I’m designing a mixte with dual tubes from the top of the headtube to the dropout connecting to the seattube on its way. Classic mixte you know…
I’m wondering what diameter, thickness and material to use for the tubes.
It’s going to be a super flexy bike so I’m shooting for something similar to a 25.4 0.7-0.5-0.7 butted top tube. Maybe even a bit more flexy 
I’m not a very experienced in CAD and engineering and have no idea how to calculate what I should use factoring in the different points of attatchment and the double tubes compared to a “regular” top tube.
Any experiences or calculations or ideas are welcome! 
/Tore
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Here’s a drawing of a mixte for I had produced in Asia.
The long stays are 12.7 x 1.4mm
The upper stays are 16 x 1.2mm
To keep cost down we specced hi-ten steel in most places except for the down tube.
If you’re using cro-mo I think you can easily get away with 12.7 x .9mm
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This one used Reynolds 1/2” 0.9 tubing, as dictated by the 1960’s French lug set. Rider is 5’0” so the wheels used were 559/26”. Raana's mixte | Flickr. Sorry, too many pictures…
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So cool! Thanks @ElysianBikeCo and @markbeaver ! The drawing and the process photos together is basically a manual for building a mixte!
Do you know anything about the reynolds tubing? 4130 equivalent or?
Also a very beautiful bike! Very inspiring for my process to see 
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This was years ago, I don’t think Reynolds is currently listing this tube. But 4130 steel in 1/2” in 0.035 would be fine.
Mark Beaver
Tamarack Cycles
Hello Tore,
no, Reynolds does not list any suitable tube for such cases.
But have look in the thread Suppliers for straight gauge thin walled tubing in Europe? and find some small diameter tubes. Attention: Aircraftspruce Europe does not display the actual prices for the tubes, they’ll add import taxes.
If not, maybe you might change your design to a single top tube with additional stays? Yes, that’s a different look, I know …
And don’t hesitate to ask Reynolds for infos about their tubes.
Johannes
