Old School Chopper Frame Fabrication - Knucklebuster

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While this might seem like a simple operation it pays to stay alert when you’re knocking this little item out in the garage
some late evening. I admit that on more than one occasion while under the influence of certain health beverages I have cut the
fish-mouth in the top of the seat post tube backwards relative to the angle cut made in the base. What’s worse is that I welded
it together and didn’t notice the problem until the next day.
The slope angle on the seat post relative to a vertical line is 17 degrees. The stock angle is just a hair under 16. The extra
degree of slant combined with the 19” length gives the frame room enough to house Evolution motors. In effect the top of the
seat post has been moved .625” towards the rear and the backbone has been raised .625” in relation to a stock Panhead frame.
I think the easiest way to build the seat post is to first make the fish-mouth cut with a hole saw mounted in a tubing notcher or
by using a miter template and die grinder. The angle between the seat post and backbone is 97 degrees. If you’re using a
notcher set the angle to 7 degrees and make the cut.
Figure 7 shows my cheap imported tubing notcher with the seat post set up for the cut.
Do not trust the protractor built into your notcher. Always check the actual angle with a good accurate angle finder. In this
picture as we set the tube up for the cut our little yellow angle finder said we were right on 7 degrees but the factory supplied
dial on the face of the notcher read 5 degrees. Had we turned the tube to the 7-degree mark on the notcher we would have
actually cut a 9-degree angle.