BONES BACON ON SUSPENSION: HOW TO PROPERLY SET SAG
By Bones Bacon
The one thing that every rider has done or tried to do is set his race sag. Sounds easy, doesn’t it? Yeah, right. If you lined up 10 race team mechanics and asked them to set the sag, the result would be 10 different numbers. How could something so simple be so complicated? First, the theories on how to do it vary from sitting down to standing up to pushing down to pulling up to feet-on-the-pegs to feet-off-the-pegs and so on. Second, the multiple variables that affect the sag are the temperature of the shock, whether the ground is level or slanted, where the rider sits on the seat, rear axle position, full gas tank or empty gas tank, greased or dry linkage bearings‚Äîwant me to keep going?
Race sag is very critical to the overall balance of your bike and how it handles. It is very important that you do it correctly and consistently to keep the proper amount of load on the front of the bike for the track conditions you are riding. For example, a lower race sag may work better on sand tracks because it takes the load off the forks, which slows down the steering and calms the shock. Taller race sag might be best for hard-pack conditions, because it puts more load on the front for more precise steering while freeing up the shock’s action for the choppy bumps.
“ON AVERAGE, ONE TURN OF THE SHOCK COLLAR EQUALS 3MM OF RACE SAG. IF YOU WANT TO HAVE 110MM OF SAG, LOOSEN THE SHOCK COLLAR.”
I accept that there are different theories on how to set race sag, but I believe in my own method. Truthfully, consistency is what’s most important, even if you don’t follow my advice. I start with the bike on its stand. I use a small metric tape measure to measure from the rear axle up to the rear fender‚Äîtrying to follow the arc of the rear wheel. Typically, the juncture where the fender and side panel meet is a convenient spot. It helps to put a Sharpie mark on a spot that lines up with an even number on the tape measure. Even numbers make the math simpler. Be consistent with where you are measuring, because different tape measure angles will produce different numbers. Then, I remove the bike from its stand and set it on level ground. The rider should have all of his gear on, sit where he actually rides (normally directly over the footpegs), with his toes lightly touching the ground, the back of his boots contacting the front of the footpegs, and his hands on the handlebars. If you have a second person hold the front of the bike so the rider can lift his toes off the ground, that’s even better. Now, repeat the first measurement (from the exact same spots on the axle and rear fender) and subtract the second number from the first number. If the first measurement was 610mm and the second measurement was 505mm, you have 105mm of race sag. If you want 100mm of sag, tighten the spring collar to compress the sag and raise the rear of the bike. On average, one turn of the shock collar equals 3mm of race sag. If you want to have 110mm of sag, loosen the shock collar.
What does it all mean? Let me give you some basic examples of why you would run more or less sag. If the front of your bike feels very vague, light and washy in the corners‚ especially when applying the throttle, you could help alleviate this problem by changing your race sag. If you have your sag on the low side (say 110mm), tightening the spring collar one turn and raising the sag will put more weight on the front end and help the front tire bite a little better. The opposite would be if your front end were diving in the turns and oversteering on exit while the rear end felt light under braking or loose in flat corners. In this case, your race sag might be too high, so loosening the spring collar one turn would transfer some weight to the rear of the bike and relieve it from the forks.
Here are some tips. (1) Keep track of how many turns you move the collars, and the next time you check sag you will know the correlation between the number of turns on the spring collar and the number of millimeters of race sag per turn. (2) How much you weigh affects the sag number, so if you are a binge eater, be forewarned. (3) Adding fuel to your gas tank can lower your sag (a full tank of gas weighs 12 pounds). (4) If you move the rear axle forward to accept a larger rear sprocket, you will take leverage away from the swingarm and the sag will rise. (5) Use caution when working around the shock immediately after riding the bike. It will be very hot. (6) Clean the shock collars before working on them and cool the shock or you will run the risk of galling the threads on the shock body.
Race sag is a valuable tuning tool. It can bring your bike into balance, which is more than half the battle in getting it to work.
Jim “Bones” Bacon has tuned the suspension of the biggest names in motocross, including Jeremy McGrath, Ricky Carmichael and Ryan Villopoto. If you have a suspension question, send it to [email protected].