Here's a RH Liberty sidecar frame on a Harley-Davidson, conversion by Liberty Sidecars in Seattle a couple of years ago.
The H-D sidecar uses a single curved upper strut, but a rigid sidecar axle. The Liberty uses two upper struts for increased rigidity, and a swing arm for sidecar suspension.
The standard sidecar frame mounts by H-D are very strong and a tried-and-true design.
Liberty replaces the stock H-D triple clamps with new ones to reduce trail, so the front end looks stock, but steering is much lighter.