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The second installment in our suspension articles will cover springs and their importance in proper handling on a motorcycle. Lets start out by squashing a myth. Springs do not help bottoming resistance. They do not make a bike stiff or plush. They do not give you more lift off of a jump. Their only purpose is to set ride-height, nothing more, nothing less.
I used to believe that springs did so much more, but as I studied suspension from an engineering standpoint, their function became more clear to me. A spring technically is a device to absorb energy. The problem is that all the energy a spring absorbs, it gives it back. A spring left uncontrolled will oscillate, or cycle up and down. An example would be a shock without oil. You push down 4 inches and let go, the bike not only returns back to its start position, it trys to rise another 4 inches, then comes back down to 3 1/2 below center, rises to 3 inches above center, so on and so on until it finally settles back at center. This is why we need hydraulic dampening-to control the springs oscillations. We will cover this aspect in our next article.
Each bike was designed with a certain geometry. The only way a bike will handle as it was designed is to have the ride height set properly. This requires not just a spring, but one with the correct strength. There are two numbers that need to be recorded to know if you have the correct spring or not. This applies to both the shock and the forks.
The first number is race sag. To get this number you place the bike on the stand, take a measurement from the rear axle to a spot on the rear fender straight up from the axle. Mark this spot with a sharpie and write down the length. Next have the rider with full gear get on the bike, stand in the attack position. A third person will be needed to balance the bike from the front without adding their own weight into the equation. Take another measurment from the same two points. The difference from the first measurement and second is your race sag. General guidelines are as follows: full size MX bikes = 100mm sag. 85cc bikes = 85mm. 65cc bikes = 75mm. If your numbers are less than this you need to lower the shock by loosening the threaded collar, and check again. If your number is larger than this, you need to tighten the collar to raise the bike up.
Now that your race sag is set, one more measurement has to be made. Let the bike sit on the ground under its own weight. No rider. Measure from axle to your mark again. This number is subtracted from your original measurement. This is your static sag. Every bike should be between 25-35mm. (Except for KTM).
If your static sag is less than 25-35mm that means your spring is too soft. It required you to tighten the spring too much to get your race sag correct. This scenario will create a situation where your bike will be prone to headshake under braking, the rear end will kick up on braking, and will also have the back end spring up higher coming off of jumps. The reason is that the hydraulic part of the suspension will have a harder time controlling the additional stored energy of the overly compressed shock spring. This tranfers too much weight to the front end. You always want to acheive a balance from front to rear.
Conversely, if your static sag is larger than the recommended amount it means your spring is too stiff. This will result in a ride that will be harsher than it needs to be. You will not get much lift off a jump. It will not turn because it will not transfer enough weight to the front end when braking into a corner.
You always want to balance the fork springs with the shock spring.Race-tech and Mx-tech offer online spring generators. they are usefull tools but they vary greatly, which is why I suggest you learn to check this for yourself. So what do you do when your numbers are all correct and it still doesn't handle like you want it to? Lets play with the geometry of the bike. Lets say the bike doesn't turn sharp enough, you could raise the rear sag a few millimeters to transfer more weight to the front end. This also has the added effect of raking in the front forks because of how it lifts the back, this will make the front stick better and turn sharper, but it makes it less stable at speed. Now lets say the front end of your bike bites in too hard and is real twitchy, you would want to lower your sag a few mm to compensate for this. Tight, technical tracks usually benefit from raising the sag. Sandy tracks, or highspeed tracks, benefit from lowering the rear.
Proper spring selection and setup is half of the equation. Maybe more. The best valving in the world is useless without the right springs and the correct sag measurements, but any bike is pretty good with a proper spring setup, regardless of the valving specs. In my opinion this is the first thing that should be addressed on any motorcross bike. It usually takes a backseat to a shiny new exhaust, but is for more important in terms of performance and safety. Take some time and check your sag numbers, I promise it is worth the time.
The next article will cover the hydraulic part of the suspension. A good understanding of the springs role is a prerequisite to understand the valving so read this article twice if you need to, and feel free to ask questions.
Chad Rager
*Moto-Tech Performance Industries*

SUPERCROSS 2010 ATLANTA GA