I'm wondering if there's a reason this hasn't been tried. Probably the low quality of the donor bikes. But I wonder if this is worth discussing.
Looking at some cheap stretched cruisers with forward pedals, long chainstays, long top tubes, and long swept back bars, it occurred to me that with more standard bars and a forward offset seatpost, you'd have the beginning of a cargo frame.
Something like this:
Micargi Bicycles - Beach Cruisers, City Bikes, Mountain Bikes
I suspect that one couldn't go all the way to a standard seat angle, so I guess that would be less than ideal. But for just a neighborhood hauler, would it be out of the question?
Mtbr's 2016 Winter Biking GearReviews and Roundups
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