Understanding Your Suspension Sag with Dr. Dirt

Originally posted on September 13, 2012.

The most important thing you can to do to improve the handling of your bike is to set your sag. When you sit on your motorcycle your bike 'sags' or compresses under your weight. The amount of sag affects handling by changing the ride height of your motorcycle fore and aft. Understanding how sag works lets you fine tune your bike so it will perform it's best.

To determine rider sag put your bike on a stand with the wheels off the ground and measure the distance from the rear axle to a fixed point near the rear fender/side panel. The fixed point should be along the arc that the axle travels when the shock is compressed.

Stand on the footpegs as it gives a more consistent measurement than sitting on the seat. Bounce lightly on the bike to overcome stiction and measure the compressed distance. The difference between the two is the rider sag. The correct amount is approximately 33% of the bikes total available travel, usually between 95-115mm on full size bikes. The correct amount varies because of bike geometry, rider preference, and type of riding. The rider sag should be set with the rider in full gear. The bike should have correct levels of fluids, be free of excess mud, and the shock should be cool to the touch as heat build up changes sag levels.

Once you have determined your rider sag it is a good idea to check your free sag. The free sag is the amount the bike compresses under its own weight. The free sag determines if you have the correct spring rate for your weight. On full size bikes this can range from 25-45mm. If you are towards the extremes of this range you should consider a different shock spring. If you have around 25mm or less free sag your spring has too much preload on it to get the proper rider sag and your spring rate is too soft. If you have around or more than 45mm you don't have enough preload on your spring and should consider a softer spring.

On most bikes you change the sag of your bike with a hammer and punch. After loosening the shock springs lockring you can turn the spring preload ring to increase or decrease sag. Usually 1 complete turn changes the sag by about 3mm. If your shock is hot from riding it is best to note sag changes by the amount of turns in or out since the rider sag was set. For example, rider sag at the beginning of the day is 102mm and the best setting is 1 turn softer than that.

It is to your benefit to test different sag settings for various conditions as it has a huge cause and effect. Decreasing the sag of your bike generally makes it quicker handling but reduces high speed stability. Decrease sag on tight courses to improve steering accuracy and in muddy conditions to handle weight build up. Increasing the sag of your bike usually makes it more stable but decreases front end traction and cornering ability. Increase your sag on high speed and sandy tracks to improve stability. Too much sag will cause your front end to be too light and deflect off bumps and too little sag will cause your shock to be too stiff on bumps. A soft spring will initially ride high in the rear because it needs to be preloaded more than a stiffer spring. Conversely, a stiff spring will have a lower initial ride height but will feel more firm on bigger bumps because of it's rate. Changing rider sag can also correct a motorcycle chassis if it is unbalanced. A bike that has a stink bug stance can be leveled out by increasing sag to produce an even, predictable ride height. You should check your sag before every ride or as a minimum at least once a month.