Calculating the sag (amount of compression) of a bicycle suspension is an important step in properly setting up the suspension for a specific discipline.
For cross country riding, a general guideline is to set the sag at around 15-25% of the total suspension travel. This allows for efficient pedaling and good small bump compliance.
For trail riding, a sag setting of 25-30% is a good starting point. This provides a balance of pedaling efficiency and good handling in technical terrain.
For enduro or downhill riding, a sag setting of 30-35% is recommended. This allows for maximum absorption of big hits and improved stability in rough terrain.
To measure sag, start by setting the suspension to the recommended pressure or damping for your weight. Sit on the bike in a normal riding position and have a friend measure the distance between a fixed point on the suspension and the same point on the bike while you are seated. Then, stand up and compress the suspension by pushing down on the handlebars and have your friend measure the distance again. The difference between the two measurements is the sag.
It's important to note that these are general guidelines and your personal preferences, riding style, and the specific bike and suspension setup can also affect the ideal sag setting. It is recommended to experiment with different sag settings and find what works best for you.