Backbone trail/Farmer's Fire Road.

This ride goes south on the "Backbone Trail" ending at Will Rogers Polo Field, and then returns north via Farmer's Fire Road, back to Mulholland.

You start from Reseda Blvd - take the Reseda Blvd off ramp from the 101 and head south through residential neighborhoods, Thomas Guide® ref pg 590 G7. After a couple of miles, it opens out and starts a gentle ascent. You can either continue to the top where there is a parking lot, or just park as I did at the Caballero Canyon Trail Head, which you'll see on the left hand side - look for this notice. The advantage of this is you add the Caballero Canyon single track to the ride, and you get a longer stretch of continuous downhill on the way back. If you park at the top, the penalty you pay is having to do some uphill at the very end of the ride.
The Caballero Canyon single track trail goes for about 2 miles and gets you up to Dirt Mulholland. It's quite steep in places, and pretty bumpy throughout. About halfway through, it widens quite a bit. I first discovered this trail from Sierra Club hikes - they do it every Friday night at 7pm. Once you're up on Dirt Mulholland, you get a great view south across the Santa Monica mountains.
Now you turn right which shortly starts up a gentle hill. Within a mile, you'll be able to look down at the top of Reseda Blvd (where you could have driven to) on the right. Shortly after this, there's a gate to the left labelled "Temescal Canyon", which you take. Now you start heading south across the Santa Monica Mountains proper, and the trail is pleasantly flat, with just gentle undulations. Quite a bit of loose sand though as well. After a couple of miles, there's a sharpish ascent for about 1/4 mile near a rocky outcrop which brings you to Hub Junction. I've frequently come across groups of bikers resting at this spot - and of course there's the portapotty.
You take the left fork labelled "Temescal Canyon" and the trail continues fairly flat. After about 1/2 mile, there's the northern entrance to the Backbone Trail, which is the single track you've come all this way for. The sign says it goes for 6 miles. You have to carry or walk your bike down the first few feet of the trail, and then you can just blast down this wonderful single track. I passed nobody on this trail, until about the last mile, where I met up with hikers who had started from the other end.
The colors at this time of year were spectacular along the trail. The trail has a bit of everything - plenty of rocky descents, some smooth fast and shady stretches, a bit of climbing.
Part way through, there's some intense downhill - the trail is clearly a streambed in the rainy season. It's just rough rough rough. At this point, you've got to carry your bike again.
At about mile 5 of the trail, you come across a sharp descent followed by a bridge and then more intense down hill, complete with occasional steps which are quite fun just to jump down. You reach civilization as you enter the Will Rogers State Park area where there are tons of sign posts everywhere and some kind of nature trail. Eventually, you end up on a wide dirt road which leads you to the Polo Field area, where you can drive to after paying admission and have a picnic in the grounds.

As you descend this stretch, you can look over to your left, and see where you want to be for the return, however there is apparently no way to get over there without first dropping down to Sunset Blvd. So now, you ride out of the park, past the Ranger's booth, and down the entrance road to Sunset. The is truly the low point on the ride!

You head left (inland) on Sunset for a couple of miles. It's a gentle climb most of the way. There are actually 3 ways you could return from this point: Sullivan Canyon, the Farmer's Fire Road, or Mandeville Fire Road. All are fun rides. I chose Farmer's Fire Road which is mostly shady. So you turn left on Mandeville Canyon off Sunset, then left again shortly after on Westridge Road. It climbs and climbs through neighborhoods of fancy houses. You make a left on Bayliss Road and you're suddenly losing height, then right on Queensferry Road.

You come to a solid gate across the road, except there's a way around it to the left, for pedestrians and bikes. To your right, you see this amazing house, complete with bronze sculptures in the sloped front yard.
The Farmer's Fire Road is fairly smooth and pleasantly shady - it does a gradual ascent all the time. After about 4 miles, it starts to climb more steeply and eventually wiggles up to the left, to join the Sullivan Canyon Fire Road.
The ride becomes much flatter, and the views much more open. Within 5 minutes, you're back at Dirt Mulholland as you pass by this gate

And now it's just about down hill all the way back - you turn left on Dirt Mulholland and descend back to the top of the Caballero Canyon Trail, then take this back to your starting point. Plenty bumpy. For me, this was one of the most fun rides I've ever done since it features so many different types of terrain.