The backbone assembly from Part 2 represented the major part of the frame build so far. The next critical joint is the down legs to the steering neck. After the steering neck height is determined the top of the down tubes are rough trimmed to length. By this point, you need a frame jig or at least a platform where the bottom rails can be spaced and anchored. The down tubes need to have a bit of the center section removed so they move inwards towards the centerline of the frame. In this case, ½” was removed. This was done by plumbing up one down tube, measuring in ½” at the top and using a square with a built in level to draw a vertical line. The same process is done for the opposite side. Drawing a flat file between the rough-cut tubes can produce a nice tight fit. The cope was rough-cut with a die grinder and carbide double cut bur. As a few trial fits are done with the neck, the cope is further refined until all gaps are eliminated. Note how the tube ends are squared off and not sharp knife-edges. The square edge along with a bit of a chamfer allows for a full thickness weld between the tube and steering neck.
The picture to the right shows how the down tube to neck joint looks. A little more chamfer needs to be added for weld penetration and the dark coat from the tubing will be sanded off to keep the weld clean. Although it is possible to build a frame without a jig, it makes fabrication of a frame much easier. Here you can see one of the neck centering cones holding the neck in place. If the centering cone rod is replaced with a shorter rod that has been center drilled on one end, a 2” hole saw can be used to cut the down tube cope for the neck.
Here is the connection at the bottom rail to the bottom rear wishbone. This is going to be a spot requiring an exceptional weld. Adding a gusset for reinforcement is also a good idea. On a typical frame build, the tube that acts as a crossbar just forward of the rear fender under the seat tends to pull the axle plates together making it impossible to get the wheel and spacers installed until the width is corrected. This weld will have an opposite effect and tend to widen the axle plates. The jig or a threaded rod keeps the dimension while this weld is done, reducing the problem. Short welds can help reduce shrinkage on various parts of the frame.
Here are the rest of the parts that need to go into the frame for a HD type engine. (Left to right) Steering neck, front engine mount with quarter round shaped gussets and bosses for forward controls, a rear engine mount, front transmission mount, seat post crossbar, 2 pieces to fabricate a rear transmission mount and the axle plates. Fitting an tacking in these parts will take about as long as setting up the basic frame. Cutting these parts out also takes considerable time since they are cut from ¼” to ½” thick plate depending on the part.