These trikes include a cargo area consisting of a steel tube carrier, an open or enclosed box, a flat platform, or a large, heavy-duty wire basket. These are usually mounted over one or both wheels, low behind the front wheel, or between parallel wheels at either the front or rear of the vehicle, to keep the center of gravity low. The frame and drivetrain must be constructed to handle loads several times that of an ordinary bicycle; as such, extra low gears may be added.