A folding e-bike is supposed to make your life smaller and simpler: it tucks into a closet, rides the elevator with you, and slips into a car trunk instead of onto a roof rack. That promise only holds up if you can lift the bike without dreading it. The 40-pound mark has become the practical dividing line for urban riders, and for good reason — it is roughly where a folded bike still feels like luggage instead of a small motorcycle. Below that line, you carry the bike; above it, the bike starts carrying you. Why the 40-Pound Threshold Changes How You Actually Use the Bike The number on the…