I left off last post with the old down tube completely removed from the frame and reinstalled into the Anvil frame fixture. The old down tube was 1.75″ in diameter, and the new one will be 1.375″. Not only is the new tube much lighter, but the coupler for the new sized tube is substantially lighter too. The first step in S&S coupler installation is to accurately mark the butting of the tube. For those reading this that are not familiar with tube butting; essentially the wall thickness of the tube is thicker at the ends and thinner in the middle. The outside diamter of the tubes is the same (save for some seat tubes), but the inner diamter changes. Each tube has a specific length of the thick section (called the butt) and a specific length for the transition to the thinner/middle section of the tube. Tubes have butting to add strength to the joints and remove material in the middle of the tube to reduce weight. You can see from the picture below that the butting length (indicated by the left most line drawn) is not extremely long; and on the particular tube shown the transition (the area/length between the two lines) is the same length as the butting. Also on this tube, one of the butts is longer than the other. For this frame we will want to absolutely use the longer butting length on the end where the coupler will go; near the bottom bracket.
The first cut of the new tube is done without the S&S coupler installed. It is done at 90 degrees and uses a 1.5″ hole saw to cut the miter for the outside of the bottom bracket.
The first cut is made so as to remove as little as possible while still maintaining a fully engaged cut. You can see from the picture below just how little is removed by the crescent shaped piece dangling.
The next cut is made at the seat tube / down tube interior angle. On this partricular frame the angle is 58.4 degrees, so I set the rotary table (that the tube clamping system is bolted to) at the correct angle. Most lugs are cast at 60 degrees for this angle, so if you were watching a lugged frame builder do this an intermediate lug cold setting picture would be inserted. With TIG welded construction I don’t really care what this angle is, I just need to know what it is to see the cut angle.
A piece of steel turned to 1.5″ is used to make sure the down tube is in the correct phase for the next cut. This cut is done with a hole saw of the seat tube diameter. On this frame the seat tube is 1.25″, so I make a cut in the down tube at that size. I have developed a good eye for knowing where to make this cut so very little file work is needed after, but it does take practice. It is better to err on the side of caution and make the cut a bit too shallow, it easy to file to the right size. Making the cut too deep will create a nightmare of a weld bead to run later.
You can see here the hole saw making it though the cut with very little burr and via a very clean cut. Take your time making this cut and use as new a hole saw as possible. It is very easy to catch/tear your tube making this cut.
The sharp acute points of the down tube miters need to be rounded to flow over the seat tube / bottom bracket weld. The picture below shows the down tube fully mitered at the bottom bracket end. Notice the amount of butting left. We need all the real estate we can here, so do everything you can to cut as little as possible.
The next step will be to install the S&S coupler. The down tube / bottom bracket miters are done first for a couple reasons; which I will highlight in the next post.