Trail loops around the perimeter of the Frick park nature trail. You can ride on this trail, but not on the 'actual' nature trail which runs in the center of this loop. Signage clearly indicates where bikes are not allowed.
Additionally, you can access one of the best singletrack trails, Bradema near the intersection on the southern side of the loop. New signage has been installed to clearly identify the trailhead, you can't miss it.