The average rider can enjoy a dirt bike fresh off the showroom floor until skill and experience warrant adjustments to the bike set-up that matches their comfort and allows them to push their boundaries.
However, one aspect to the bike, regardless of make and model, that riders ignore or think works just fine, involves the suspension. Dirt bike suspension setup includes a number of components that affect the front and rear of the bike. A poor suspension set up reduces your ability to control the bike and eventually leads to fatigue, slower lap times and/or hitting the dirt.
Of the various components to dialing in the suspension, most riders overlook setting the sag, one of the most important aspects that impacts the bike's handling. This most simple process looks a bit complex on paper, but of everything involved with the suspension, it takes little time (most of it test riding) and a few adjustments using a spanner wrench.
Setting the sag matches the rider's weight and riding style to how much the suspension compresses, therefore, a bad sag setting results in riding on a brick wall or a trampoline. For a radical example of sag setting, check out photos of a rider landing a jump where the rear fender squishes down close to the rear tire. Too soft, the suspension pops you off the seat. Too hard the bike lands like a rock. A properly adjusted sag impacts performance, comfort and safety.
Two Types of Sag
When it comes to setting the sag, you have static sag (or free sag) and race sag. Static sag adjusts compression with the bike on a stand without the rider. Conversely, race sag adjusts compression with the rider on the bike in full riding gear. This fine tuning determines how well the dirt bike absorbs bumps, maintains traction and corners. Generally, on faster tracks run more sag to help stabilize the bike and on technical tracks less sag helps the bike turn better.
Using Motool Digital Scale to Measure Sag
How to Measure Sag on a Dirt Bike
First measure the static sag. The Motool Slacker Digital Sag Scale makes this easy process, easier. A sag scale from HP Tools costs far less and might remind you of geometry class but also an easy alternative. Otherwise, grab a tape measure (plus pen and paper if you failed geometry or simply can't remember things) and with the bike resting on a traditional stand measure the distance from a fixed point on the swingarm directly above the rear axle to a point on the rear fender or frame directly above it. This measures the dirt bike's fully extended suspension length. Now sit on the bike in full riding gear in normal riding position. You'll need a friend to hold the bike and measure the racing sag as above using the same fixed points.
The difference between the race sag and static sag represents your current race sag. Depending on riding style, track conditions and rider preference, race sag between 100mm and 105mm works for Motocross and up to 110mm for off-road and enduro. Once you dial in the race sag, measure the static sag again which should measure between 25mm to 35mm, again rider preference plays a role. Changing the sag measurement requires an adjustment to the preload.
Measuring Sag with measuring tape
Adjust the Preload
Adjust preload by tightening or loosening the spring on the shock absorber. An increase in preload (tightening) decreases sag (raising the rear end) while decreasing preload (loosening) increases sag (lowering the rear end). Use a spanner wrench to tighten or loosen the shock absorber. Just a few turns should do it. For a quick adjustment at the track we recommend a spanner punch which turns hard-to-reach lockring and preload ring on the rear shock.
Using the above measurements as a guide works for most riders, especially on trails. Those who race regularly or want to take their skills to the next level should experiment with different sag settings to find the best measurement for their bike, riding style and riding conditions. Investing in that Motool Digital Sag Scale now might be a good idea. Eventually, a practice lap around a Motocross track gives you enough detail to make a quick sag adjustment in the pits prior to gatedrop.
If you cannot make the necessary sag adjustments to fit in between the recommended sag settings or adapt to your comfort, consider changing to a softer or harder spring. Like routine maintenance, checking the sag often keeps you within preferred measurements. Conversely, if you cannot get the right pre-load (check manufacturer's recommended settings) to fall in line with your desired sag settings you also need a different spring (softer or harder) depending on your weight while wearing gear.
Keep in mind, changing the sag often affects other elements of the suspension like the damping setting. And you thought riding dirt bikes would be fun! You certainly can get deep in the weeds on adjusting this and that to the suspension but ultimately you need to find the adjustments that work for you, even when measurements sit outside general guidelines, and finding a suspension setup that feels intuitive and responsive to enhance the riding experience.
