cutting the seat tubes. For some reason, that’s the point where every amateur bike build goes to pot.
My plan all along was to lay up a fiberglass seat pan and tail section to fit the contours of my frame. I wanted something compact, with a bit of a butt-stop, but I didn’t want one of those thinly padded racing seats. I want the bike to look like a ’70s factory street bike, and I want it to be big and comfy enough to actually ride.
I devised about a dozen ways to work around the intrusive frame tubes; each one was deficient in some way or another. Since I still didn’t know exactly how the seat was going to work, that whole end of the bike was sort of vague in my mind’s eye. I cut up the Yamaha Seca 400 tail section I had purchased, trying to integrate it into my mock-up, but because the width of the frame tubes didn’t match, there wasn’t much of it that I could actually use. I then bought the tail from a first-gen Honda CBR900RR off Ebay, but when I actually got the physical pieces they were ridiculously oversized in comparison to the compact Bultaco frame. This was getting me nowhere.
One day I looked at a Honda Express II and realized that the seat was exactly what I was looking for. It was the right size and shape, well padded, plentiful enough that I can get a replacement if this one starts looking shabby. Suddenly, the idea of starting with a production seat and fabricating a tail to accommodate that made a lot of sense. I went out on Ebay and quickly found a slightly different Honda NU50 Urban Express that had some cool detailing in the vinyl cover, to boot.
Bingo! It was the perfect dimensions, exactly what I was wanting. Unfortunately, this did nothing to resolve the problem of my snaky, bumpy frame. I’d have to mount the seat much higher than I wanted or cut into the plastic seat pan. In addition, the kinked tubes and big gussets the shocks originally bolted to were now vestigial, and due to the chain run on the left-hand side, asymmetrical as well. The whole arrangement was going to look half-assed.
I played around in Photoshop, which confirmed that the only acceptable solution would be cutting the frame after all. So, as you can see in the lede photo, I screwed up my courage and reached for my reciprocating saw and pneumatic cut-off tool one morning about two months ago. Whack. Whack. Then, looking down at my amputated frame, I couldn’t help but hear OMD lyrics playing in my head: Oh my God, what have I done this time? Oh my God, what have I done this time…
Fortunately, having access to your own metal lathe is about the coolest thing in the universe, and magically makes everything alright again. The Bultaco frame tubes are metric: 20 mm OD x 1.5 mm wall thickness. That works out to .787″ dia. x 0.055″. I did a bunch of measuring, calculating and sketching, then drove downtown to Metal By The Foot for some 3/4″ x 0.120″ wall DOM tubing. It took me three test pieces to get the exact diameter that provides a snug, slop-free slip fit inside the frame tubes, but I was then able to make up two tube extensions that fit perfectly.
I now need to drill the frame tubes in a number of spots and rosette weld these with a slight gap at the joint. Then, they can be welded around the circumference of the joint and ground down. The OD of the new pieces is just slightly less than the frame, so the joint can’t be made invisible, but that’s a rather minor aesthetic concession.
There’s a lot more to do to finish the rear of the frame, but I was very happy with the result when I was able to put the seat in its proper place for the first time.
And, flush with good feelings from the progress I’d made, I’ve embraced a bit more scope creep. I’ve decided to integrate these rubber engine mounts from a Suzuki triple into my latest sketch for rear engine mounts.

“Do a podcast!” they said. “It’ll be fun!” they said…

Things will probably continue to progress much more slowly than I’d like. Just as I finished up a huge project that had been weighing me down at work, I got this crazy notion to get together with two other bike guys once a week and record our conversation. And then edit the audio. And then try to find photos of all the bikes we discuss. And then upload it to the Internet. Just like a garage project, the False Neutral podcast has required a whole lot more time than I expected. Also, my 88-year-old father turned his fifth novel over to his editor/proofreader—me. Unfortunately, when it comes to my free time, those two endeavors take precedence over project fabrication. But, I am happy to report that my resolve is strong, progress has been made, and the State of the Bike Projects is good.

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  1. Alff Avatar

    You’re not particularly short. How well does this frame fit you?

    1. Tanshanomi Avatar

      It will be compact, but not cramped. As if I’m ever going to actually be on it.

  2. jeepjeff Avatar

    Double down when fabricating/making/etc has produced pretty much all of my best life decisions. My tendency to almost always step over the point of no return on a project has gotten me my most important professional skills and a shocking percentage of my fondest memories. Also, a couple custom guitars, a working, reliable Jeep, a lot of confidence at work, and so on. I’m hoping to soon add “awesome custom motorcycle” to that list too. (SOON. I’m so close to being ready to dock the tail of the FZ-07 and build a custom tank for it.)
    So. Hallelujah and pass the sawzall. Good job on those seat posts.

    1. anonymic Avatar

      What can be welded can be ground off and tried again.

  3. theskitter Avatar

    Been trying for three days to think of something to add.
    This is awesome.
    Don’t lift.

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