Sag Explained

Sag is measured from a fixed reference: the distance from the axle to a point on the chassis with the wheel hanging fully extended. The same distance is measured again with the bike on its wheels, and again with the rider aboard. The difference from the unloaded figure to each loaded figure is the sag.

Two readings come out of it, and both matter:

• Static sag is the bike settling under its own weight.
• Riding sag is the bike plus the rider in full gear. The manuals read for this guide take it with the rider seated, feet on the footrests, after bouncing a few times to let the suspension settle.

Manufacturers specify sag in millimeters. Divided by the travel, it becomes a percentage, which is what lets one bike be compared with another. Across nine manuals from two independent manufacturers, rear riding sag works out to between about 29% and 37% of rear travel, and static sag to between about 11% and 13%.

That is a band, not one number. Husqvarna and KTM sit at 33–37% and Sherco at 29–30%. The figure is also set in millimeters, not as a ratio: Husqvarna and KTM give the same 110 mm riding sag on bikes with 300 mm, 310 mm, and 330 mm of rear travel, so one figure works out to anywhere from 33% to 37%. The percentage is a way to read and compare specs, not a rule any manufacturer states.

The extra distance the rider pushes the suspension down is set by the rider's weight and the wheel rate, not by preload. Preload shifts where the spring starts, which moves static sag and riding sag together by about the same amount. It does not change the gap between them. Only the spring rate, through the wheel rate, sets that gap. (This is exact for a wheel rate that stays constant. On a linkage bike the wheel rate changes through the travel, so "the same amount" is close rather than exact.)

The consequence is that preload can bring riding sag into its band on almost any spring. On the wrong spring, static sag then lands outside its band, or the preload needed runs past the adjuster's range. The pair of readings, not either one alone, shows whether the spring suits the rider.

The manufacturers build their procedures around this. Husqvarna and KTM set the spring by a rider-weight chart, with preload covering small weight differences and a spring change for large ones. Sherco states it most directly: if static sag is wrong, adjust preload; if riding sag is wrong, change the spring.

Sag decides three things at once.

How the travel is split. At rest, the wheel has the sag's worth of travel to extend into holes and to follow the ground as the chassis rises, and the rest of the travel to absorb hits. Sag at a third of the travel leaves a third to extend and two thirds to compress.

Where the suspension recovers to. After a hit, the spring pushes the wheel back toward its resting point, and rebound damping slows the return. Sag sets where that resting point is. Preload moves the resting point but not the push back toward it: that push comes from the spring rate, through the wheel rate. A rear set too deep on the right spring returns as quickly as before, to a lower point. A rear on a spring that is too soft returns more slowly at the same rebound setting, even with preload bringing its sag into range. This is analysis from the force balance, exact for a wheel rate that stays constant and approximate on a linkage.

The chassis attitude. Rear sag sets the height of the rear of the bike. A lower rear tilts the chassis nose-up, which slackens the head angle and lengthens trail. A higher rear does the reverse. The direction is geometry and holds on any bike; how far the head angle moves depends on the model and its generation.

Sag can be wrong in either direction, and each direction has its own profile.

Too deep (too little preload, or a spring too soft). The rear starts each hit lower in its travel, with less stroke left to absorb it, so it reaches the end of the travel sooner. When the next hit arrives before the wheel has returned, that hit starts deeper still, and over a run of hits the rear squats lower and the stroke left shrinks. A spring that is too soft adds a slower return on top of the lower start. On the trail it presents as the rear squatting and not coming back up over repeated hits. The steering slackens as the rear goes down.

Too shallow (too much preload, or a spring too stiff). The rear sits high, with less room left below the resting position for the wheel to drop into a hole or follow the ground away from the chassis. A spring that is too stiff also returns faster and resists small hits more. On the trail it presents as a bike that is harder to control and skips over terrain instead of staying on it and keeping traction. The steering quickens as the rear rises.

Sag sets the starting point, and the spring rate sets the force returning the suspension to it. Rebound damping meters how fast the suspension returns. Compression damping decides how deep each hit goes, and so how far the suspension has to recover. Squatting over repeated hits can come from sag set too deep, a spring too soft, rebound damping too slow, or a combination. A rear that kicks can come from sag set too shallow, a spring too stiff, or rebound damping too fast. A spring or sag change is therefore followed by a rebound check: after a spring change, the manufacturer's order is static sag, then riding sag, then rebound damping.

The fork does not get a sag figure. Husqvarna and KTM state that no exact riding sag can be determined for the fork, and Sherco gives a sag figure for the rear shock only. The fork is set by its spring rate, chosen from the same kind of rider-weight chart, with spring preload covering smaller differences in rider weight. A fork that bottoms frequently needs harder springs. Why spring rate rather than sag sets the fork is covered in the spring-rate guide.

The manufacturer's figure is a starting point inside a band, not a target to hit exactly. One reason riders settle at different points is that sag is measured in one position and ridden in another. The manuals measure riding sag seated, feet on the footrests. A rider who rides standing, further back on the bike, puts more load on the rear than the measuring position does. (Analysis: the static load on each axle follows where the combined mass of bike and rider sits between them.) Terrain is another reason: deeper sag leaves the wheel more room to follow uneven ground, at the cost of stroke for big hits.

A worked example from Two Blips' own setup, in both directions. The 2018, 2019, and 2021 Husqvarna FC 450 manuals specify 105 mm of riding sag on 300 mm of rear travel.

• Deeper, for rocky terrain. Set at 110 mm, about 37% of the travel, the rear presents as softer and more compliant over rocks. The spring rate has not changed. What has changed is that the wheel has more room to drop off rocks and into holes, so it stays on the ground more of the time, and the lower rear slackens the steering. It likely also feels softer under throttle because the lower rear flattens the swingarm, so the drive force does less to hold the suspension up and the rear stays freer to move over rocks. The cost is stroke: big, fast hits reach the end of the travel sooner.
• Shallower, for steep climbs. Set below the 105 mm spec, as low as 100 mm (about 33–35% of the travel), the rear tire presents as staying pinned on steep climbs where the rider has to lean forward. A steep climb and hard acceleration both shift load onto the rear wheel, which pushes the rear deeper into its travel and tilts the chassis further nose-up. A rider who stays too far back adds to that and lightens the front. Less sag holds the rear higher, keeps more stroke in reserve, and keeps the chassis closer to level, so the rider can stay forward over the bike. The pinned feel is likely due to the swingarm angle. With the rear held up, the swingarm slopes more steeply down from its pivot to the axle, and the drive force at the rear tire then pushes the suspension toward extension instead of letting it squat. This is anti-squat. The higher rear also steepens the head angle and quickens the steering, which helps the front hold a line on a steep climb. The cost is less room for the wheel to follow the ground, and less stability where the trail turns fast.

Both feels are explained here as analysis: how much the swingarm angle changes anti-squat depends on each bike's pivot position and chain line, which the anti-squat guide covers. Each setup leans toward one kind of terrain and gives something up on the other. Neither is a figure to copy.