I've seen a real one, and I think it's 1" or larger steel tube.
(If they were copper the scrap dealers would have ensured that every single one would have been traced and melted down by now.)
I'm fairly sure the museum of technology in Milton Keynes (UK) has got an original.
Must have a visit some time.
(Apparently the tyres are an odd size and "difficult"; you have been warned.)
:-)>
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