If you have been around motorcycles very long, chances are you have come across an article, forum post, or a random person talking about setting the sag on a bike. The problem is, no one really talks about WHY you should care about the sag on your bike. So if you are like me (and lots of other people) you most likely have never adjusted and probably won't since the bike handles good anyway. The truth of the matter is your bike probably does handle pretty well for the most part, but chances are with a little adjusting it could handle much better and allow you more control in those situations that gets your heart pumping in your ears.
First things first, what is sag? The simple answer; sag is distance that your rear shock compresses when you are sitting or standing on the bike in a stationary position. I know not really very useful, but here is the interesting part. That amount of rear shock compression has a huge impact on how much pressure is on the front wheel. Which in turn has a big impact on whether or not your front wheel is going head West while you are turning East, causing your head to go South right into the ground (lovingly known as "washing out the front end"). It also effects how your bike handles in a straight line. If you are getting a "death wobble" going straight it may have to do with a sag adjustment (but don't assume that's the issue, check the wheel and stem bearings before your next ride). Another way to look at sag is how level your bike sits with you (and your gear) on it.
Now that you have an idea what sag is, the next question is how do you adjust it? The process is pretty similar for most modern bikes. The specifics can vary quite a bike from one bike to the next, so keep that in mind as you read. To start with, a rear shock on a motorcycle is not the same as a shock on your average car or truck. A motorcycle shock has two parts, an adjustable shock absorber and a coil spring. The coil spring is the part we want to take a look at, the rest of the shock is for another blog. The coil spring is usually held in place by some sort of ring that can be screwed up or down to adjust the amount of pressure being put on the spring (known as the pre-load). That ring often has another ring above it which is known as the lock-ring. The lock-ring's only job is to keep the adjusting ring in place once it has been set. For sake of time, we are going to pretend the lock-ring doesn't exist once it has been loosened and moved out of the way.
The first thing you need to know before you begin is how much sag you want. This is the tricky part. Your owner's manual will have a number, when you start reading forums you will see other numbers. Often people refer to this as "race sag". On most modern dirt-bikes that number is somewhere around 100 mm. This is a good place to start, but by no means the golden ticket for everyone. Once you have a number in mind, it's time to find, rent or steal a friend; get your handy tape measure and put on your riding gear (including your backpack if you use one). You also need to find a hard, level place and something you can lightly lean your handlebars against while you are on the bike.
Once you have everything in place (BTW this is not the part where you start drinking beer with your buddy); put the bike on a stand and measure from the rear axle to the fender (keeping the tape measure vertical) and record that number (this is call the "unloaded dimension"). Next take the bike off the stand and hold it upright, pull up on the rear fender and let go (don't bounce the bike), measure from the rear axle bolt to the finder, making sure the tape measure is vertical. I typically put a small mark on the fender so I know I am measuring to the same spot each time. Once you have that measurement, write that number down. Subtract small number from the big number; this is known as your "Static Sag".
Next get on the bike, with your gear on; keeping the bike upright and standing on the pegs (or sitting slightly forward) have your buddy (assuming he took my advice about the beer) measure from the rear axle to the same spot on the fender; record that number, this is the "loaded sag". Subtract the loaded sag from the unloaded dimension and that will give you your "race sag".
Now that you have your race sag and your static sag you can start the adjustment process. In very general terms, your static sag should be 30-40 mm. Your first step is to adjust your race sag by turning the lock ring either tightening or loosening the pre-load depending on which direction you need to go. It will most likely take a few tries before you get the number you are looking for. Once you have your race sag set check your static sag again. If your static sag is way off it's an indicator that you need a different shock spring. Most bikes are designed for 180 pound riders with minimal gear. They are not generally setup out of the box for a rider with gear or if you are like me a heavier rider with gear.
It's a good practice to check your sag periodically throughout the year. If you are racing it's a good idea to check it before every race. This also a good time to take a close look at the bearings in the rear-end and shock. Once you have done all that, go ahead and grab a beer, relax and tell each other lies about that monster hill you climbed or just barely avoided an agonizing death on that cliff-edge.
Some final thoughts on sag. Your race sag is an important adjustment that shouldn't be ignored. It's not a bad idea to ask around and see what other people are running on their bikes. Most articles that give a specific numbers are written with track riding in mind. Depending on if primarily sit or stand while riding, what type of riding you do (trail riding, hare scrambles, etc.) and what type of terrain you ride can have an impact on your best race sag number. You may find that 90 mm race sag makes your bike handle much better than 100. Don't be afraid to experiment. Finally, if you haven't had your rear shock serviced recently it's probably time, not enough pressure in the shock can throw your sag numbers off and cause poor handling.