Inclusion and exclusion of the coupling male (on bicycle) in and out of the clutch females (the trailer) is often stiff. You have the male hone a few tenths of a millimeter thinner, so it misses slightly. When pressing the lock button for opening the coupling tears you scih regularly the fingernail. It is incomprehensible why so short designed the button and do not make you stand out a few millimeters. Note: We use four bicycles with two trailers with this system.