Among the regions network of flume trails, logging roads and ore-car grades, theres a 63-mile counter-clockwise loop around Mount Shasta that begins and ends in the eponymous town. Much of it is on forest service roads.
Follow FS 31 to the junction of FS 19. Turn left and then immediately left on 42NO2 for 6.7 miles to 42NI6. Turn left at tree cutting sign 42N34; right at 42N33 down to railroad tracks. Follow tracks out to south of Weed or arrange for a shuttle. There is a lot of opportunity to make variations at the beginning and end of the mountain bike ride, so be sure to study a map. The scenery is spectacular with views of Mt. Shasta to the left with glaciers on the eastern slope and lava flow fields in the valley to the right.
To reach the mountain biking trailhead from US 5, exit onto SR89 just before Mt. Shasta City and follow to Forest Service Road 88. Turn left and follow 88 to the intersection of FS 3 1. The mountain bike ride begins here.
Overall, Mount Shasta Loop is the 33rd most popular mountain bike trail of all 132 mountain biking rides in California.