The following pics are of a bicycle crank arm, manufactured in reaonably small quantities around 20 years ago. What makes them different is their shape.
Made of Chromoly steel, hollow with no visible seams. hexaganol cross section, tapering from one end to the other in 2 dimensions (one is obvious, the other is widthwise end to end - wide end at crank spindle, narrow at pedal)
- 2 Halves Stamped from sheet steel, welded together and then finished.
- A Custom tube - drawn and tapered to exact requirements, with ends formed together in a tool, brazed and finished.
The trouble with 1 is that I can's see any evidence of the weld - the 'seam' between the two halves is absolutely 100% - straight and sharp. The rest of the visible welding though is not so good, so I am not convinced by this.
No. 2 seems possible, particularly given that the ends (where the 'seam' betweenm the two halves are) looks definitely 'manually' finished (not absolutely straight and true). But, I can't see how tubing can be shaped hexagonally and then tapered in this way.
Any light anyone could shed would be very welcome!