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Without being a pain (commence being a pain) it's for the seat manufacturer to say not us. Having said that, with the very same seat tube, and much sillier rear end I have ran one with a ~15kg child, as well as trailer for the whole of coast to coast and it didn't catch on fire. I would suggest having a nicde long post and doing exactly as the manufacturer of the seat tells you to, as we haven't specifically designed them with this in mind.
would the seat tube hold a cantilever baby seat ?
or best to rack it ?