Checking your sag and spring rates is easy. There are a lot of different springs you can use on your bike depending on your weight and riding preferences. They reckon a bike straight out of the crate has a spring rate suitable for your Joe Average 75 - 80kg rider. So, if you are weighing in at over 100kg and you have a standard spring fitted, chances are your donkey won't be riding as well as it should be.
Here's a guide to help you find out if the sag and spring rates on your bike are right for you.
These are guides only - for a more detailed setup of your suspension, refer to your owners manual.
If the laden sag does not meet these requirements, you will need to adjust the spring preload. Do this by using a hammer and punch to release the top collar on the shock spring. Then gently tap the lower collar to compress the spring (more preload), or to release the spring (less preload). Make sure you tighten the top collar once you have finished to lock it in place.
So, after a bit of fiddling around, you have the correct laden sag. But if the static sag is way out, e.g. not enough sag, then the rear spring is too soft for you. And of course if there is too much static sag, the spring is too hard.
The reason this is so, is that if the spring is too soft you have to apply too much preload to get the desired laden sag, which will offset the static sag. If you put a harder spring in, you wouldn't have to apply so much preload. And then both the static sag and laden sag would be balanced.