South of Bozeman in the Gallatin National Forest, the Gallatin Crest Trail (aka Devils Backbone) is a challenging 26.5 mile route. Across the route, the elevation varies from 6,800 feet up to 10,100 feet. Keep an eye out for moose, bear and mountain goats.
To reach the dirt biking at the northern trailhead from Bozeman, head south on 19th Avenue to Hyalite Canyon Road. Follow Hyalite Canyon Road to Hyalite Reservoir and continue across the dam. Follow the dirt road along the other side of the reservoir to the Grotto Falls trailhead.
To reach the southern trailhead, follow US191 south towards Big Sky. About 6 miles before Big Sky, look for the Portal Creek trailhead.
Starting from the Grotto Falls trailhead, you will need to follow Trail #427 up to the ridgeline. Then turn south on Trail #96 running along the ridge with a few serious climbs all the way to Windy Pass. The Portal Creek Trailhead is about two miles past the pass.
Overall, Gallatin Crest Trail is the 18th most popular dirt bike trail of all 30 dirt biking in Montana.