Northwest of Fort Collins and southwest of Red Feather Lakes, there is a network of Forest Services roads and trails open to OHV users. The Green Ridge route is a high altitude route through dense pine forest.
To reach the dirt biking at the Green Ridge trailhead from Fort Collins, head west on SR14 to near milepost 71. Turn right heading north on CR103 for 1.4 miles. Turn right into the trailhead parking area. Keep in mind that this trailhead does not open until at least mid-July every year. There is a gate and it is periodically patrolled prior to the official opening (very wet and/or snowy before opening).
Note: Colorado residents must register their off-highway vehicle with the Colorado State Parks (303) 866-3437. Non-residents are not required to register in Colorado, if they are registered in their home states. All off-highway vehicles must have an approved muffler and spark arrester. These requirements are subject to change so make sure to contact the Colorado State Parks prior to using your off-highway vehicle.
Overall, Green Ridge is the 70th most popular dirt bike trail of all 93 dirt biking rides in Colorado.