In the southern Willamette Valley there are many spectacular places for mountain bike enthusiasts. I am by no means an expert in the topic. I just started mountain biking myself. Oakridge, OR located about one hour East of Eugene on Highway 58 is considered by many to be the mountain biking capital of the Northwest. Inquire locally at Oakridges main bike store, off highway 58. I have found many trail maps, and descriptions through "singletracks.com".
Also, In the area is the pristine Mckenzie River Trail, dubbed "America's Favorite Trail". The MRT is located in the Western Cascades and can accessed through many trailheads off of Highway 126. The entire trails extends for about 26 miles, starting just before the Ranger Station and running all the way up the Cascades to it's headwaters at Clear Lake. For more information and recreation on the Mckenzie visit the Willamette National Forest website, or mckenzierivertrail.com.