1/2mm wall tube on its own seems excessively thin, but yes a 28.6 OD seat tube, to fit a 27.2mm seat post in ,

[math] would have an 0.7mm tube wall..

my BiFri uses a 1.5mm wall (32-29) seat mast.. 12" up 305mm, from its lower bracing portion..

going oversize OD on the seat tube? it gains in stiffness, through increased diameter ..

you could sleeve inside the steel tube with carbon , epoxy wet wrap cloth , inflate a balloon. to expand inside it, let it cure..

or buy composite tube and slip it in..






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