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Step 1

RIDER SAG is the distance the suspension compresses under the weight of the rider on the bike, in gear, in the riding position. The objective is to set (for both the rear suspension and the front suspension) the spring/shock preload so that we have the optimal Rider Sag. In addition, we want an optimal FREE SAG, which is the distance the suspension compresses under the weight of the bike alone, without any rider. Finally, we want to measure STICTION, which is the variance in suspension compression that is attributable to friction of the suspension components. If the idea of stiction sounds confusing, read on and it will be very clear once you take the measurements.
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Step 2

REAR SUSPENSION: Begin with the rear suspension. First you need to measure Full extension. Without a center stand, you must support the bike by the rearsets or have your helpers lift the bike. You want for the bike’s swingarm to fall and extend freely and fully. Then, measure from a specific point on your axel nut to a specific point on your subframe or rear body work. Have the same person measure each time, and be very precise. Maybe put a piece of tape on the tail above the axel and mark a line on it so that you can measure from this same point. If your body work is race plastics and is a bit flimsy, try to find a place on the subframe instead. You want your measurements to be very accurate. Always have the same person take each measurement (to ensure consistency).
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Step 3

Measuring Rear Sag

Next, you need to measure rear suspension Free Sag, which is the sag of the bike under its own weight. If you were to simply stand up the bike and measure it, you would get inconsistent numbers because the rear suspension would settle in slightly different positions each time. This is due to the stiction in the suspension. To account for stiction, you must take two measurements and average the two. First, with the bike resting on its own weight and someone supporting it in the front to keep it level, press down very hard on the rear and let it rise up VERY slowly/gently until it settles. Take a measurement once it settles. Next, lift up on the tail and then let it settle back down VERY slowly and gently. Measure again in this position. Average these two to get your Free Sag. The difference between your two measurements is the rear suspension stiction. If the stiction is more than about 5mm on the rear, that's too much stiction and you should have the suspension serviced.
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Step 4

Finally, you need to measure rear suspension Rider Sag. This is done just like you measured Free Sag, but with you sitting on the bike (in full gear, in an agressive riding position). Again, someone must support the front. Then have your helper that is taking the measurements take the two measurements to account for stiction. Follow the same procedure as you did for Free Sag. Again, be very precise.
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Step 5

Click for Example

Click image for example numbers. For the rear suspension, these are numbers you want to see... ~5-10mm Free Sag and ~30mm Rider Sag. Of course, your actual measurements will differ, but that's the type of sag you want to see, in the end.
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Step 6

To adjust your Rider Sag, vary the shock preload by turning the large coupler that presses against the rear shock spring. You will probably need a special tool to turn this coupler, and it will require considerable force since the spring is under a lot of pressure.
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Step 7

Measuring Front Sag

FRONT SUSPENSION: On your front forks, it's the same process. You'll want to measure from a fixed point on the fork (for USF, or upside-down forks) or the steering head. Use tape to mark a line, if it helps. And measure very precisely, just as you did for the rear.
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Step 8

Using Calipers on USF

For USF front ends, you can also use a set of calipers to just measure the inner fork tube (shiny chrome area) on the fork. This is the easiest place to mesure if you have USF forks.
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Step 9

For the front forks, you are shooting for ~15-20mm Free Sag and ~35mm Rider Sag.
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Step 10

To adjust your Rider Sag on the front, vary the fork preload by turning the large nuts on the top of each fork tube. Be sure you adjust each tube in the same direction by the same amount. Don’t worry if it’s not EXACTLY the same, but try to keep them equivalent.
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Step 11

Regarding stiction on the front, we’re told by the suspension gurus that anything >10mm is a red flag. Note that this is for Free Sag stiction. When measuring Rider Sag, and the fork is more compressed, you’ll likely see more stiction, and that’s okay.