Every air fork and rear shock ships with a pressure chart on the stanchion or printed in the manual. That chart takes your weight, points at a number, and says: put this much air in. Riders do exactly that, ride the bike once, and never touch the settings again.
The chart is not wrong. But it is incomplete. It cannot account for your riding style, your terrain, the weight of your kit, the temperature at the trailhead, or the fact that you prefer to sit and pedal through technical sections rather than stand and attack. Those variables matter. They are, collectively, the difference between a bike that feels alive and a bike that feels like it is working against you on every root and rock.
Proper suspension setup is the single biggest ride-quality improvement available to any off-road cyclist. Bigger than tyres. Bigger than wheels. Bigger than a fork upgrade. And it costs nothing but a shock pump and the patience to do it properly. Our Shock Pressure Calculator will give you a personalised starting point — but this guide explains the reasoning behind those numbers so you can fine-tune with confidence.
What Sag Actually Means — and Why the Percentage Matters
Sag is the amount your suspension compresses under your body weight when you are sitting on the bike in riding position. It is expressed as a percentage of total travel. If your fork has 150 mm of travel and it compresses 30 mm under your weight, that is 20% sag.
This sounds like a dry technical specification. It is not. Sag determines where in its stroke the suspension sits at rest, and that position changes everything about how the bike behaves.
Too little sag (too much air pressure)
When sag is too low — say 10-12% on a trail bike — the suspension is sitting right at the top of its travel. The initial part of the stroke, where small bumps are absorbed, barely gets used. The fork and shock cannot extend downward to follow holes and dips because there is almost no room below the resting point. The result: the bike skips across rough ground, the tyres lose contact with the surface, and grip vanishes precisely when you need it most.
This is the most common setup mistake in amateur off-road cycling. The bike feels firm and fast on smooth fire roads, which gives the impression of a good setup. But on anything technical — roots, rocks, off-camber turns — that stiffness becomes a liability. The suspension is fighting you instead of working for you.
Too much sag (too little air pressure)
When sag exceeds 35%, the suspension is sitting deep in its travel before you have hit anything. You have used a third of the available stroke just by getting on the bike. What remains is not enough to absorb hard impacts, so you bottom out frequently. The geometry sags too — the bottom bracket drops, the head angle slackens further, and the bike feels sluggish and wallowy. Climbing becomes a battle against constant bob and squat.
The targets
For trail and enduro riding, the standard starting points are:
| Component | Target Sag | Notes | |---|---|---| | Rear shock | 25-30% | Measured at the shock body, not at the wheel | | Fork | 15-20% | Some riders and manufacturers prefer 20-25% | | XC fork and shock | 15-20% | Firmer for pedalling efficiency | | Enduro fork and shock | 25-30% | Softer for rough terrain confidence |
These are starting points. Your ideal sag depends on your weight, riding style, terrain, and personal preference. A rider who favours technical descents and is willing to sacrifice some climbing efficiency will lean toward the higher end. A rider who values pedalling performance and rides smoother terrain will sit lower. Neither is wrong.
Rider Weight, Kit Weight, and Air Pressure
Air pressure and sag are directly linked, but the relationship is not linear and it varies between fork and shock models. A Fox 36 and a RockShox Lyrik at the same pressure will not produce the same sag for the same rider because their air spring volumes, seals, and negative chamber designs differ.
This is why sag percentage — not a fixed PSI number — is the correct target. The pressure is just the tool you use to get there.
That said, starting points by weight are useful. For a trail fork with 140-160 mm of travel:
| Rider Weight (kitted) | Approximate Fork PSI | Approximate Shock PSI | |---|---|---| | 55-65 kg | 55-70 | 120-145 | | 65-75 kg | 65-80 | 140-165 | | 75-85 kg | 75-95 | 155-185 | | 85-95 kg | 85-105 | 175-205 | | 95-105 kg | 100-120 | 195-225 |
The ranges are wide because every fork and shock model behaves differently. Run your numbers through the Shock Pressure Calculator for a tighter recommendation based on specific hardware. Then measure sag on the bike and adjust from there.
