Just outside the southern boundary of Yellowstone, the old Reclamation Road (FS261) makes for a great 45+ mile mountain bike ride from Flagg Ranch westward to Ashton, Idaho.
From Flagg Ranch just south of the South Entrance on US89, this dirt road is signed as Grassy Lake Road (FS3261). The road does appear on some maps as Ashton-Flagg Ranch Road. It heads west to Grassy Lake and side roads lead to Lake Of The Woods (both in the Targhee National Forest). From Grassy Lake to Indian Lake, there are several side trails and road, especially before reaching Indian Lake on the Idaho border. Beyond Indian Lake, the route parallels the Falls River.
There are several established campsites with pit toilets along the road.
Overall, Grassy Lake / Reclamation Road is the 12th most popular mountain bike trail of all 19 mountain biking rides in Wyoming.