This unit is the basis for all of the attachments for the Sport-Link. You will first need to purchase and install the Sport-Link and then purchase this unit along with your choice of attachment (such as the bike rack attachment, one of the several Tow-Link attachments, the Bike Rack attachment or the Spare-Link attachment). This modular approach to the Sport-Link and its attachments, is designed to give you flexibilty in how you approach your set up. It will also probably save you some money along the way. The basic idea is that you can buy the Sport-Link Attachment Base once and bolt on any of the above accessories to that single base. You can of course buy a new base for each if you do not want to have to unbolt one to bolt on another. This bolting is done with (2) half inch bolts.