WARNING: This page is about a different car, the 2016 BMW Z4, 2015 BMW Z4, and 2014 BMW Z4. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Identifying Frame (000226)
Courtesy of BMW OF NORTH AMERICA, INC.
NOTE:
(Side frame) Only required once for constructing the base frame Deletion, only available via tool set