There is an adapter, part # HTAD that can be used to convert the 1-1/4 inch receiver of the Class I hitch so that it will be compatible with your Thule rack intended for use in a 2 inch Class III or greater receiver.
Using an adapter can be problematic for a few reasons. First, using the adapter will add another piece in between the rack and the hitch, moving the load further away from the vehicle. The further away from the vehicle the load is placed, a greater amount of leverage is applied... view full answer...