South of Souel, Kwanak-san (14km) is a classic, a must-mountain bike ride. The top is the payoff with a 4 km ridge descent that begins much like Acha, with granite steeps. As you descend, however, it gets technically easier and much faster, with a few rocks thrown in along the way to keep you honest. Expect speeds beyond your comfort zone.
To reach the trail from Seoul, head towards the imposing mountain between Seoul and Kwachon. It lies to the west of Umyon-san, although the eastern part of Kwanak is militarized and impassable. The mountain bike ride starts at the western part of Kwanak-san (Samsung-san) behind Novotel Toksan Hotel (find it on a tourist map).
Start at Toksan-dong or near Nambu Hospital (on the Nambu Ring Rd), which is roughly a 25 minute mountain bike ride from the Han River. Find the Novotel Toksan deluxe hotel, and wander through the little neighborhood behind the hotel to the mountains foothills. Its almost impossible to explain exactly how to get there, but with enough wandering you should be able to find some dirt. As usual, find the main ridge trail. Once youre in sight of the temple, there should be stairs in front of you going straight up. Carry your bike up this trail and at the top just keep following the ridge from helicopter pad to helicopter pad. This ridge should be almost all mountain bike rideable and very scenic, with views out to Seoul in the north and Shiheung to the west. Riding from the top, within a kilometer you re going to have to turn right, but there are so many trails its once again too difficult to describe. Once youve located Dog Rock, youre ready to go. Just stay on the main ridge as you descend. There will be a fork in the trail (not T-intersections); at each fork, make a soft left. The bottom should put you very, very close to Shiheung Daero (Shiheung Grand Street), depending on the actual exit trail you took. Heading north on this street will put you on Yoido and the Han River Bikeway, about 35 minutes away by bicycle.
Info courtesy of Steve Danyo.
Overall, Kwanak-san is the 5th most popular mountain bike trail of all 7 mountain biking in Korea.