The Pinkham Ridge Trail is one of my favorite single-track trails in this area. It is short (by my standards), but fun and it provides some nice vistas.
Getting there requires some grunt effort since every possible route to the trail head is uphill which is at a wee 5000-ft elevation (for reference the starting elevation of Asotin is 780-ft and the starting elevation of Headgate County Park (up Asotin Cr. Rd.) is about 1500-ft. There are other starting places that are closer if you're truly that lazy. From Headgate Park it is about 18-miles via the easiest route (all uphill) to the trailhead. The trailhead is not labeled or easy to find and maps (including GPS) are not accurate - truly you just have to know where to go or be willing to do some bushwhacking and hike-a-bike exploring, which is how I found it several years ago. Now I have 3-rideable routes in from the top - no more bushwacking for me.
I've found 5-rideable routes to this trail and 8 different ways down (some of which could also be routes up, but I highly recommend against all but 3 due to the shear pain you would endure climbing them). You can loop this trail down into the Asotin Cr. Trail on 2-different trails down, but the down hills are very steep and technical until you reach the creek far below, so beware and make sure your brakes are in good condition if you ever decided to take this route (I do this route a few times a year just for fun and a good adrenalin rush).
The remaining routes (loops) down are not too technical, but do require some braking if you're fearful of too much speed. There is also the occasional log-hop (or stop, get off, and walk bike over the log) on the way down on 2 of the best single-track routes.
You do have to be aware that the the neighborhood bear may be on the trail (yep, I've met the bear family more than once) so making noise with a bell, talking loudly to yourself, or whistling (which is what I typically do) on the fast down hill is a good idea I've learned. I'm sure the bears do not appreciate the sudden surprise any more than we do, so unless you really get a thrill from running into bears make some noise.