Southwest of Hilo and southeast of Mauna Loa on the Big Island, there is a 8.2 mile one-way route along an old 4WD road heading up Mauna Loa that is open to dirt biking. This 4WD road eventually connects with the historic Ainapo Trail leading to the summit (foot traffic only). The 4WD road starts at 2,600 feet and climbs to 5,500 feet.
To get to the trailhead from Hilo, head southwest on SR11 past Volcano House. Between milepost 40 and 41, there is a yellow and brown Na Ala Hele sign near a locked gate. This is the start of the 4WD road. To get the gate unlocked, you need to call several days ahead of time to the Kapalala Ranch (808) 928-8403.
Note: there is very limited trailhead parking. There is not much room for trailers or turn-around.
Overall, Kapapala Forest Reserve - Ainapo Trail is the 4th most popular dirt bike trail of all 5 dirt biking rides in Hawaii.