The Timber Trail is a singletrack that starts from the parking lot on the north end of the park, and climbs about 1200 feet to the back Arthur's Rock, where it intersects with the hiking trail to the summit. Timber can be ridden as an out and back (8.8 miles), or as a loop with other trails.
The lower portion of the trail is a fairly smooth and gradual climb by way of a series of switchbacks across scrub and grass covered hills. This section is an intermediate level trail with only a few technical obstacles to clear. There are nice views of Horsetooth Reservoir at every turn. The lower section ends at the intersection with Kimmons Trail. Many riders climb Timber and descend on Kimmons.
Beyond this intersection, Timber Trail really steps up the difficulty level. After a short rolling section of technical riding, the trail heads into the forest. Here, the climb gets much steeper and the switchbacks become extremely tight and technical. Between the switchbacks, there are enough technical challenges to keep any rider busy. All but the superhuman should expect to stop to rest frequently on this climb. Near the top, the trail intersects with the Westridge Trail, where it turns into a dirt road for the last mile to the Arthur's Rock Trail.
The descent is fun and challenging. The narrow trail, and fairly high consequences of a crash tend to keep speeds down.