Kitted weight matters. The figures above assume you are wearing helmet, shoes, gloves, and carrying the kit you normally ride with. A hydration pack loaded with two litres of water and a set of tools adds roughly 3-4 kg. That shifts the correct pressure noticeably — possibly 5-8 PSI in the shock. Always set up with everything you will carry on the ride, not in shorts and a t-shirt in the garage.
How to Set Sag Properly: Step by Step
You need three things: a shock pump, the o-ring on your fork stanchion (or a zip tie as a substitute), and ideally a second person to hold the bike.
Fork sag
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Set a baseline pressure. Use the manufacturer's chart, the Shock Pressure Calculator, or the weight-based table above. Attach the shock pump to the fork's air valve (usually on top of the left stanchion), set the pressure, and disconnect carefully. You will lose 3-5 PSI on disconnection — account for this by slightly overshooting.
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Push the o-ring down. Slide the rubber o-ring on the fork stanchion all the way down to the dust seal. If your fork does not have one, wrap a zip tie loosely around the stanchion instead.
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Mount the bike in full kit. Wear everything you would on a ride — helmet, shoes, pack, hydration. Have a friend hold the bike upright, or lean against a wall or fence. Settle into your natural riding position. Do not bounce. Do not lean forward aggressively. Just sit as you would on a flat trail.
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Dismount carefully. Step off without pushing down on the bars. The o-ring will have slid up the stanchion, marking how far the fork compressed.
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Measure. The distance between the o-ring and the dust seal is your sag in millimetres. Divide by total fork travel and multiply by 100 for the percentage. A 150 mm fork with 30 mm of sag is at 20%.
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Adjust and repeat. If sag is too high, add 5 PSI and try again. If sag is too low, release 5 PSI. Re-check after each change. This process typically takes three or four attempts to dial in.
Rear shock sag
The process is identical, but the measurement is taken at the shock body itself — not at the rear wheel. Push the o-ring against the air can seal, mount the bike, dismount, and measure.
Rear shocks have less physical stroke than forks (typically 50-65 mm on a trail bike), so the same sag percentage translates to a smaller absolute measurement. On a shock with 57.5 mm of stroke, 30% sag is just 17.25 mm. Precision matters here — a cheap tape measure is fine, but read it carefully.
A note on metric vs trunnion shocks: the stroke length you need for sag calculations is the shaft stroke, not the eye-to-eye length. Your shock will have this printed on it — something like "210 x 55" means 210 mm eye-to-eye and 55 mm of stroke. Use the 55.
Rebound: The Setting Most Riders Get Wrong
Rebound damping controls how quickly the suspension extends after being compressed. Turn the red dial (it is almost always red) clockwise to slow rebound, anti-clockwise to speed it up.
Why rebound matters
When your fork or shock compresses over a bump, the air spring stores energy. Rebound damping controls how fast that energy is released. Get it right and the suspension recovers smoothly, ready for the next impact. Get it wrong and problems cascade.
Too fast: The suspension snaps back immediately after every compression. On a single bump, this feels lively. On a series of bumps — a rock garden, a root section, a washboard fire road — the fork and shock bounce off each impact and launch the wheels upward. The bike bucks and kicks. Tyre contact is inconsistent. Braking traction disappears.
Too slow: The suspension compresses on the first hit but does not recover before the second hit arrives. Then the third. Then the fourth. Each successive impact pushes the suspension deeper into its travel because it cannot extend between hits. This is called packing down. You lose travel, the geometry steepens, the bottom bracket drops, and by the bottom of the descent you are riding on something closer to a rigid bike than a suspended one.
The drop test
This is the simplest and most reliable way to set rebound.
For the fork: Stand next to the bike with both brakes locked. Push down firmly on the handlebars, compressing the fork roughly a third of its travel. Release. Watch the fork extend.
