roswheel saddle bag for bicycle offers enough room for the gear that won't fit in a jersey pocket. It attaches easily onto a seatpost with secure hook-and-loop straps, has a reflective light clip if you need to ride at night, and is splash-resistant, so your gear won't get ruined if you go for a ride in the rain. Nothing beats the convenience of an under-seat pack. Not that they're very large: the road bike bags but once you have your important items stashed in the seat pack, they're there when you need them. Stash a spare tube, a patch kit, a multi-tool, and some extra chain in case of the typical mechanical failures.