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AfterBurner Chopper - Section 1

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*** Featured on and ***

Take a basic mountain bike and transform it into a 1970s style chopper in a weekend.

This classic 1970's style chop was named and built by Enzo and Nathan (father and son team) from an old mountain bike that was found laying in the mud at the city dump. Instead of the extended front forks, this chopper is roughly styled after the classic Raleigh Chopper bicycle with the straight frame and tiny front wheel. This project requires no extra tubing in the frame, and except for a few small add-ons, only requires cutting and welding two frame tubes.

Any mountain bike with 26 or 24 inch wheels can be used for this project. In addition to replacing the seat and handlebars, you will only need a front wheel of any size, but it should be smaller than the original rear wheel. A 16-inch wheel was chosen for this build.

The donor bicycle (Photo 1) is the typical garden variety steel framed mountain bike. This frame is described as "uni-sex", but looks kind of "girlie" to me. Don't worry about it, dude, when we are done, that won't be the case. The overall condition of this bike is fairly good considering it sat in a mud hole at the local dump, and was probably run over a dozen times by the bulldozer guy. This is a good thing, since we will be using everything but the front wheel, seat and handlebars.
Photo 1 - A typical steel frame mountain bike with 26-inch wheels.


All of the parts are stripped and checked for damage (Photo 2) before the building process. Everything looks good so far, minus a rusty chain which will be used in ways a chain was not intended for later.
Photo 2 - Taking it all apart.


To install a small 16-inch front wheel yet keep the frame looking somewhat like the original, something has to be altered or the pedals will certainly be dragging on the ground. Longer front forks would do the trick, but since that isn't the focus of this project, the only other option is to raise the bottom bracket.

To get the bottom bracket up higher, cut the frame cut in half at the seat tube where both the main tube and down tube were connected (Photo 3). Cut the tubing as close as possible to the seat tube, then grind any remaining metal from the seat tube.

Photo 3 - The frame is cut in half.


Modification of the frame simply involves turning the front half upside down as show in Photo 4. Because the down tube (now the top tube) is shorter than the top tube (now the down tube), this places the head tube farther down, or the bottom bracket higher, depending on how you look at it. Basically, now the pedals have enough ground clearance.

This frame modification also adds more rake because the head tube angle is pushed forward. This will add a nice look to the final design.

Photo 4 - Inversion of the front frame half.


Here is the newly rejoined frame, with the front half turned upside down and re-welded (Photo 5). If you didn't look closely, you may not even notice how different the new frame is compared to the original, although the new head tube position and angle would make an almost impossible bike if the larger front wheel were to be used. Also, the top tube is larger in diameter than the down tube - another thing you normally don't see on "non-freak" bikes.

Yes, the front forks are already painted! Since there were three of us working on this chopper, things progressed at an alarming rate.

Photo 5 - The frame is rejoined, upside down.


Originally, the plan was to weld the screen from an old satellite dish into the frame triangles in order to give the bike a cool "filled in look", but unfortunately, the satellite mesh was aluminum. Welding thin aluminum to mild steel is not something that a simple AC welder can do, although I gave it a shot anyhow - oops. The mesh quickly turned into toxic vapor as the welding arc blew it away.

Enzo, being the creative genius that he is, came up with the idea of welding the original rusty chain into the frame instead, since it would have to be replaced anyhow due to mega-rust (Photo 6). In the end, it looks pretty cool, almost military-like.

Photo 6 - Don't toss away that rusty chain


Finding a decent banana seat in a few hours around here was impossible, so Enzo came back with this humungous seat from what was probably an exercise rowing machine (Photo 7). Although the freaky dude at the pawn shop charged him $20 for the thing the rest of the bike was free. This thing was just plain sick. This seat is at least twice as wide as a regular bicycle seat and three times as long.
Photo 7 - One extremely wide seat.


To accommodate the new mega seat, a pair of chain stays were cut from one of the many bent and twisted frames in my scrap pile (Photo 8). The idea was to weld the stays to the metal bit under the fat seat and then weld the other end to the rear dropouts on the chopper frame. Just about any old tubing would have worked, but these were handy at the time.
Photo 8 - Butchered chain stays from an old frame.


With the new seat support welded to the frame (Photo 9), it would be easy to just bolt the rowing machine seat back onto the bike. Another solution that would work involves cutting the original seat stays from the chopper frame, move them backwards and then weld a small tube from the top of the seat tube back to them. We wanted this chop done that same afternoon, so this method worked just fine.

Because the seat was an after thought from our inability to scrounge a banana seat, the frame was already painted when I did the welding. Oh well, paint is cheap.

Photo 9 - The seat support made from chain stays


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