The SART is roughly 30 miles of singletrack. I'm pretty sure I've ridden all of it at different times and there's numerous ways to make different loops.
Option #1 - Post OfficeThere's one loop in particular that I think of when SART is mentioned. I've ridden it well over 30 times and if you're looking to ride SART and aren't looking for too much mileage it's the way to go.
Directions -
From Redlands: Take Highway 38 about 15 miles. When you get to the town of Angelus Oaks you're there. They'll be a restaurant, general store, and post office on the left side of the road. There's a turnout you can park in about 1/4 mile past the restaurant.
The Ride -
This ride is only about 9 miles. The only bad part about the ride is that it finishes with a 3 mile uphill. The good thing is that it feels like it's 6 miles of downhill to start with only the 3 mile uphill finish. In reality there's bits and pieces of climbs at the beginning, but nothing that would be at all strenous.
Start on the road and head back to the restaurant. Take the dirt road that goes past the Post Office. You'll follow it up about 1/4 of a mile and they'll be a trailhead on your right. You can take it if you want to cut out about 2 miles of good riding. The best way is to continue on the dirt road about another mile. There will then be a turnout on the right to get to the trail. You can go left at the singletrack and get good riding, but for this ride you'll take a right. After about a mile of good winding singletrack you'll see a SART sign. This is where the real fun starts (it's also where the first turnoff that you didn't take meets up with this one). The next 3 miles will be some of the best downhill singletrack you'll find anywhere. After the fun, the singletrack will then climb for about a 1/2 mile and then it'll meet up with Middle Control Road (a dirt road). Take a right on this road and you'll have a solid climb back to your car. The road comes out at the pullout where you parked your car.
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Option #2 - South ForkThis ride is a nice 12 miler. You'll start at Glass Rd. and head up to the South Fork Campground on a dirt road. You'll then come back to Glass Rd. on the SART.
To get there turn off highway 38 onto Glass Rd. Take Glass Rd. down about a mile and you'll see a trail head on the left. There's also a trail head on the right, this is where you'll be finishing. Park anywhere in this area.
Start off riding down Glass Rd. and make a right at the first turn you come to - about 1/4 mile down the road. This road will soon turn to dirt and start heading uphill. You'll have a pretty gradual climb for about 5 miles. This road runs along a stream (possibly the Santa Ana River?) and is among a lot of trees. Just keep an eye out for cars. You won't see many, but you will see some.
This road takes you right to Hwy 38 and the South Fork Campground. From there you'll pick up the SART and take it 7 miles back to the car. The first 3 miles is a mixture of up and down with slightly more down than up. Then the final 4 miles is nearly all downhill on fast fun singletrack with a nice steep drop on your right side.
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Option #3 - ShuttleIt's been awhile since I've shuttled SART and I don't do it very often. If you're looking for mostly downhill there's one good shuttle option.
Start in Angelus Oaks by the Post Office. Head up that dirt road and at the 2nd trailhead make a left. It's a little hard to see that the trail goes left, but it does. If you go right you'll be doing the ride in my first description. From this point on you'll stay on the SART for quite awhile. The first mile from here is a gradual climb, then there's an area that'll be mostly flat. This is the one spot where it gets real sandy and you might have to walk a stretch. After the sand you ride over a hump and then it's all downhill from there. There are a couple times where you cross a dirt road. This is the same dirt road that you started on by the post office, and it's also the dirt road that you'll meet up with at the finish.
This singletrack is different than most of the SART. This section of it is nearly all downhill for quite a ways, plus it's more technical in most places. There will be one point where you drop down on a short, but steep dropoff onto the dirt road. Make sure you look to your right, the singletrack continues on for another mile or so. After this section you'll be on the dirt road for the rest of the way.
There will be one short climb after a long downhill on this road. If you want another fun option, make a right at the top of this climb. There'll be a gate you'll have to go around. You'll have a steep fireroad climb for about 1.5 miles. Once at the top you'll be at the top of Morton Peak (where the fire lookout is). Hop on the singletrack there and it'll take you down a fun 6 miles into Mentone. This section isn't quite as fun as it was 1-2 years ago before the fire, but it's still a good way down.
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Both of these videos are primarily on the South Fork/Glass Road option.