You've loaded the bike for a familiar loop, checked the tyres, and rolled into the first rough section. Instead of holding its line, the fork dives under braking and the rear end feels low and vague through the turns. That behaviour often starts with one basic setting, suspension sag.

Sag is the amount of travel the fork or shock uses while supporting your weight in normal riding position. Set it correctly and the bike has room to compress into bumps and extend into dips. Set it poorly and the bike can feel harsh, wallowy, nervous, or reluctant to turn. The right process isn't about chasing a perfect internet number. It's about measuring ride height, balancing the chassis, then checking how the bike behaves on the trails you ride.

Why Sag Matters Before You Touch the Bike

Before the first root, rut, or braking bump, sag determines where the suspension sits in its travel. Too little sag keeps a fork high, which can make it skip across small chatter. Too much removes support under braking and pedalling, leaving the front to dive and the rear to feel low.

Ride height also changes the bike's geometry. Excessive fork compression steepens the steering position, while a compressed rear suspension lowers the back and affects cornering. The difference may be hard to spot in a car park, then become obvious on wet Nelson roots, steep approaches, and long rough descents. Correct sag lets the chassis stay balanced, so the rider is not constantly correcting its attitude.

For a practical introduction to the chassis as a whole, see Rider 18's guide to full-suspension bikes. Sag is not only a comfort setting. It influences traction, braking control, support, and handling together.

Start with a measurement, not a feeling

Scott NZ describes fork sag as the distance between the O-ring and dust seal. On a 100mm fork, 20mm of sag equals 20%, while 25mm equals 25%, according to its New Zealand suspension setup guidance. The same guidance places a longer-travel fork for light trail riding around 25%, with rear shocks commonly set at 25% to 30% sag.

Use those figures as starting points, then match them to the riding. A family trail loop with smooth climbs may benefit from more support. Wet roots often reward a suspension system that settles into the ground and maintains grip. E-bike commuting adds motor and battery weight, while a pack, water, or child seat changes the spring load on any bike.

Workshop rule: Set ride height first. Use rebound and compression damping to control movement afterwards, rather than hiding an incorrect spring setting.

The result is easy to feel. The tyres remain connected over small bumps, your hands and feet take less punishment, and the bike behaves more predictably while braking, cornering, and accelerating. That makes sag the right setting to establish before turning the damping dials.

Tools and Prep Before Adjusting Sag

A reliable sag check starts before you touch the air spring. Gather a shock pump with a bleed valve, a clean rag, a cable tie or the fork's rubber O-ring, a fine-tip marker, and a notebook or phone note. Record every pressure change and measured result. After several adjustments, memory is a poor reference.

Work on level ground, or use a stand that allows the suspension to reach full extension. Set both tyres to your normal riding pressure first. Tyre pressure changes how the bike settles, so a nearly flat front tyre will give you a false starting point.

Back off the rebound adjusters before measuring. The fork and shock need to settle without heavy damping masking their natural movement. Wipe the fork stanchions and shock shaft, then check the seals for visible dirt or damage. Stiction can hold the suspension high and make the reading look lower than the bike's actual loaded position.

A quick safety check now prevents wasted setup time. Look for air loss, oil around the seals, loose hardware, or a shock pump connection that does not seal properly. Fix those faults before adjusting sag.

Measure the rider, not the garage version of the rider

Put on the clothing and equipment used for the ride, including your helmet, shoes, pack, water, flat kit, and anything that remains attached to the bike. An e-bike commuter carries a different load from a rider on a light family trail loop. Wet Nelson roots may reward a slightly more settled setup, while extra support can feel better when carrying a pack or climbing under motor power.

Begin with the bike at full extension and avoid measuring immediately after a long descent. Heat and repeated movement can change how the suspension feels. Let the bike cool and settle, then wipe the contact surfaces clean before starting.

Keep the bike accessible for repeat checks. If it lives in a cramped flat or shed, practical facility bike storage ideas can help keep it upright and ready to inspect. Sag is not a one-time setting when your load or riding changes. Note the initial fork pressure, shock pressure, rebound positions, and measured sag so you can return to a known setup.

For replacement seals, pumps, hardware, or other setup items, Rider 18's range of mountain bike parts in New Zealand can help identify compatible components.

Setting Fork Sag Step by Step

Here's how to measure fork sag without disturbing the reading. With the fork unloaded, clean around the air cap and wiper seal, then check that the O-ring slides freely on the stanchion. Push it down until it touches the dust seal. That gives you the zero mark.

