How to Make a Chopper Motor Bike
Chopper motor bikes, also known as mini-choppers, are smaller versions of the Harley-Davidson-based choppers. Commercially available, mini-choppers generally fall into two classes, serious and novelty. The serious variety may cost $2,000 to $5,000 (based on the level of sophistication) and have 50cc to 250cc engines with up to 20 horsepower. They are suitable for casual road use, look like the real thing and have the quality and craftsmanship of the real thing. The novelty variety is available in the $350 to $600 range. However, their small size, construction and low power limit them to mostly off-road use by young riders. If you have some stumps to pull, or entirely too much spare time, and want to do 0 to 20 mph in a heartbeat, this is your machine.
Look at real choppers on the road or on the Web. Start to sketch ideas of your own. Pick colors for paint and designs that will offer contrast. Design drawings of the engine mounting and drive train, brakes and front fork/handlebar assembly. (The drawing tool in MS Word can be enormously helpful if you aren’t a draftsman. By “formatting” the dimensions of each piece, you can get a great idea of how it will lay out, both in scale and location. Try it! You can color it to see how it will look). Mark each separate piece in the final drawing for the exact location and direction it must face when the frame is welded together.
Cut steel tubing and other pieces, holding them in a vise and using a portable band saw. File or grind rough edges smooth. Mark pieces with non-erasable marker, according to location code. Plan to draw each as they are cut and shaped. Mark left/right/or upper or lower, or inner or outer. This step is extremely important; as some pieces will look similar later. Marking them now saves hours of wasted time later. Measure and drill as many of the holes you will have to drill in the drill press, before pieces are welded together and you are forced to use a hand drill.
Take the time to jig/clamp/brace the pieces prior to welding. Measure assemblies and use squares liberally. The best way to get a straight bike is to weld a straight bike. Tack pieces securely in place in at least three places first, then go around and finish the entire weld bead. Welding one side entirely first tends to pull the top of the piece towards the weld, and the joint will be crooked. Chip any weld slag if using sticks; use wire wheel in a high speed hand drill to buff weld clean. Inspect each weld carefully; this is where the strength is. Continually check measurements and squareness or straightness as each piece is added. Try to work symmetrically so that frames will self-straighten as welds cool.
Finish frame and test for rigidity. Add drive line components, making sure drive components are lined up using a large steel framing square. Build adjustment points into the drive line system by using long bolts, several washers and nuts, or threaded rods, and turnbuckles. Install brake discs on wheels; make sure wheel bearings are lubricated, and assemble wheels. Add brake and throttle cables. Start bike and test ride. Make mechanical adjustments, such as alignments, chain tension and cable adjustments for brake and throttle.
Disassemble bike, marking pieces and fasteners carefully. Clean and degrease frame, fork and all components that will be painted or otherwise finished. Sand and fill where necessary. This is critical to a professional looking end product. Take time here and pay attention to detail. Degrease and clean all surfaces with a tack cloth. Spray paint bike according to your finishing scheme. Allow to dry completely, then do detail finishing. Reassemble bike carefully. Stainless steel fasteners are well worth the extra cost. Locktite 242 works well to keep bolts tight without lock washers. Take the time to align wheels and drive line carefully. Inspect every joint and every bolt. Test ride again and readjust as needed. Finally, make sure someone special takes a picture of you, the proud owner, sitting on your new mini-chopper.