Many bikes just don't fit on the mast-style, hitch bike racks because of the spaces created by today's bike frame geometry.  Frame adapters work very well to carry bikes like this and are very easy to use.  Yes, they are an extra step but are practical. 

If you find that all of your bikes will require a frame adapter to mount them, we suggest using a tray-style, hitch bike rack, like the 916 T2 that doesn't carry the bike by its frame.