The Xtracycle now has a bicycle rack mounted on it to carry a second bicycle. A Rocky Mounts Lariat, it mounted onto the wideloader with no modification to the rack or bicycle - just a few shims (old tubes). Bracing is necessary to stabilize the passenger bicycle, but can easily be constructed. Check out the pictures, including a couple of close ups, in the Xtracycle section of the photo gallery.
Two Fewer Cars