Wear the kit you normally ride in, including shoes, helmet, pack, and water. An e-bike commuter's load differs from someone riding a light family trail loop, while a wet Nelson roots ride may call for a more settled front end. Place your hands on the bars and stand on the pedals in a neutral position. Let the bike roll forward a few centimetres so the seals and bushings settle, then stop without repeated bouncing.

Step off carefully from the back of the bike. Pulling up on the bars or dropping your weight onto the fork can move the O-ring and spoil the measurement. Measure the gap between the O-ring and dust seal, then compare that distance with the fork's total travel. Use the fork's actual usable travel, not the visible stanchion length.

Adjust pressure in controlled changes

If the fork sits too deep in its travel, add air with the shock pump. If it sits too high, remove air through the pump's bleed valve. Make one pressure change at a time, repeat the same measurement, and record the result. Pressure figures from another fork model are only rough guidance because air-spring design and volume spacers change how pressure affects sag.

The table below is a workshop starting reference, not a verified pressure chart. The O-ring measurement decides the setup.

These figures come from the supplied setup brief and remain approximate because the exact fork model is not specified. A taller or heavier e-bike rider may need another 5–10 PSI to support the extra riding load, but the O-ring position remains the deciding check.

Ride the bike after the first adjustment, then measure it again after two rides. Air springs may settle and seals may bed in, so the initial reading can shift slightly. If reaching a sensible sag position requires a large pressure change, check the manufacturer's air-spring guidance or ask a suspension technician. Do not use damping to mask an air-spring setting that is wrong.

Setting Rear Shock Sag the Right Way

A wet Nelson root loop, an e-bike commute, and a family trail ride place different demands on the rear shock. Set sag from your normal riding position and load, then judge whether the bike remains supported without losing grip.

Start with the shock fully extended. Slide the rubber band or O-ring against the shock seal, sit on the bike in your usual riding gear, and place your feet evenly on the pedals. Settle into a normal seated position. Avoid pulling hard on the bars or bouncing repeatedly, since both actions add load and distort the reading. Dismount without pressing down on the saddle, then measure how far the O-ring moved along the shock shaft.

Calculate that movement as a percentage of the shock's total stroke, not the advertised rear-wheel travel. The frame's geometry means a small change at the shock can produce a larger change at the wheel. Check the shock specification or frame documentation if the stroke length is unclear.

Balance matters more than matching numbers

Shockcraft NZ recommends balancing the front and rear suspension before setting rebound or fine-tuning ride height. Its suspension setup process says the front and rear should compress evenly when you bounce the bike on level ground. The chassis should remain settled rather than pitching forward or rearward.

The rear may need a different target from the fork. Its kinematics, chainstay length, rider position, and frame design all affect how the bike sits. Check the bike while pedalling, braking, and shifting your weight instead of forcing identical percentages at both ends.

For a trail or enduro bike, the supplied guidance places rear sag broadly around 25% to 30%. Scott NZ also notes that rear shocks are often set at 25% to 30%. Use that range as a starting point, then choose the bias that suits the ride. A firmer setting can improve support for commuting or smooth family loops. More sag can keep the rear tyre tracking wet roots, though too much may reduce pedalling support and make the bike sit low.

Change air or preload in small steps and repeat the measurement.

If excessive pressure or preload is needed to hold the correct ride height, the spring may not suit the rider and bike combination. If the shock still sits too deep with the available adjustment, stop adding preload blindly. A spring-rate or air-spring change may be the proper fix.

Tuning Sag for Weight, Style and NZ Conditions

A useful sag setting starts with the complete riding load, not body weight written on an old medical scale. Include shoes, helmet, pack, water, tools, lights, child-carrying equipment, or the extra mass associated with an e-bike. The heavier the total system, the more important it becomes to check the O-ring after every pressure change.

The following reference gives approximate starting pressures from the supplied setup brief. It isn't a universal chart, and it shouldn't replace a measured sag check.

E-bike riders or riders carrying heavier cargo may need an additional 5–10 PSI, according to the supplied setup brief. That doesn't mean the bike should automatically be made firm. The extra load may require more pressure to preserve ride height, while the terrain may still call for enough sag to maintain grip.

