Two-Wheel Tuesday: Adaptation

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Every one of us has dreamed up a cool swap at least once or twice or fifty times. Engine swap, suspension swap, perhaps just swapping some car’s cool bucket seats into yours: customizing is swapping. After all, unless you’re  a bona fide OEM, you can’t make a vehicle from molten metal and steel sheet. Even if you’re casting your own crankcases, you’re still utilizing production parts to some extent. So we select the good stuff we want from different models to create that recombinant mutant we call a hot rod: Mustang II suspension under a ’32 highboy, or a SHO motor in an Austin-Healey.

Sometimes, all the parts simply bolt up, thanks to corporate parts-raiding, industry standardization, or just serendipitous fortune. But the vast majority of the time, things from different vehicles don’t work together so perfectly. Bolt centers are spaced on differently. Shaft diameters are different. Gears don’t line up. Things don’t fit.

The solution might be to have one of the parts massaged to fit the other: a shaft turned, a flywheel re-drilled. But quite often, you need a go-between: a single, newly fabricated part that can speak the native languages of two parts that can’t interface directly. You need an adapter.

On my never-ending ongoing Bultakenstein project, there were a few hiccups mating the Suzuki GS650 front end to the Bultaco frame. But figuring out how to overcome those obstacles is the fun part. Or at least the creative and rewarding part.

Adapter plates are quite common in the world of performance cars, and motorcycle chassis components are even more adaptable from one machine to another. There are a still a number of variables to consider, but they’re rarely as numerous as, say, putting a Celica GT-4 running gear in a Austin Mini. In the case of a front-end swap, the whole front wheel and suspension pivot around the steering axis, therefore the only critical dimensions are the two steering head bearing sizes and the distance between them. The Suzuki and the Bultaco shared the same upper steering bearing ID, so half the battle was done.

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The Bul’s lower bearing, however, was smaller and the steering head tube was shorter than the Suzuki’s, putting the two bearings closer together. The solution was to have the steering stem machined down and a spacer ring (an adapter!) made.

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And we now have a front end ready to be married to my frame. A piece of proverbial cake. But we’re not quite done yet.

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OH NOES! The steering stops don’t line up. The Bul’s lower triple clamp had stops that contacted that tab you see sticking out of the front of the steering tube. The Suzuki’s has those two cast-in posts on either side that were designed to hit limiting stops on the frame behind the steering tube. And even if they did work the same way, the plane of the triple clamp is well below the bottom of the steering tube. I thought through all the options that would solve the functional problem, and then condensed the list down to those I could do with the money, tools and skills I had available. What this called for was a special adapter!

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A trip across town to Metal By The Foot provided me with a chunk of round aluminum bar sliced to an appropriate thickness. I sketched out the shape I needed on my Mac, printed it off at 100% scale, and glued it to one side of my new Metal McMuffin. Careful application of a hole saw, a hack saw, and a couple of hand files resulted in a part that fit snugly over my bearing spacer and engaged the faces of both of the Suzuki steering stops.

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A couple of holes drilled and tapped for 6 mm socket-head screws, plus some unused battery bolt spacers from my “odd hardware” tin, and—ta da!—functional steering stops (as shown in the lede photo). I like this solution a lot: Stress is transmitted to the triple clamp and the frame at the points each was originally designed to receive it. I now have a filler to block road debris from the hitherto unshielded lower bearing. The cash outlay was negligible.

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As a bonus, I have the option of easily tapping different holes to fine-tune the amount of steering lock available once the project is on the road.

(Hey, you in the back—stop snickering!)

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