Material: The bicycle trailer hitch attachment is made of steel with a black enamel coating, made of robust iron, strong and durable, corrosion-resistant and not easily damaged. Size: The couplings are of an integrated type. The attached integrated part: axle diameter is 0.58 \", hole diameter is 0.27 \", length is 0.76 \". The connection of the bicycle part: the diameter of the mounting hole is 0.4 inches, the diameter of the fixed screw nut is 0.87 inches. Each bicycle trailer hitch is equipped with a connecting pin. The pins are difficult to open, and once installed, they will not fall off easily, which can guarantee the safety of your children. The set includes 2 bicycle trailer hitch sets. With this extra trailer hitch accessory, mom's and dad's bikes can be connected to the children's trailers. As a bonus, you can attach it next to your bike when your children get tired of cycling. {xjxj}