Choose the bias that suits the ride

Cross-country riders and commuters often prefer the firmer end of their usable sag range. That gives stronger pedalling support and reduces excessive movement on smooth roads. The trade-off is reduced sensitivity if the setting becomes too firm for rough surfaces.

Gravity riders commonly accept more sag to keep the tyres working on steep, loose ground. On wet Nelson roots, a slightly more settled suspension can help the wheels follow uneven surfaces rather than skipping across them. Loose climbs can also reward grip, particularly when motor assistance encourages the rider to climb seated over slippery terrain.

Rotorua's clay-heavy tracks can expose the same compromise. More support helps prevent the bike from feeling vague, but excessive firmness can reduce contact on wet, irregular ground. Smooth Canterbury flow trails often favour a firmer feel because the surface provides fewer sharp traction problems and the rider may value quick direction changes.

Family trail loops sit between those extremes. A comfortable, controlled setting usually works better than chasing maximum softness, especially when a parent carries extra equipment or rides an e-bike. Make one change at a time and judge the bike on a familiar section, not on a completely new trail where every sensation is difficult to interpret.

Putting It All Together in the Workshop

A wet Nelson root section, an e-bike commute, and a family trail loop can expose different weaknesses in the same setup. Use one repeatable order so you can identify whether the problem is spring support, damping, or chassis balance. Set fork and shock sag first, then check the bike on level ground before adjusting rebound or compression.

Use this sequence every time

• Set sag: Measure with riding clothing, pack, and any normal e-bike load in place. Adjust air pressure or coil preload until the fork and shock sit at a practical ride height.
• Check balance: Pedal seated, then stand on the pedals. The bike should remain supported at both ends rather than pitching forward or settling heavily into the rear.
• Dial rebound: After a firm compression, each end should return smoothly. Too fast can kick the rider or lift the front wheel, while too slow can leave the suspension packed down through repeated bumps.
• Add compression: Use compression damping for support after the spring and rebound settings are working. It can reduce dive, but it cannot correct incorrect sag.

Before heading onto the trail, roll over two small kerbs, sprint briefly out of the saddle, brake firmly from a controlled speed, and make a sharp turn. Watch for a front wheel that tracks straight, a rear end that returns without kicking, and a neutral position through the turn. These checks reveal obvious problems without the distractions of a new trail.

A comfortable bike still needs support. If it sinks under pedalling, braking, or cornering, add spring support rather than relying on compression damping. If it skips across roots, give the suspension more freedom to move by reviewing pressure, sag, and rebound.

Write down pressure, rebound position, weather, and load. A setup that works for a light family loop may feel too soft during an e-bike commute, while a firm commuting setting can lose grip on wet roots. Recheck the bike after a major load change, and use Rider 18's guide to when to service your bike if inconsistent performance points to worn seals, contaminated oil, loose hardware, or overdue maintenance.

A short demonstration can reinforce the measurement and adjustment process before you work on your own bike.

Common Sag Myths Worth Forgetting

Myth one, 25% works for everyone. A fixed percentage cannot account for fork travel, frame design, rider weight, terrain, or riding style. Scott NZ uses 20% sag on a 100mm fork as a calculation example, while noting that 25% can work well for a longer-travel fork used for light trail riding. Rear shocks commonly sit around 25% to 30%, but the useful setting is the one that balances grip with support on your bike.

Wet Nelson roots may reward a little more compliance and traction. An e-bike commute, with battery weight and extra load, may need more support. A family trail loop can sit between those demands. Use the percentage as a starting point, then judge braking support, climbing height, and how easily the suspension uses its travel.

Myth two, sag only needs setting once. Recheck it after adding an e-bike battery, carrying more equipment, changing riding clothing, altering the spring system, or switching between commuting and trail riding. A change in normal load, or handling that feels noticeably different, is enough reason to measure again.

Myth three, rebound can fix incorrect sag. Rebound sets how quickly the spring returns. It does not set ride height or replace spring support. Too much rebound can make an under-supported fork pack down through repeated bumps, while faster rebound cannot stop an over-soft shock from wallowing.

Practical takeaway: Start with a pressure reference, measure sag, make one small change, and ride a familiar loop. Keep the setting that improves control, even if it does not match a target percentage.

If the issue could be sag, damping, spring rate, or worn hardware, have the suspension inspected before adding random adjustments. Rider 18 provides components, workshop support, and setup advice for trail riding, commuting, and family use at Rider 18. Provide your bike model, fully equipped rider weight, and usual terrain.