ASK THE MXPERTS: WHAT’S THE DIFFERENCE BETWEEN RACE & FREE SAG

Dear MXA,
       I know very little about setting up the suspension. I tend to just ride my bike and live with whatever happens. But, as of late I’ve started to read MXA and realize that stupidity is not a positive attribute. I want to start with the basics and learn more about making my bike better. What do I need to know about setting the sag? And what is the difference between race sag and free sag?

Setting sag is the one thing that every rider thinks he can do but normally does wrong. You would think that making two measurements and subtracting one from the other would be simple. Not so! Here are some tips.

(1) Do not measure the sag with the rider standing up. (2) Bounce on the suspension before measuring. (3) Sit where you actually ride, not some dream position. (4) Measure from the exact same location every time.

How do you measure sag? Put your bike on a stand with the rear wheel off the ground, and measure from the rear fender to the rear axle. Write that number on your buddy’s forehead. Next, remove the bike from the stand and climb on board. Once you are positioned on the bike, have your buddy make the same measurement again. Subtract the new number from the one written on his forehead. The result should be 4 inches or 100mm, whichever comes first. Some bikes, most notably KTM work best with 105mm, but you can choose your own race number number by experimenting.

What should you do if it isn’t 100mm? Loosen the shock’s preload ring and turn the collar in the proper direction to make the spring stiffer or softer. Then, measure again and again until you get the desired number.

When to check sag? We know this sounds like a broken record, but that is better than a broken bone, check your sag before each race! It changes. One caveat: do not check the sag when the shock is hot — and it gets hot very quickly. If you’ve just ridden your bike in the first practice and think that you should check the sag before the second practice, save yourself the trouble. The hot shock will not give you an accurate measurement. Wait until the shock cools down. You can use sag as a tuning tool. Jeremy McGrath liked to set his sag at more than 100mm. Why? He wanted the rear shock to feel dead in the whoops. On the other hand, setting the sag at less than 100mm can make your bike handle quicker in the turns, because it raises the rear of the bike, which steepens the head angle. As a rule of thumb, every MXA test rider stays between 100mm and 105mm.

What is free sag? Free sag is a measurement of how much the bike sags under its own weight (without you on it). Free sag can only be checked after race sag is set.

How do you check free sag? Once you have the preload set at 100mm, or your favorite number, for your weight, position the bike on level ground, grab the bike under the rear fender and lift up—like you are going to pick the bike up off the ground. Did you feel the shock move? How far could you lift the rear fender up before the rear wheel tried to leave the ground? Was it 25mm? Was it 3mm? The amount of slop in the rear end is called “free sag.”

How much free sag should there be? Free sag should be between 30mm and 40mm. If you have more than 40mm of free sag, your shock spring is too stiff. Less than 30mm and your spring is too soft. This is so important and so basic that you need to repeat after us: if you have excess free sag, your shock spring is too stiff. Now ask yourself, why? That’s simple; you haven’t been eating enough. Gain weight and you will have to preload the shock spring more, which will take up the excess free sag. If you don’t want to pork up, you should consider a lighter shock spring. If you have very little free sag (less than 30mm), your shock spring is too soft. Why? Because to hold your body up, the shock spring has to be preloaded way down into its stroke. It is overstressed, so overstressed that it is working overtime to hold up the bike; thus it has no jangle left in the rear end. The solution? Buy a stiffer shock spring (or go on a diet).

What if you have less than 30mm of free sag? Many times a rider will set the race sag at 100mm and find out that he has no static sag at all, but he wants to race the bike anyway. What should he do? Back the race sag off until there is at least 10mm of static sag. Free sag trumps race sag in this extreme condition.