Northeast of San Bernardino and Lake Arrowhead in the San Bernardino National Forest, the Forest OHV System (aka Big Bear OHV) consists of 36 miles of OHV trails and 169 miles of forest roads (open to green sticker/red sticker usage) that are open to off road motorcycling. The dirt biking trails are primarily in the Cajon, Arrowhead and Big Bear areas. There is a smaller system in the San Jacinto area.
The best dirt biking trailheads for the Forest OHV System / Big Bear OHV trail systems are: Pinnacles Staging area near Lake Arrowhead, Cactus Flats Staging area near Big Bear, Big Pine Flats Campground, Horse Spring Campground, and Crab Flats Campground
Note: The forest does not have open areas for hill-climbing activities, cross country travel or motor cross tracks for racing.
A good place to start exploring is the Pinnacles Staging Area. The Pinnacles Staging Area is located north of Lake Arrowhead on SR173. Note: As of September 2007, a rough section of SR173 has been reported closed -- that section is about 8 mile west of the trailhead. Make sure to approach the trailhead on SR173 from the east.
Overall, Forest OHV System / Big Bear OHV is the 4th most popular dirt bike trail of all 120 dirt biking rides in California.