Spring Rate Explained

Spring rate is the force required to compress a spring by the unit of distance named in its rating. A spring rated 45 N/mm, 45 newtons per millimeter, requires 45 N of force to compress 1 mm. The same logic applies the other way: compressed 1 mm, the spring pushes back with 45 N.

Compressed 10 mm, the same spring pushes back with 450 N. Read the other way, it took 450 N of force to compress the spring 10 mm.

Manuals give the rate in N/mm, often with lb/in alongside; 1 N/mm is about 5.71 lb/in, and 1 kg/mm is about 9.81 N/mm. The rate is marked on the outside of the spring.

Preload is compression the spring already has when it is installed, so it counts toward the spring's total compression. A 45 N/mm spring preloaded 10 mm is already pushing back with 450 N at full extension, and the shock does not move until the load on the spring passes 450 N. The first millimeter of shock travel takes 495 N, and each millimeter after it another 45 N. Preload therefore changes how much force the spring holds before the suspension moves, and with it where the bike sits. It does not change the rate, the 45 N each further millimeter adds.

The rear wheel does not push on the spring directly. It pushes through the swingarm, and on a linkage bike through the linkage too, so the wheel travels further than the shock does. That lever divides the spring's rate twice: once because the wheel has leverage over the spring's force, and again because the spring moves less than the wheel. The rate at the wheel is the spring rate divided by the leverage ratio squared.

That is why the numbers look so different between the ends of the bike. A coil fork's spring compresses by about as much as the fork travels, with almost no lever in between, so fork springs are rated around 4–5 N/mm. A shock spring moves a fraction of the rear wheel's travel, so shock springs are rated around 40–70 N/mm for the same riders.

It is also why a spring rate does not carry from one bike to another. The same 45 N/mm spring gives a different wheel rate on every bike whose leverage ratio differs, and a 10% higher leverage ratio needs a 21% stiffer spring to hold the bike up the same way. The manuals show the same thing from the other side: the same model can list different charts for different markets and variants, such as the 2021 FC 450 in its EU and US versions. A spring chart belongs to one bike.

A linkage changes the leverage ratio as the suspension compresses, so the wheel rate changes through the travel even on a spring with one rate; where the leverage ratio falls through the stroke, the wheel rate rises. Every linkage shock in the manuals read lists a single rate per rider weight band.

A PDS rear end has no linkage. The one PDS manual read, the 2022 KTM 350 XCF-W, lists its shock spring as a range rather than one number (60…66 N/mm for a 65–75 kg rider), and the range sits well above the linkage shocks' figures. The manual does not say what the range describes. What follows either way is that a PDS figure and a linkage figure cannot be compared: each is divided by its own bike's leverage ratio.

The spring is what holds the bike up, so its rate shapes most of what the suspension does.

• How far each load moves the suspension. Braking, landings, and the rider's own weight compress a softer spring further and a stiffer one less.
• Hold-up and bottoming. The energy a spring absorbs over the stroke it has left grows with its rate, so a stiffer spring stays higher under the same hit and takes a bigger hit to reach the end of the travel.
• How much of a bump reaches the chassis. For every millimeter the wheel moves up into the travel, the spring pushes the chassis by the wheel rate times that millimeter. A stiffer spring passes more force into the chassis for the same wheel movement; a softer one lets the wheel move further for the same force.
• How fast the suspension returns. The force pushing the wheel back out after a hit is the wheel rate times how far the suspension moved, and rebound damping meters it. At the same rebound setting, a stiffer spring returns faster and a softer one slower, so rebound damping is set to the spring. The manuals re-check rebound after every spring change.
• Where the bike sits. On the right spring, preload brings both sag readings into range. On the wrong one, it cannot: the sag guide covers why.

A spring outside the rider's range does what would be expected. Each item below is how it presents on the trail, then what the spring is doing underneath.

Too soft.

• The rear squats, and everything gets harder. Every load compresses a softer spring further: compression is the load divided by the wheel rate. At the same rebound setting it also returns more slowly. When the next hit arrives before the rear has recovered, it starts deeper in the travel.
• The head angle rakes out, and the bike weaves and wanders. A rear sitting lower pitches the chassis nose-up, which slackens the head angle and lengthens trail. A slacker head angle on its own steers more slowly; what the soft spring adds is movement. Because every load moves a soft rear further, the chassis pitch, and with it the steering geometry, changes more from one moment to the next.
• Traction becomes unreliable, and climbing suffers. On a climb, the slope moves more of the weight onto the rear wheel, and a softer spring gives further under that load. The rear sits lower, the chassis pitches further back, and less stroke is left for the next bump, so more bumps reach the end of the travel, where the wheel can no longer move on its own.
• The bike feels sluggish and unpredictable. It sits away from the attitude it was designed around, and that attitude changes with every load.

Too stiff.

• The bike gets locked into lines, and small corrections take a fight. A stiffer spring moves less for the same force, so the chassis responds less to the rider's own inputs, such as weighting the pegs or braking into a turn, and more of each bump's force reaches the chassis.
• The rider squeezes the bike much harder to hold on, bringing on cramps and arm pump. Each millimeter the wheel moves pushes the chassis, and the rider on it, harder than on a softer spring.

Too soft and too stiff are each relative to the rider's weight in gear, not to a number. The same spring can be right for one rider and wrong for another.

The manufacturer's chart is the starting point. Every manual read lists the spring by rider weight in 10 kg bands, from 65 to 95 kg, and the bikes are delivered for a 75–85 kg rider in full protective clothing. Gear, a hydration pack, and tools all count toward that weight. Small differences inside a band are covered with preload; larger ones need a different spring. After a spring change, the order is static sag, then riding sag, then rebound damping.

The calculation behind a chart is the same on linkage and PDS bikes:

Spring rate needed ≈ leverage ratio squared, times the rider's load on the rear wheel, divided by the gap wanted between static sag and riding sag.

The rider's load is set by weight in gear and where it sits on the bike; the gap is set by the manufacturer's sag figures. The leverage ratio is the input a rider cannot look up: it comes from the bike's linkage design, and owner's manuals do not list it. That is why a calculation that does not ask for the exact bike cannot give a spring rate. On a linkage bike, the leverage ratio to use is the one around the sag point, so the result is a close estimate rather than an exact one.

The formula is more useful for what it shows than for the number it gives: the leverage ratio counts twice, so it moves the answer furthest. The table under "Spring Rates Across Manufacturers" sets out how far each input moves it.

The fork does not get a sag figure. Husqvarna and KTM state that no exact riding sag can be determined for the fork, "for various reasons" they do not list, and Sherco gives sag figures for the shock only.

What the manuals give instead is a spring rate. A coil fork is set the same way as the shock, by a rider-weight chart, with preload covering smaller differences. It is checked by bottoming: a fork that bottoms frequently needs harder springs.

An air fork, such as the WP AER 48 and XACT forks on the 2018–2021 Husqvarna FC 450, has no coil spring to change. A sealed air chamber in one leg does the spring's job, with a small steel spring only as an end stop. It is set with a pump to the rider's weight, the surface, and the rider's preference, within the manufacturer's minimum and maximum pressures, and a fork that bottoms frequently gets more pressure. Air stiffens as it is squeezed into a smaller volume, so an air spring's rate rises through the travel by its nature.

Braking, acceleration, and landings pitch the bike against both springs at once, so how far it pitches depends on the rates at both ends. A spring change at one end changes that balance and is a reason to check the other end.