I rode the bike like this for a few months until towards the end of last year the "Mullet" brake set up (Disc/V Brake Combo) annoyed me and I decided I to do something about putting a disc on the back.
I didnt want to weld a tab on the frame. I have always had this feeling that one day I would like to restore her to a period correct build. (More on that later.) There doesnt seem to be much in the way of adapters available, especially for skinny steel tubed bikes with small drop outs. The only option I had was to make something myself. Good thing I am an engineer, although not in the bike industry.
We have a really nice high tech 3d Scanner at work that is used for metrology. One day during lunch I decided to scan the frame so that I would have a CAD model to work with.
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Next step was to design something simple to make that would bolt on. Its basically a bent plate that is connected to the brake boss and sandwiched between the frame and wheel. Its also bolted to the rack mount to hold it in place when the wheel is removed. The plate is 5mm thick but machined down to 2mm at the point where it is sandwiched. This minited how much flexing of the frame is needed to still get the wheel in and also meant there was still some axle in contact with the drop out.
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I did some FEA analysis to check its strength. To be honest I have no idea what forces go through something like this but made some assumptions and I am still alive.
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And finally, here is the finished product. Its not pretty but it works.
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