As basic as it may seem, one shock spring does not fit all riders. The MXA wrecking crew wants to help you find the correct shock spring for your bike—without you having to spend money, take advanced college mathematics or seek help from a fifth grader.
One thing we know for sure is that if you weigh more than 185 pounds or less than 150 pounds, the odds are good that the spring on your box-stock motocross bike is not right for you.
HOW CAN THE SPRING ON YOUR BIKE BE WRONG?
The answer is simple: Honda, Yamaha, Suzuki, KTM, Kawasaki and Husqvarna didn’t build the bike for you; they built it for their “target audience.” And sadly, that isn’t you—in terms of weight, speed, riding style or track conditions. Once you accept that the shock spring on your bike was designed for someone else, you can go about finding the right one for you.
HOW TO GET THE BASE MEASUREMENT
Before you can accurately measure race sag and free sag, you need to put your bike on a stand—so that the rear wheel is in the air—and take a measurement from the edge of the rear fender to the hole in the rear axle. Don’t take this measurement in a straight up-and-down alignment, but with a slight forward angle that mimics, to some degree, the movement of the swingarm and rear wheel. Once you have this measurement, write it down. This is the “base” measurement.
HOW TO SET YOUR RACE SAG
Setting race sag is something that every rider can do—even if he does it wrong. All you have to do is sit on the bike and have a friend measure from the fender to the rear axle and then subtract this number from the “base” measurement. Typically, the difference between these two numbers should be 100mm (give or take 5mm).
HOW TO MEASURE STATIC SAG
Once you have the race sag set for your weight, you need to get off the bike and get ready to measure the static sag. Static sag is a measurement of how much the bike sags under its own weight. Static sag can only be checked after race sag is set. With the bike sitting under its own weight on level ground, take another brand-new measurement from the fender to the axle. Then, subtract this measurement from your “base” measurement. The difference will be the static sag number.
HOW TO USE STATIC SAG TO CHOOSE A SPRING
Static sag should be between 30mm and 40mm. If you have more than 40mm of static sag, your shock spring is too stiff. That may sound wrong, but it is right. Conversely, if you have less than 30mm of static sag, your spring is too soft.
SPRING RATE BASICS
“Spring rate” is a numerical measurement of the stiffness of the spring and is measured in kilograms per millimeter (kg/mm). The printed number in the owner’s manual is the spring rate for the first inch of travel. A 5.4 kg/mm rating doesn’t represent how stiff the spring is at the end of its travel, only in its first inch. As a spring is compressed, it gets stiffer and stiffer; thus a 5.4 kg/mm spring can be an 8.4 kg/mm spring before its effective coil length is used up. Logically, putting more preload on a spring does not make the spring stiffer at the end of its stroke, only in its first inch of travel; thus putting more preload on a 5.4 spring does not necessarily make it a 5.6 kg/mm spring.
If you have to go stiffer—or lighter—you can’t do it with preload; you need a new spring.