West of Grand Junction and Fruita in the Rabbit Valley Recreation Management Area, there is the popular West Rim Trail which combines buff singletrack and slickrock riding.
To reach mountain biking for the West Rim Trail at the McDonald Trailhead from Grand Junction, head west on US70 to Exit #2. Head south on access road generally staying to the right. At about a mile in, the road forks -- follow the left fork which is signed with a large caution rough road ahead (its actually easier than the right fork). Continue for about 3 miles to the signed McDonald Trailhead parking area.
Overall, West Rim Trail is the 116th most popular mountain bike trail of all 175 mountain biking rides in Colorado.