West of Lyons in the North Foothills Open Space, the Hall Ranch has a 10 mile trail network open to mountain biking. Except for an extended rocky climb near the start, the trails are mainly packed dirt with some loose turns. These are nicely designed mountain bike trails with switchbacks, winding open sections and water bars. The trails can get pretty hot mid-day since they are exposed and have little shade.
To reach the Hall Ranch mountain biking trailhead from Boulder, head north on SR36 for abou 14 miles. Turn left heading northwest on SR36/66 for 1.4 miles into Lyons. Turn left heading south on SR7 (South St. Vrain Drive) for 1.4 miles and look for the trailhead on the right.
From the trailhead, follow the Bitterbrush Trail by mountain bike climbing up 3.7 miles to the Nelson Loop. The 2.2 mile loop is best ridden clockwise. Then return downhill via the Bitterbrush Trail back to the parking lot. The total mileage is 9.6 miles. There is one additional trail open to mountain biking, the Antelope Trail. It is a one mile trail from the middle of the Bitterbrush Trail that leads to an alternate parking area off Antelope Drive just north of Lyons.
Note: This trail system is very crowded on the weekends, especially from 10am to 4pm.