Kuroyama 「黒山」　MTB circuit (8.5km, 445m climb, 1hr)　

Category: Road: 4.5km, Hike trail: 4km

Route: Kami-Nariki, Nariki hill-climb, Kuro yama, Kosawa tunnel, Kami-Nariki

Map:

Profile:

Notes:
Onishi-san of KFC triathlon (http://www.kfctriathlon.jp/html/event_bike.html) organizes a race up this route to the end of the paved road (4.5km). I managed to clock 18min15 in 2008 race. There's good drinking water at this point.
The climb on the road is close to 10% average and for the most part the gradient is close to this average, so there are no easy or very steep sections.
The hiking trail section (4km descent) is steep in just a few sections and has only 2 small sections requiring some climbing which can be cycled. You should be able to cycle about 85% of the descent. See photos for samples of the surface - some of the upper areas are smooth and long with constant gradient of about 8% - just perfect!
The only real difficult section is by the stepping logs - apart from that this is even OK for a beginner I reckon, as long as you don't mind pushing/carrying the bike for a few meters.

For a short excursion, you can leave your bike where dirt road and trail meet and walk 600m up to Kuroyama top (see photo1 below). Much of this section can be cycled, but you'd be faster without the bike.

Starting from Ome, you can combine this route with any route that passes by Kosawa tunnel, eg the Takamizu MTB circuit
　

Photos:

Kuroyama top - A short excursion off the circuit (about 500m one way).

There are some roots on the trail but not a major obstruction

You can see quite far ahead here, so don't need to worry about hikers suddenly appearing out of nowhere.

A fallen tree - possible to jump over if you're adventurous!

The trail is very clear.

Naguri is visible through the trees - unfortunately there's no open area to give a great view.

However, there's an excellent view of Takamizu-san and Iwatakeishi yama

The trail is amazingly smooth considering the number of trees right beside it.

There are many places to sit and rest.

You meet some steep sections as you get close to Kosawa-toge

Stepping logs - This is the most difficult part and does not last very long

Follow the sign for Nariki

The trail is soon good again

If you follow the track under the tunnel, you come out near the start at this bridge.
Alternatively, just use the main road from near the tunnel entrance to complete the loop and avoid some steps where the trail goes under the main road.