If the front wheel bounces off the ground, rebound is too fast. Slow it down two clicks and repeat. If the fork oozes back up like it is moving through treacle, rebound is too slow. Speed it up two clicks and repeat.
The target: the fork extends as quickly as possible without the wheel lifting off the ground. One clean, controlled return to full extension.
For the rear shock: Straddle the bike, push your weight firmly through the saddle and pedals, then release. Same principle — the bike should return to sag height quickly and settle without oscillating.
Common rebound mistakes
The most frequent error is running rebound too slow. Slow rebound feels smooth and controlled in the car park. On trail, it means the suspension packs down through any sustained rough section. If you find yourself running out of travel halfway through a descent but your sag is correct, check rebound before reaching for the air pump.
The second error is setting fork and shock rebound at the same speed. The fork typically wants slightly faster rebound than the rear shock because it needs to recover quickly to track the ground ahead. If both are matched, the front end can feel sluggish relative to the rear.
Compression Damping: Less Is Usually More
Compression damping controls how the suspension behaves as it compresses — the other half of the equation from rebound. Most mid-range and higher forks split this into low-speed compression (LSC) and high-speed compression (HSC).
Low-speed compression governs the fork's response to slow, sustained forces: pedalling bob, brake dive, body weight shifts in corners, and G-loading in berms. Adding LSC firms up the bike's platform and reduces wasted movement. Too much and the fork stops reacting to small bumps — precisely the impacts you bought suspension to absorb.
High-speed compression governs the response to fast, sharp impacts: rocks, roots, drops, hard landings. Adding HSC prevents the suspension from using all its travel on a single hard hit. Too much and the bike feels rigid and deflective.
The rule of thumb: start with both wide open (fully anti-clockwise). Ride your normal trails. If the bike bobs excessively on climbs, add LSC two clicks at a time until the bobbing is controlled but the fork still responds to trail chatter. If you are bottoming out regularly on drops or rock gardens, add HSC two clicks at a time.
Most trail riders end up with a few clicks of LSC and leave HSC wide open. That is fine. Compression damping is the seasoning, not the main course — get your air pressure and rebound right first, and compression becomes a subtle refinement rather than a rescue operation.
Volume Spacers: Progressive Spring Rate Without Adding Pressure
Volume spacers — called tokens by RockShox and bottomless nuts by Fox — are plastic inserts that sit inside the air spring chamber, reducing its volume. They do not change the beginning of the stroke. Your sag stays the same. What they change is the end of the stroke.
With no spacers, the air spring has a relatively linear rate. It takes roughly the same increase in force to compress each additional centimetre of travel. With spacers installed, the spring rate becomes progressive — soft at the top for small-bump sensitivity, increasingly firm as you move deeper into the travel. The suspension ramps up resistance as it approaches bottom-out.
When to add spacers
- You are using full travel on hard rides but the bike feels good for everything else — you do not want to add air pressure because it would ruin the top of the stroke
- You want more pop and support from the suspension without sacrificing initial compliance
- Hard impacts or drops send you to full bottom-out even with compression damping adjusted
When to remove spacers
- The fork or shock feels harsh in the last third of its travel
- You cannot access full travel even on the biggest hits your terrain offers
- The suspension feels dead and unsupportive when compressed deeply
Most trail riders benefit from one or two spacers in the fork. The rear shock often runs well with its stock spacer configuration. Heavier riders (90+ kg) or those riding aggressive terrain may want an additional spacer in both.
Installing spacers requires removing the air spring — a job that involves releasing all air pressure, unthreading the top cap (fork) or air can (shock), and physically inserting the spacer. It is a 15-minute job with basic tools and a clean workspace. If you are not comfortable with it, any bike shop will do it while you wait. The spacers themselves cost a few pounds for a pack.
Temperature, Altitude, and When to Re-Check
Air pressure changes with temperature. This is physics, not opinion. As air heats up, it expands and pressure increases. As it cools, pressure drops. The magnitude is roughly 3-5 PSI for every 10 degrees Celsius of temperature change — enough to shift your sag noticeably.
