West of Grand Junction and Fruita within the Rabbit Valley Recreation Management Area, the Zion Curtain Trail is a 23.5 mile lollypop loop with 2,700 feet of climbing. This long route combines roughly 7 miles of singletrack with sandy doubletrack. The highlight of the route is the three miles along the rim overlooking the vallies below and onto the Colorado River gorge.
The Zion Curtain route starts with a stout climb and then 4.5 miles of sandy dirt road before reaching the final trailhead where the singletrack starts. This dirt road crosses a couple washes which could flood if it rains. Unless you are a masochist, it is recommended to drive to the final trailhead, especially if you have a 4WD vehicle. It will save you 9 miles through pretty uninteresting desert along with 1,400 feet of climbing. Some locals will park at the first trailhead near the highway since they extend the loop with a stretch of the Kokopelli's trail and the West Rim Trail, which winds up back near the highway trailhead (40+ mile loop).
To reach the Zion Curtain mountain biking trailhead from Grand Junction, head west on US70 to Exit #2. Head south on access road for about 500 feet and park on the left at the signed trailhead. Start riding back north across the highway to the Trail Through Time and wind up the ridge to the west. After a couple miles, follow the route south under the highway to the final trailhead and up to the start of the loop. The loop is usually ridden clockwise.
Note: The Zion Curtain trail is actually just across the border in Utah, but is listed here in Colorado since it is usually accessed from the services in Fruita, Colorado.