By Bones Bacon
Static sag (also know as “free sag”) is a very useful tool in selecting the proper spring rate’if you understand it correctly. Over the years, I have seen it all. Riders would come to my truck and tell me that their bike wasn’t handling right, but when I lifted their bikes up to put them on the stand, I realized that their rear shock springs were completely topped out. They had no static sag whatsoever. Just as often, I would grab the rear fender to lift a bike and feel like I had to lift it a country mile before it topped out. They had way too much static sag
Static sag is important. How important? First and foremost, static sag, when set properly, can help you determine if you need a stiffer or softer shock spring. And, it goes without saying that a bike with too much or too little static sag is not going to work up to its potential.
“IF YOU ARE ALONE, YOU CAN LEAN IT LIGHTLY AGAINST SOMETHING TO HOLD IT UP (PREFERABLY NOT YOUR MOM’S OR WIFE’S LEXUS).”
Static sag is simpler to measure than race sag because there is no rider input involved’although it is best to set your race sag before measuring the static sag. To measure static sag, take your first measurement with your bike on its stand, just as you do when you set your race sag. Get an accurate measurement from your rear axle upward (at a slight angle in the direction the rear wheel’s arc) to the junction between the fender and side panel. It’s best to mark your fender at an even number (say it is 630mm) and write it down. Next, take the bike off the stand and have someone hold the bars. If you are alone, you can lean it lightly against something to hold it up (preferably not your mom’s or wife’s Lexus), but it’s best to elicit the help of a friend to hold the bike. Push down on the seat to compress the shock slightly and then let it rise to its natural height without help. Now, take the measurement again (say it is 600mm this time). Subtract the second measurement from the first. In our sample, it would be 630mm – 600mm = 30mm.
What do you do with this number? First of all, be glad that your second number isn’t the same as your first, because that would mean that you either have to lose a lot of weight or your spring rate isn’t in the ballpark. As a rule of thumb, for big bikes static sag should be between 30mm and 40mm. If after setting the race sag your static sag is more than the recommended 40mm, your spring may be too stiff for your weight. In this case, the spring is not compressed enough to allow the suspension to extend far enough on its own. A spring that’s too firm does not allow the rear tire to hook up under acceleration and transmits more bump energy into the rider.
If the static sag is less than 30mm in the rear, the spring may be too soft for your weight. In this case, the spring requires so much preload to achieve the proper race sag that it makes the rear suspension closer to being topped out. As a result, the weight transfer is incorrect and the rear end tops out under even light braking into corners or down hills and may feel loose and wallowy, especially when accelerating in flatter turns.
Remember, this is just a guideline. For Supercross, I often push the static sag past 40mm, while for heavier or taller riders I may fudge the number lower. Another example of when I might recommend straying from these guidelines a little is when I worked with James Stewart. I was able to get away with more static sag because James rides very far forward on the bike and rarely loads the rear too much. There are also light riders who can get away with more static sag. For example, Blake Baggett gets the same benefits of more static sag as James Stewart, even though he is lighter and rides in a more neutral position. On the other hand, Ryan Villopoto loved to move around a lot on the bike, steer with the rear wheel and hammer outside turns. He could get away with less static sag on his shock spring.
Once you understand the correlation between race sag and static sag, and you feel like you’re close to having the correct rear sag rate, you can then more easily pick your front spring rate or air pressures. Remember, for whatever kind of riding you are doing, and whatever kind of rider you are, static sag is a helpful guideline to make your bike feel better balanced.
Jim “Bones” Bacon has tuned the suspension of the biggest names in motocross, including Jeremy McGrath, Ricky Carmichael and Ryan Villopoto. Bones has a rehular column in MXA every month—where he hands out valuable advice. If you have a suspension question, send it to firstname.lastname@example.org.