Once you’ve worked out how much travel your bike has, put a small amount of air back in the shock or remount the coil and put in a single turn of preload.
The next step is to calculate how much sag you’re aiming to have. Different bike shapes and shock models work best with different amounts of sag, but as a rule, for cross-country racers it’s 15–25 per cent of total travel; for general cross-country, it’s 20–30 per cent; and for all mountain/freeride, it’s 30–35 per cent. These are guidelines only – refer to your shock manual for recommendations. This gives you a starting point to use to tune to your preferences. Don’t worry about that now, we take that into account at the test stage. Now work out what sag you would like.
You’ll need a friend to help you with this. Measure the distance between the shock mounting bolts. Sit on your bike, in your normal riding position (it helps to lean against a wall for this), and get your friend to measure the same distance again. Get your friend to repeat the measurement between the centers of the shock eyelets. Subtract this new measurement from the original unloaded shock length, and you have the sag. If the amount is more than you expected, add preload to the coil spring. If it’s less than you expected, back off the coil spring, or bleed out air, until you have it about right.
Air springs are adjustable throughout the range of what you need, but coil springs have a much narrower range. For example, Fox Vanilla springs are designed for up to two turns of the preloadadjusting ring. Crank them up too much and they won’t work properly. Leave them too loose and they bang around. Ideally, with the exact spring rate, you shouldn’t need to use preload at all. If you can’t get the adjustment you need from the spring you have, get a softer or a harder spring. Give the bike shop the details of your bike (make, model and year) and spring (spring rate and travel as printed on the spring), as well as your weight, so that they can work out the correct spring for you.
You can work out the sag on an air shock without press-ganging an assistant to measure for you – the travel O-ring on the shock shaft will get pushed down as you compress the shock, and will then remain there when the shock re-extends, making it simple to measure how far you squashed the shock by sitting on the bike. You’ll need a shock pump to add air and to measure the pressure inside the shock.
Step 1: For air shocks, push the travel O-ring right up the shaft of the shock, so that it rests against the air sleeve.
Step 2: Sit in your normal position on the bike in normal riding clothes. Just sit, don’t bounce or twiddle. Get off the bike. Your weight will have compressed the shock, pushing the travel O-ring along the shaft. Measure the gap between the air sleeve and the O-ring. This is your sag.
Step 3: If the measurement is less than you expected, release a little air from the shock. If it’s more than you expected, add a little air – screw the shock pump onto the valve, enough so that you can hear a little air escaping, then half a turn more. Add a little pressure. Remove the shock pump and test again. Make a note of what
pressure you ended up with!
10% sag = total travel divided by 10
15% sag = total travel divided by 7
20% sag = total travel divided by 5
25% sag = total travel divided by 4
33% sag = total travel divided by 3
“Ideally, with the exact spring rate, you shouldn’t need to use preload at all”
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Suspension: measuring total travel”