According to the Big Basin State Park Brochure, this trail is closed from the end of the Butano State Park to Big Basin State Park near the entrance from China Grade. If I am mistaken, please update. Thanks!
This is a great, easy fire road trail ride. You access the Butano fire road via a locked gate on the east side of Cloverdale Road. The entrance is easy to miss as there is a small access road off of Cloverdale that leads up a 50 meters or so to the locked gate. There are only spots for two cars along this access road so it is often better to park off the side of Canyon Road. From Canyon Road ride a few hundred meters south (towards Butano Park) and you'll see the access to the fire road on your left.
There is an entrance gate for bikes and hikers that goes around the locked gate and you start a very gradual, five mile climb at 5% - 7% that tops out at a small airstrip. There are a few parts that are more than 5% - 7% but this is a pretty moderate climb that you can do in less than an hour or even faster on a road bike.
From the airstrip you can either turn around and go back or cross the airstrip and keep going. You are now on the Butano Ridge, and there are some steeper stretches but in less than a mile you'll come to the Olmo Fire Trail on your right. You can descend this road. The upper part of Olmo is pretty gnarly and I wouldn't do it on a road bike! Even on a mountain bike it is steep, with deep ruts and gravel - so tricky to descend. Once you get past this chalky, gnarly stuff though you end up in Butano State Park and have a beautiful descent through the Redwoods and out the park entrance. Make a right on Cloverdale and ride back to the starting point. Altogether this is a 12.5 mile loop with about 2200 feet of climbing.