If you didn't know, I'm riding a 2013 Diamondback Sortie 29er. See my bike review here:
One of the advantages of this new machine was full suspension. For the techies out there, here's the back story from Diamondback:
Working closely with Fox, we’re able to optimize the wheel-rate to be linear-to-slightly progressive. This results in superb small-bump compliance, more perceived travel and excellent bottom-out protection. For our Sortie 29 series, we’ve chosen to run a Fox Float CTD Air Shock with Boost Valve. The extra volume “LV” 190×51mm air shock with rebound adjust 3 position Propedal Platform On/Off lever provides a feeling of endless travel and plushness like that of a downhill rig, but with XC pedaling efficiency. Dial in climb, trail, or descend mode instantly.
The Sortie 29 1 has taken quality componentry and placed it in an economical package… starting with the Fox 32 Float CTD 29 RL 120mm travel air fork. This fork comes with an open bath cartridge, rebound, lever actuated lockout, butted alloy tapered 1.5 steerer, 32mm Easton aluminum stanchions, magnesium lowers, and a standard 9mm quick release drop out. Just like the rear shock, this fork can handle anything you’ll see on an epic ride.
Getting the suspension to meet your trail needs is a big part of the dual-suspension advantage. Here's a few quick tips to help first-timers adjust the settings:
1) Get a shock pump. This is a special piece of equipment, not the same as a tire pump. They run $30 to $40 at your local bicycle shop.
2) Benchmark your shock pressure. Take a ride and note how the suspension feels. It is mushy, or stiff like a rock? Then screw the shock pump to the fitting on both the front fork and rear shock. The shock pump has a pressure gauge, so you can determine the pressure. Write it down for both fork and rear shock.
3) Fox tech support is excellent and responds quickly, but they'll tell you to work with sag...that's sitting on the bike and seeing how much your rear shock and front fork compress. That's troublesome for me. I also read online and some have suggested 1 psi per pound of body weight. I'm 155 so that would be way stiff for my taste.
4) I don't' think I'll be jumping off of a 20 foot cliff anytime soon, so here's the setting I found that works best for me. I ride slow and easy, on moderate difficulty trail. I sit, a lot. If you're going for the marshmallow feel that I like, try running 90 psi in the rear shock and 100 psi in the front fork. That seems stiff enough to take the hits - I ride one section in Alabama with a mile of downhill on softball size rocks. But the 90/100 setting is also compliant enough to soak up the smaller aberrations. I tested at 80/90, 85/95, 95/105 and 100/110, then came back to my sweet spot - 90/100.
5) I run 30 pounds psi in my 29er tires. There's a big spread of possible tire pressure, as low as 20 psi all the way to 65 psi, so in some ways tire pressure is also part of the suspension equation.
There's no one perfect setting for every rider, so I'd suggest you use my method to test and tune. A well set up dual-suspension mountain bike makes for a wonderful woods experience. Enjoy.