Sag is how far the suspension compresses under the weight of the bike and the weight of the rider. It sets where the suspension sits within its travel, and that in turn sets ride height, steering geometry, and how much travel is left to soak up bumps. Setting sag comes first in any suspension setup, because damping adjustments only make sense once the bike is sitting at the right height. The job needs a tape measure, a helper, and about half an hour.
Static sag and rider sag
Static sag, sometimes called free sag, is how much the suspension compresses under the weight of the bike alone. Rider sag, also called race sag on dirt bikes, is the compression with the rider aboard in riding gear and in a normal riding position. Both are measured as the difference between the fully extended length and the loaded length. Rider sag is the number you set with spring preload, and static sag is the cross-check that tells you whether the spring rate suits your weight.
Measuring rear sag
Put the bike on a stand so the rear wheel hangs free and the suspension is fully extended. Pick a fixed point on the frame or tail section directly above the rear axle, and measure from the center of the axle straight up to that point. Mark both spots with tape so every measurement follows the same line, and write the number down as the extended length.
Take the bike off the stand and let it settle on its wheels while someone holds it upright, then measure again for the static number. Next, have the rider sit on the bike in full gear with feet on the pegs and hands on the bars, with a helper balancing the bike from the front, and take the third measurement. Subtract each loaded measurement from the extended one to get static sag and rider sag.
Friction in the suspension makes the bike settle at a slightly different height depending on whether it's sinking or rising. For a more accurate number, push down on the seat and let the bike rise slowly before measuring, then lift it and let it settle slowly and measure again. The average of the two readings is your sag.
Measuring front sag
The front works the same way, using the fork. On a conventional fork, measure the exposed fork tube between the dust seal and the bottom of the lower triple clamp, and on an upside-down fork, measure between the dust seal and the axle bracket. Take the extended number with the front wheel off the ground, then repeat with the bike on its wheels and again with the rider aboard. A zip tie snugged around a fork tube against the seal is a handy extra check, since it gets pushed along by the seal and shows how much travel the fork used on a ride.
What the numbers should be
Your owner's manual or the suspension maker's setup guide gives the best starting point for your bike. As a general reference, many street setups put rider sag at somewhere around a quarter to a third of total suspension travel, and long-travel off-road bikes typically run more sag in absolute terms. Riders focused on the track often run a little less, and touring riders carrying a passenger and luggage need more preload to keep the same ride height. Treat any general figure as a starting point and adjust from there for how the bike feels.
Adjusting spring preload
Preload changes how much the spring is compressed before the bike's weight goes on it. More preload raises the bike and reduces rider sag, and less preload lowers it and increases sag. Preload doesn't change the spring rate, so a soft spring with a lot of preload still behaves like a soft spring once the suspension is moving.
Rear shocks adjust preload with a threaded collar and lock ring, a stepped ramp collar, a remote knob, or on some bikes an electronic setting. Collars usually need a spanner wrench, and the manual lists the range of positions. Many forks have a preload adjuster on top of each fork cap, and some designs need internal spacers changed. Adjust both fork legs by the same amount, and recheck sag after every change.
When the spring is wrong for your weight
Static sag tells you whether the spring suits you. If you have to wind in nearly all the available preload to reach your rider sag target, static sag will come out very small, which is a sign the spring is too soft for your weight. If you reach the target with almost no preload and static sag comes out large, the spring is too stiff. Either way, the fix is a spring of a different rate, sized to rider weight, bike weight, and the kind of riding you do, because damping adjustments can't make up for a spring that's the wrong rate.
After sag, damping
Once sag is set, the compression and rebound damping adjusters fine-tune how the suspension moves through its travel. Start from the manual's standard settings, change one adjuster at a time by a couple of clicks, and ride the same stretch of road after each change. Keep notes on the settings that work for solo riding and for riding loaded, so you can return to either one.
Tire pressure changes how the bike feels too, so set pressures cold before any suspension session, using the method in the chain, belt, and tire inspection guide. If an exhaust or fueling change is also on the list, the intake and fuel tuning guide covers that side of the bike.
Specs in this guide are general. Fluid grades, capacities, torque values, and service intervals vary by model and year, and the owner's manual or factory service manual for your exact machine takes priority.