If you like choppers, you may have thought about building your own. Welding a bike frame is not rocket science, but it does require the right tools. You need a gas metal arc welder or a gas tungsten arc welder, which creates a high level of heat and a very tight bond, and the right type of steel. Most motorcycle frames are built with DOM (drawn over mandrel) steel tubing, which is strong enough to hold the heavy engine and take the physical stress of road travel.
Build a frame jig to the size of your frame. Then cut your steel pipes to the proper length for the bottom rails, steering yoke and centre brace. Cut the two rear axle frames to length. Leave an extra 4 to 5 inches on each frame piece in case you make a mistake on the weld; you can cut the weld off and start over, instead of rebuilding the entire frame. When you build the set for the rear axle, make sure you account for the extra length that the axle holders will add. These you will have to buy separately.
Cut the lower rails to the length of your design and lay them out. Attach the metal engine plates to these rails with a strong weld. Be sure you have measured your engine area correctly and allowed room for the seating bolts and other attachments your engine will require.
Weld the yoke section to the lower rails. This yoke comes together at the steering tube with room for the tube to be inset between the two pipes. Weld the two yoke sections together with a brace plate, then fit the steering tube to the yoke and weld this to the frame. Remember, the steering tube walls are usually 1/4-inch thick and much wider than your frame pipes. Your steering rod goes through this piece.
Cut and fit the centre bar to the steering yoke, then measure the distance to your rear bars, where the rear axle will be held. These bars can be in any shape you want, but usually fall from the centre bar in a straight diagonal line. Weld a centre support brace to secure these two pipes together. Always measure twice to be sure the wheel you want to use fits between these bars, then weld a support brace connecting both rear bars to the centre bar. You should also dry-fit your wheel and be sure it fits perfectly.
Cut and set a brace on top of both rear side tubes behind the engine block and weld it secure. Weld your cross braces to the two side tubes below the centre bar near the back of the engine.
Weld your main support behind the engine plates to the lower rails and centre rail. Now your frame is mostly complete. There are still a few more steps to take, like welding on the parts to hold the wheels and testing the overall strength of the frame.
Fit and weld the rear axle assemblies to the bike frame; carefully remeasure and dry-fit the rear wheel again before you complete the weld.
Weld on any extra fittings for the engine block, the seat and any wire harness hooks or bolt seats for the additional parts that you want on the bike, like the blinkers, gas tank and rear shocks. Hopefully, you have these all measured out on your plans.
Fit your axle carriers and gaskets for the steering and forks, just to be sure they fit, and slide your steering pipe into the tube, if you have it already. It should fit cleanly; don't use grease. After everything is done and tested for stress, you are ready to paint your frame with a high-temperature, rustproof primer and apply the final coat.
Find blueprints and follow them very carefully to build your own motorcycle frame successfully. Each type of frame (chopper, sport bike, etc.) has a very different shape and therefore carries the load of the rider, stress of the engine and weight of the parts much differently. For your first frame, try building a simple one that involves less measuring and fewer welds. Build a good jig and set the frame up correctly before each weld, for stronger and better welds. Also, position yourself to perform the most continuous welds possible, to give yourself a better bead.
Never use a motorcycle frame you built without properly testing the welds. You can weld a few sections and perform destructive testing, strength tests or bend tests and several more weld tests so you become familiar with the welding before building your own frame. There are many welding courses at tech colleges that all aspiring motorcycle frame welders should take before attempting this project.