Stainless steel hitch cup: Made of strong stainless steel, well polished surface. Strong Ball Head: Wear resistant flexible plastic ball head, Including variable diameter sleeve. Easy to install: Remove your stocked axle, and put this hitch up on and re-install it tight. Thru axle bike compatible: The axle hole on the hitch is 12.1mm, a 12mm thru axle can go through it, but we suggest you use a thru axle hitch adapter to prevent scratching the bike painting. Flex ball head conector replacement part for thule chariot bike trailers, we suggest you replace the ball head part if the plastic part getting hard obviously. With this ball head kit, you can covert your instep/schwinn/burley trailers to fit with your thule cup mount system. Ball-Cup hitch system compatible with thule burley schwinn and instep bike trailers. Allow Lay dowm flat, 360 degree rotation.