If you set up your suspension in a 20-degree garage and ride in 5-degree conditions at the trailhead, your fork and shock are running lower pressure than you intended. In summer, a bike stored in a hot car and ridden in cooler mountain air will feel different again.
The practical rule: check your pressure at the trailhead, not at home. Bring your shock pump. It takes thirty seconds and it removes the variable entirely. If you are travelling to ride — an alpine trip, a destination trail centre — re-check before every ride because conditions may vary day to day.
Altitude also affects air pressure, though less dramatically than temperature for most riders. At 2,000 metres elevation, atmospheric pressure is roughly 20% lower than at sea level. The pressure differential across your fork and shock seals shifts, and you may find the suspension sits slightly deeper into its travel. If you are heading to altitude for a riding holiday, start with your normal pressures and check sag on arrival. Adjust if needed.
Other re-check triggers:
- Start of every riding season (air leaks slowly through seals over months of storage)
- After any weight change of 5 kg or more (including switching between a day ride and a loaded bikepacking setup)
- After fork or shock service (fresh oil and seals change damping feel; air pressures should be re-baselined)
- When something feels off mid-season (slow leaks are common — a 15-20 PSI drop over weeks is not unusual on some shocks)
Trail, Enduro, XC, and Gravel: Setup Differences
The principles are identical across disciplines. What changes is where you sit within the ranges.
Cross-country and marathon
XC riders want a firm, efficient platform. Pedalling performance matters more than deep-travel plushness. Target 15-20% sag front and rear. Run slightly more LSC to control bob. Rebound can be a touch faster because the terrain is generally smoother and the suspension does not need to recover from violent impacts as frequently.
Trail
The all-rounder setup. 20-25% fork sag, 25-30% rear. Moderate compression, moderate rebound. One or two volume spacers to add bottom-out resistance without sacrificing the top of the stroke. This is the starting point for most riders, and where you should begin before skewing toward XC or enduro setups.
Enduro and aggressive trail
Prioritise descending confidence. 25-30% sag front and rear — or even 30% if your terrain demands it. Slightly slower rebound to keep things controlled through sustained rough sections. More volume spacers to protect against bottoming on drops and jumps. Less LSC than a trail setup, because the suspension needs to be fully active on technical descents even if it bobs slightly on climbs.
Gravel
Gravel suspension exists in a different space. Forks like the RockShox Rudy Ultimate, Fox 32 SC, and Lauf Grit offer 30-40 mm of travel — a fraction of what a mountain bike fork provides. The goal is not to absorb drops and rock gardens. It is to take the edge off washboard gravel, rough tracks, and the relentless vibration that accumulates over four, five, six hours of mixed-surface riding.
Sag targets for gravel suspension sit at 20-25%. The shorter travel means you need to be more precise — on a 30 mm travel fork, the difference between 20% and 30% sag is just 3 mm. Use the Shock Pressure Calculator for a starting point and fine-tune on a surface that represents your typical riding.
Rebound on gravel forks should be faster than on MTB forks. The impacts are smaller and more frequent. You want the fork to recover quickly between chattery vibrations rather than absorbing deep, violent compressions. Most gravel riders set rebound near the middle of the adjustment range and leave it there.
Compression damping, if your gravel fork offers it, is best used sparingly. A click or two of LSC can reduce bob on paved sections — useful for mixed-surface events where you spend real time on tarmac — but too much and you lose the comfort that is the entire reason for having suspension on a gravel bike.
If you are new to gravel riding, our gravel cycling beginners guide covers bike setup, tyres, and event preparation beyond suspension.
How Suspension Setup Affects Fatigue
This is the part that matters most to riders over 35 riding for three hours or more, and it is the part that gets discussed least.
Poor suspension setup does not just reduce grip and control. It increases fatigue. When the fork and shock are too stiff, impacts that should be absorbed by the suspension are instead absorbed by your body — your wrists, forearms, shoulders, lower back, and neck take the load. Over a two-hour ride, that adds up. Over a five-hour ride, it can be the difference between finishing strong and finishing wrecked.
