How To: rear frame fabrication
The Veloce project had some damage to the rear frame tubes, just ahead of the rear axle. The rear stand pivots on the dropouts (the lugs that hold the axle) and the stand has two legs which pivot up to rest on the bottom side of the chainstay tubes. Starting the bike is done by pedaling while up on the rear stand, so the chainstay tubes see a bit of stress, and mine were crushed by the strain at some point in the last 99 years.
While it would have been very simple to fill the dents with bronze and then paint over the repair, I wanted something stronger. Knowing that my kids will eventually learn to ride on this bike leads me to make it as strong and safe as possible. And the repair is fairly straightforward: remove the tubes, make new ones, braze them in. Except the tubes are not round, or square. They are D shaped in their cross section. These tubes are not easy to buy, but there are a few companies in the world that I found that make such tubes. And they were glad to quote the manufacture of tube to the correct dimensions, if I wanted to buy the minimum order of 2 tons of tubing…
So, to make some D shaped tubing: First step is to measure everything before cutting out the old tubes. I traced the frame onto paper and kept it as a reference, marking the paper with the measurements from the steel parts.
Then cut open the tube to measure the diameter and flat dimension of the D and the wall thickness too:
Buy some round tubes, and cut them with a slitting saw on the mill. Some other guys have done this also, and they cut the tube with a cutoff wheel and a grinder. That makes it tougher to get a good joint for welding, but it can work.
Buy some flat bar to weld onto the round. I used flat bar that was 2x thicker than the original D, as it made it easier to bend the tubing to shape after welding. Thank you to John Quirke in Ireland for this tip and the general fabrication ideas!
Bryan down at Advanced Welding TIG’ed these for me after I did a lousy job on the sample pieces with my MIG.
Here was when things went bad. I heated the tubing to almost red hot, and dropped it in my tubing bender and promptly crushed the tubing. Luckily I had made up several feet extra of the D tubing, so I could experiment a bit.
Eventually I found that I could fill the inside of the tubing with a solid steel bar of D cross section. They are available at steel yards and used for fabricating handrails on modern staircases. With just a touch of grinding it dropped right inside the D. I made it long enough to support the portion that was being bent, plus an inch or two on either side. Now those portions are essentially solid bar stock and are very strong. I did the ends too, the area that the rear stand support legs will hit against, so this exercise won’t have to be repeated sometime in the future.
Then the bending went fine. Fixture it all up and measure a few times to make sure that the dummy axle is square to the rest of the frame, and then brazing it all back into one big piece:
It looks easy, and it almost was. Not shown were the hours and hours of test cuts, test bends, grinding, welding, machining, making fixtures and the other fiddly little things are needed to make new parts fit old bikes.
Here is a whole different approach for making D tubing using a press.
More How To articles can be found at this link.