Sag is that initial sinking that every full house (or front) must have when getting on the saddle. This sinking, which can vary from about 15 to 40%, is used to make a suspension work optimally. For having calibrated it on your weight in a nutshell.

As for the tyres, where you have to use the right pressure, also in the suspensions one must be careful to inflate them correctly: on the other hand one cannot think that a bike is good for a 50kg girl and for a 90kg big man!
HOW TO CALIBRATE THE SAG?
Let's start right away by saying that the Sag adjustment is to be done with the registers all open, i.e. the slow rebound and the compressions (low and high speeds towards -). If your suspension is basic, chances are you only have rebound adjust.

First you need to bring the o-rings (those soft rings you find in the suspension stem) to the bottom of the sleeve, as low as possible: this to ensure that it is then possible to see how much the suspension has been compressed. Gently climb into the bike and look for suspension compression only due to your body weight. Is there no ring? We use a classic electrician's cable tie.
But how do we compress the suspension with our weight? You simply lean against a wall, with the handlebar, or get help from another person. Subsequently, you gently get off the bike (so as not to compress the suspension in question more than necessary) and look at where the o-rings (those soft rings you find in the stem of our suspension) have arrived.
After seeing where the rings have arrived, let's evaluate whether it is the case to put more air or remove air (or change the spring in the case of spring suspensions). Rockshox forks have a ferrule stamped on the stem that allows you to immediately understand the percentage of sinking, while with other brands (for example Fox) you need to have a caliper.

What are the correct SAG percentages? We present an explanatory table since the higher the excursion, the more the sag must be abundant.
CROSS COUNTRY (80-100mm) | 10-15% |
Trail (110/130mm) | 15 / 20% |
All-Mountain/Enduro (140/170mm) | 20 / 30% |
DH (over 180mm) | 30 / 40% |
HOW TO PUT AIR IN THE SUSPENSION
To change the air pressure in the suspension, special high-pressure pumps are needed: they are used like a normal pump with the difference that the common pumps with which we inflate the tires are not suitable here (being, precisely, at high pressure).

ON THE FIELD
If we make a correct Sag, at the end of the descents we will have to find our o-ring about 1cm from the bottom out (end of the stem): it is never advisable, both for the health of the fork and for safety reasons, to bottom out ride fully and use the entire hike, unless we've had a lull in life. Also because the limit switch is never at the end of the stem.

Why should this be avoided? Because when a fork bottoms out, there is a risk of ruining the hydraulics and, in technical situations, having the fork too far out risks making us stall, given that the steering angle closes and the fork stalls in the true sense of the word.
WHAT HAPPENS IF WE WRONG THE SAG?
Preamble: If we get the sag completely wrong, we can also suffer damage to the product (if the sag is too abundant over 50%).
Let's make two general considerations but know that the exception proves the rule: calm riders equal softer suspension, enraged riders equal harder suspension (always remaining within the range of correct percentage values). Because it is normal that a Jackson Goldstone, even if it weighs 70kg, will compress the suspension like a Mario Rossi (beginner) weighing 80kg.

If we have less sag on the shock absorber than necessary, the suspension becomes stiffer and makes the vehicle more nervous, certainly more pedalable uphill (less bobbing) but less bumps are absorbed. Reversing the situation (too much sag), we still have a more precise bike that absorbs all the bumps but in the event of big bumps or jumps, the bottom out is always around the corner and bobbing is amplified during pedaling.
FURTHER LINKS
Regarding suspensions, I suggest you read the following articles: How to adjust MTB suspension - The 10 mistakes not to be made in MTB -
Here you can find other articles dedicated to maintenance: Heading 365myarrangement
