Material: The bike trailer hitch coupler attachment made of steel with black enamel coating, made from heavy duty iron material, strong and durable, corrosion resistant, hard to damage. Size: The couplers are embedded type, please be sure of the size and type before you buy. The embedded part attached: shaft diameter is 0.58 in, hole diameter is 0.27 in, length is 0.76 in. The connect the bike part: fixing hole diameter is 0.4 in, diameter of screw nut fixed in is 0.87 in. Each bike trailer coupler has equipped with a connect pin, the pins can hardly get opened, also once you install them up, they will not easily fall off, can ensure your babies safety. Package comes with 2 packs bike trailer coupler sets, with this extra trailer coupler attachment, both mom's and dad's bikes can be connected to the kids' trailers. A bonus, when your kids is tired of riding, you can tie him next to your bike. Package Included:2pcs*Bike Trailer Coupler