Description for Osborne Mountain
This route is sometimes known as the Sawtooth Ridge Epic.
This ride has it all: old growth forest, pristine lakes, some great views, very narrow trail, some climbing challenge, a stretch of flowy downhill and to finish a section of steeper downhill with many switchbacks. Perhaps the one thing missing is an actual peak--despite the name, the route does not go to the top of Osborne Mountain. If you want panoramic views, do the three mile side hike up High Rock that's described below.
The ride difficulty is in the same class as, but a bit higher than the Lower Dungeness-Gold Creek trail loop. There are some tough climbs, and there are long, long stretches of very narrow trail with side slope exposure.
The adventure consists of an 11-12 mile approach on forest roads, and an 11-12 mile return that's mostly singletrack with a moderate road climb in the middle.
Head back out of the campground, and turn right on FR52/Skate Creek Road. After about 2.8 miles, turn right onto FR 84, a major road. (Signed for High Rock?). Follow this road more than 7.5 miles, then take the uphill right of a fork onto FR 8440. (A short brown post a bit into the road confirms this is 8440.) Go about a mile until you find the Big Creek Trail (#252) on the right and a small 3-4 car pullout on the left. If you get to the large parking area by the High Rock trail, you've gone too far.
Trail 252 starts with a roughly 1.5 mile tough climb to a saddle above Cora Lake. At the saddle, don't miss the view to Mt. Rainier. The trail will go about 0.6 mile downhill to a junction; stay left going uphill on Trail 251, Teeley Creek trail. (The right turn goes down to Cora Lake.) You'll climb steeply for a bit, descend a mile, climb a mile, and then descend to Granite Lake, along the lake, over a rise, and down to Bertha Mae Lake.
A bit past Bertha Mae lake you'll come to a T. You can go either way here.
Option A: The uphill/straight option continues as singletrack through very nice forest but it is little traveled, most of it is quite a grunt, and it has one big section of tough, steep, sandy hike-a-bike. When the trail skims the road, stay on the trail for another short bit. Eventually you'll come out onto the road less than 1/4 mile from the downhill singletrack.
Option B: Alternately the right turn takes you downhill to Pothole Lake and then a 2-3 mile climb after turning left on Road 8410. Downside here is that you lose an extra 700ft of elevation that you'll have to climb back up. The road climb is about 2 miles and goes up 1,000 feet; it's a little easier than the road climb at Tiger.
Either way, when the road reaches the open saddle, you'll find Trail 250, Osborne Mountain Trail.
You now have about 5.5 miles and 2800 feet of downhill coming; the first part mellow and swoopy, the later part steeper and switchbacked. The trail ends at the Big Creek campground.
Allen Mountain variation
You can add more single track by adding the Silver Creek Trail (#258) and Allen Mountain Trail (#269), then down Tr 253 (?), right on FR 8440 to the Big Creek Trail, and continue as above. There are no recent reports for this route, but reports a few years ago mentioned that the trail was brushy and some route finding was required.
Road 85
The approach route going around the west and south side of Sawtooth Ridge on FR 85 is not recommended if you're driving a car up. As of September 2009, you'll encounter road damage that's hard to navigate. Use FR 84 to 8440. Signage isn't great, bring a map and pay attention. 8440 is signed on a short vertical post a bit uphill from the intersection.
On the other hand, FR 85 makes a pretty nice climb on the bike if you're going to ride to the trail; very little or no vehicle traffic.
High Rock side trip for view
For the best view, do a side-trip hike up trail 266 to the High Rock fire lookout, starting from Road 8440 (?) about a mile south of the start of Big Creek trail. This trail is currently posted as closed to bikes.