They've got a seatpost and seat tube that are thicker and heavier than some scaffold, I expect the head tubes and forks are similarly chunky, the down/chainstay tube is a thick square-section, the handlebars seem to routinely survive crashes, the docking clamp seems pretty substantial, they've got chain and coat guards, a hefty kickstand to hold all that upright, and probably other stuff I've forgotten. It soon adds up.Where does the 51lbs weight of Boris Bikes come from?