East of Lowell in the Nez Perce National Forest, the Round Top Mountain (6,807) area has many extreme biking options. However, the challenge is the ascent -- from along the Selway River to the summit its over 3,800 feet of vertical!
The easiestmountain bike route up to Round Top Mountain is FS317 (11 mi.) from near Lowell. Other trails can be combined with FS317 to make the ascent more interesting; these include the CCC Trail (#734 / 6 mi.) and the RadCliff Ridge Trail (#702 / 6 mi.).
The steepestmountain bike route down is the Glover Creek Trail (#705), though it is poorly maintained. The next steepest routes are the East Boyd Trail (#703 / 10 mi.) and Roundtop Moountain Trail (#704 / 13 mi.); both are well maintained and end at FS 223 along the Selway River.
Overall, Round Top Mountain Trails is the 32nd most popular mountain bike trail of all 63 mountain biking rides in Idaho.