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Budget Parts-Bin Bultaco

This is a discussion on Budget Parts-Bin Bultaco within the Project Builds forums, part of the Caferacer.net Forums category; THE SWINGARM IS MOUNTED! Woo-hoo! It only took me eight years, but I have finally a completed swingarm pivot arrangement that addresses all the prior ...

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  1. #391
    Senior Member Tanshanomi's Avatar
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    THE SWINGARM IS MOUNTED!

    Woo-hoo! It only took me eight years, but I have finally a completed swingarm pivot arrangement that addresses all the prior issues with rear suspension and the rear engine mount. It's not even that much of a milestone—really just another punchlist item finished—but I won't lie, it feels really good.

    The rear engine mount crosstubes are just mocked up out of galvanized tube right now; the actual engine mounts will have to wait. Right now I am going to concentrate on getting the rear wheel aligned and mounted in the swingarm. Fortunately, the rear of the swingarm is positioned symmetrically from the centerline of the bike, so that will simplify getting the wheel properly centered.



    Compare to the original sketch I made almost three years ago:









    You might notice that the stock DT175 shock mounts are missing. The stock location would have been too narrow. I removed them and instead I am using the original passenger peg mounting bosses. Moving the shocks forward and down was also attractive in regards to wheel travel, spring rate, and ride height. The cross tubes in the swingarm are thick walled, thoroughly welded on the inside and outside of the swingarm, and frankly seem beefier than the stock shock mounts. The only potential problem I can see would be twisting of the swingarm tubes, since the force is further away from the tube centerline. but it takes a hell of a lot to torsionally twist a round tube. After all, it's not like I'm going to jumping this thing off-road, and it worked okay for Husqvarna in the early '80s:

    Last edited by Tanshanomi; 11-08-2019 at 12:05 PM.

  2. #392
    Senior Member Tanshanomi's Avatar
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    One of the big challenges of this project has been trying to accurately locate of the rear wheel. Fortunately, as far as the frame itself goes, the only truly critical relationships are between the steering headstock and the swingarm pivot: when the steering head is perpendicular to the y axis (that is, centered and vertical), the swingarm must be parallel to the y axis (horizontal and transverse).
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    The rest of the equation–getting the rear wheel centered and in-plane–is up to the swingarm and axle. So, this should be easy, right? Well, determining the effective center line of the bike is harder than it sounds at first blush, when working with chassis components that were never designed to match up to each other. There are so many possible reference points floating in space, inches or feet away from each other. The inherent loose tolerances of high-volume Japanese production and low-volume Spanish production mean that there are very few relationships that can be assumed accurate, even if they are nominally symmetrical or theoretically in alignment. The jig has helped a bunch, but still, fractions of a degree inaccuracy in projecting a datum line will provide wildly varying and useless measurements at the other end, which is exactly what I got a lot of the time. All along the way, trying to fabricate parts to complete an incomplete bike based on these measurements has involved a lot of guesswork and "we'll sort it out later" moments. Well, the time for deferring those determinations finally ran out on me. I finally had to assemble the whole rear of the chassis and try to figure out exactly what I had.



    In order to hedge my bets, I tried to employ as many different but overlapping methodologies as possible. When I installed the swingarm in the frame, I located the swingarm as accurately as I could, so that the opening for the wheel and the rear damper mounts would be reasonably well centered within the frame. I then tried to measure the rear wheel independently of the frame, calculating the offset of the hub (technically the faces of the brake panel and sprocket carrier) from the center of the rim/tire. Based on that, I scrounged up the appropriate spacers and assembled the wheel, swingarm, and frame. Looking at the bike from the rear, my eyeball assessment was that the tire's center groove looked just slightly off center to the left, but with so much of the frame welded by hand back in 1977 Barcelona, it was impossible to know what was just optical illusion. So I got out my long metal rule, my carpenter's square, and a laser level and went to work. After measuring as many different ways as I can, I've concluded with a modicum of confidence that the wheel is off to the left about 3 mm. I'm going to machine 3 mm off the spacer on the right (visible in the photo), and make up a 6 mm longer spacer on the left, since I had added a 3 mm shim washer on that side to get the width I needed. At that point, my chassis geometry will be as good as will ever matter, and I'm going to call that good enough.
    Last edited by Tanshanomi; 11-14-2019 at 01:07 PM.
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  3. #393
    Senior Member TrialsRider's Avatar
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    :I it's a Bultaco!
    take it out and do a few wheelies, if she sets back down nicely you are good to go
    woodsman likes this.

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  5. #394
    Senior Member Tanshanomi's Avatar
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    Wheel and suspension design is complete! Except for that 7-year detour trying to make an ill-conceived monoshock work, I think it's going pretty well.
    Okay, I'm joking – but remember, this whole project has been about gaining skills and experience, not having a motorcycle to ride. And I certainly have gained a bunch of knowledge and fabrication skills since this all started. If you look at the last photo, I took the rather crude collar I'd made long ago for the steering stops and beautified it a bit with the lathe, something I didn't have when I originally created it. I forgot to take a nice close-up of it, but it doesn't look like complete crap now.

    I was actually able to push it down the driveway before work this morning. I sat on it, steered it through a couple of S-turns, and bounced up and down on the suspension. It seems to behave decently enough.





    Last edited by Tanshanomi; 11-21-2019 at 11:09 AM.
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  6. #395
    Senior Member Tanshanomi's Avatar
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  7. #396
    Senior Member grandpaul's Avatar
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    Now THAT is NICE!

    Keep the ball rolling...
    GrandPaul
    Author, \"Old Bikes\"

  8. #397
    Senior Member Tanshanomi's Avatar
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    I'm seat shopping, and trying to figure out which will work with my rear loop. I tried to scale and superimpose my frame tubes into the image as accurately as I could.
    I think the Reed-Titan seat (RTS1, at lower right) will be the most suitable fit and most straightforward installation. It wouldn't have been my first choice, but I think it will suit the bike fine.

    EDIT: Longtime followers will recall that I previously intended to create my own fiberglass seat shell. At this point in the process, however, I want to hit the easy button wherever I can to (as grandpaul put it) "keep the ball rolling." There will be plenty of opportunities to try my hand at fiberglass after this bike is done!




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    Last edited by Tanshanomi; 12-02-2019 at 12:17 PM.
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  9. #398
    Senior Member TrialsRider's Avatar
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    Next Bultaco make it a trials bike lol would be a heck of a lot cheaper and easier.

  10. #399
    Senior Member Tanshanomi's Avatar
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    Quote Originally Posted by TrialsRider View Post
    Next Bultaco make it a trials bike lol would be a heck of a lot cheaper and easier.
    True, but I haven't ridden off-road since my Army days—which was a long, long time ago. That's why my first Bultaco went from this...



    To this...


  11. #400
    Senior Member Tanshanomi's Avatar
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    I had to give up on the plastic fuel tank. I watched every YouTube instructional video, read all the vintage MX forums. I shaved. I sanded. I used heat and the recommended solvents. It was all for naught. The adhesive somehow altered the plastic under it, even well below the surface. While a razor blade would smoothly curl off a layer of plastic on the rest of the tank, the plastic under the logo is puffy and granular. No matter what, this tank will have "Can-Am" visible on the sides as long as it exists. So, I'm back to the old Rex tank, which really isn't in terrible shape, it's just a bit small for the bike.



    Last edited by Tanshanomi; 12-03-2019 at 02:03 AM.

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