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I was thinking of mielec, there is a lot about them here -> https://www.lfgss.com/conversations/217358/
-> http://www.bikemielec.com/en/
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Svend, you seem to have a lot of patience and engineering skills.
May I suggest you get a steel frame of proper quality tube. Maybe a track frame with a cracked headtube or and older touring bike with horisontal dropouts.
Then you replace the headtube (with a cheap Ceeway Cromoly one) using the string method (Patent Pending).
You can buy a disc tab for 10gbp on ebay and build a cheap fixture yourself.
Then all you need is to find a person with tig welding skills and half an hour to put it together..
Slightly unrelated, but any recommendations for cheap frame builders?