East of Seoul, Acha-san trail network has it all -- steep ups, steep downs, steep granite fields, steep dirt singletrack, and a steep walk. Plus, almost all of it is rocky! The trail network actually consists of two mountains, Acha and Yongma, which run roughly parallel to one another. At the north end of Yongma-san is Manguri, the biggest mountain cemetery in Korea. This side of the mountain is not so difficult, and novices may be content to mountain bike ride the paved ring road in Manguri, from which you can access many of the trails. Manguri also makes a good place to start the real mountain bike ride.
To reach the trails from Seoul, follow the Han River Bikeway on the south side of the river. Head northeast up the main road towards Kuri. At the top (halfway between Seoul and Kuri), turn into the paved entrance road to Manguri. Follow the ridge from this point back to the Sheraton Walker Hill Hotel.
Starting at Manguri head over Yongma via the main ridge trail to Acha, down the Acha ridge, heading off past the pagoda to the granite fields. The best trail to take down is a steep, tight granite singletrack winds its way back to the bottom of the granite field. Back up at the pagoda, I turn right at the ridge and continue up the other side of the saddle, where at the top I turn left at a fork and follow a decidedly unscenic barbed-wire-lined tight singletrack down to a quarry-like area. This eventually opens up to reveal a road, which leads back to the Olympic Bridge and the Han River bikeway.
Info courtesy of Steve Danyo
Overall, Acha-san is the 3rd most popular mountain bike trail of all 7 mountain biking rides in Korea.