The hands are the first warning sign. If your fingers go numb, your palms ache, or your grip strength fades on long rides, the fork is almost certainly too stiff. Suspension that is correctly set up dissipates impact energy through the air spring and damping circuits. Suspension that is too stiff passes that energy straight through the bars and into your skeleton.
Lower back pain on rough terrain is another indicator. When the rear shock cannot absorb trail chatter, your spine does the work instead. Riders who have dialled in their sag and rebound consistently report that back pain they attributed to fitness, age, or bike fit was actually a suspension pressure problem.
This is relevant on gravel too. The cumulative vibration load on a long gravel ride is significant. A properly set up 30 mm suspension fork does not make the bike feel like a mountain bike — but it does reduce the micro-impacts that fatigue muscles over hours. Combined with correct tyre pressure — use our Tyre Pressure Calculator or the complete tyre pressure guide — it transforms the endurance equation.
If you are also tracking your weight for performance purposes, our Race Weight Calculator can help you understand how body composition changes affect both your power-to-weight and your suspension setup needs. And for a broader picture of power relative to mass, the W/kg Calculator puts your numbers in context.
The Common Mistakes, Collected
These are the errors we see most frequently. Every one of them is fixable in minutes.
Running too much pressure. The number one mistake across all off-road disciplines. The bike feels fast on smooth ground, which creates the illusion of a good setup. On rough terrain it is harsh, skittish, and slow. If you are not using at least 70-80% of your fork travel on a proper ride, there is too much air in it.
Setting up in the garage instead of at the trail. Temperature differences between your garage and the trailhead can shift pressures 3-5 PSI. Always bring the shock pump.
Ignoring kit weight. A 3 kg pack changes your correct pressure by 5-10 PSI in the shock. Set up wearing what you ride with.
Setting rebound by feel in the car park. The car park test tells you if rebound is wildly wrong. It does not tell you how the suspension behaves through 50 consecutive impacts at speed. Set rebound with the drop test, then validate on trail. If the bike packs down on rough descents, speed the rebound up regardless of how it felt stationary.
Matching fork and shock rebound speed exactly. The fork typically needs faster rebound than the shock. Match sag percentages, not rebound click positions.
Adding air pressure to fix bottoming out. Bottoming out is a symptom with multiple causes: not enough compression damping, not enough volume spacers, or too much sag. Adding air pressure addresses the symptom by ruining the rest of the stroke. Diagnose the cause first.
Never re-checking after the initial setup. Air seals leak. Temperature changes. Your weight fluctuates. Your riding evolves. A setup that was right six months ago may not be right today.
Bringing It Together
Suspension setup is not complicated. It is methodical. The entire process — sag, rebound, compression, balance check — takes twenty minutes the first time and five minutes every subsequent time. The improvement in grip, comfort, control, and fatigue resistance is out of proportion to the effort involved.
Start with the Shock Pressure Calculator for baseline numbers. Set sag with you on the bike in full kit. Run the drop test for rebound. Leave compression wide open unless you have a specific reason to add it. Check your pressures at the trailhead. Ride, assess, adjust by small increments, and repeat.
The difference between a bike you tolerate and a bike you trust is hiding in those twenty minutes. And unlike a component upgrade, this one is free.
For the complete picture on mountain bike setup, read the full suspension setup guide and the fork-specific guide. If your tyres are not sorted either, the MTB tyre pressure guide covers that half of the equation — because tyres and suspension work as a system, and getting one right while ignoring the other leaves performance on the table. For road and gravel tyre setup, the complete tyre pressure guide covers those disciplines in detail. And if you are working on bike fit alongside suspension, the bike fit guide addresses the contact points that sit on top of all of this.
Join the Conversation
Got a question about your specific fork, shock, or riding conditions? Want setup advice from riders who have dialled in the same hardware? The Roadman Cycling community on Skool is where we discuss setup, training, racing, and everything else that matters to serious amateurs. Bring your questions — and bring your shock pump readings.