Portuguese Bend is a nice ride within few minutes ride of most Los Angeles areas. It offers about 20 miles of fire road and singletracks and some steep downhills with some great sceneries and views on the ocean and Catalina Island.
The starting point is at Del Cerro Park, located a the very end of Crenshaw boulevard. There is usually plenty of parking space close by the little park entrance.
From here you can get across the gate and start descending on the main fireroad, which is Burma rd. Almost from the beginning there is a narrow singletrack on the right side of the road, that runs along the road.
As you approach the first turn close to a large water tank, there will be a few options to go down on singletracks or continue on the main fireroad.

This park has been subject to discussions on its shared use among hikers, bikers and equestrians, so please be very careful coming down the steep downhills, always maintaining full control of your bike and yield to equestrians and hikers. It is always a good idea to stop on the side and let horses pass by, especially on narrow trail sections.

There have been several announcements that this trail would be closed to mountain bikes. As far as I know the trail is still open, but please double check at the gate for signs limiting MTB access.

A good source of information is Corba Palos Verdes.

You can find a city approved trail map on their web site indicating which trails are open to mtb.

Geoladders offers step by step directions for a nice singletrack route, but there are so many options and variations that it will take a wile before you get bored with this area.
A short review of this trail is available on mtbr.com