→ Full technical version: Dual Chamber Physical Model — White Paper v3.0
A poorly configured MTB suspension is not a comfort issue. It represents a loss of traction, a loss of control, and accumulated fatigue on every meter of the trail. The first parameter any suspension engineer adjusts before anything else is SAG, and there is a precise physical reason for that: it defines the operating point of the pneumatic spring, and everything else —rebound, compression, progressivity— works based on that point.
This article explains what SAG is, why it matters, and provides you with the most complete calculator available online: 132 fork and shock models from Fox, RockShox, SR Suntour, Öhlins, Manitou, Marzocchi, DVO, Cane Creek and more, calibrated against official manufacturer tables and validated with 10,000 statistical simulations per model.
SAG is the static compression of the pneumatic spring under the rider's weight in a neutral position on the bicycle. It is expressed as a percentage of the component's total stroke:
SAG [%] = (static compression [mm] / total stroke [mm]) × 100
A Fox 36 fork with 160mm of stroke at 25% SAG will be compressed 40mm when the rider mounts the bike in a neutral position.
When the rider sits on the bike without pedaling or moving, the weight compresses the suspension towards its equilibrium point. That equilibrium point is the SAG. If the SAG is too low (excessive pressure), the suspension is too stiff: it won't absorb small impacts well, and the rider will feel the unfiltered terrain. If it is too high (low pressure), the suspension is already partially compressed while resting and loses available travel to absorb large impacts.
The pneumatic spring is non-linear: the more it compresses, the more pressure it exerts (polytropic law PV^γ = constant). This means that the behavior of the rebound, high-speed compression, and the bottom-out feel depend directly on what point of the stroke the suspension normally operates. Setting rebound or compression before SAG is optimizing the dynamic response on an incorrect foundation.
Figure 1. SAG diagram for Fox 36 160mm. A 25% SAG positions the piston 40mm from the start of the stroke, leaving 120mm available to absorb impacts.
SAG ranges are not arbitrary. They derive from the balance between small impact sensitivity (requires high SAG) and available travel for large impacts (requires low SAG). The discipline defines which side of the balance to prioritize:
| Discipline | Front fork SAG | Rear shock SAG | Rationale |
|---|---|---|---|
| XC / Cross-Country | 15–20% | 25–28% | Pedaling efficiency, minimal parasitic compression |
| Trail | 20–23% | 28–30% | Balance between efficiency and mixed terrain absorption |
| Enduro | 23–26% | 30–33% | Absorption priority, technical descending |
| DH / Downhill | 25–28% | 33–38% | Maximum absorption, pedaling is not a decision variable |
Within the above ranges, fine-tuning depends on two variables that you know better than any official table:
The question that appears in every forum: "how many psi do I need?". The direct answer is: it depends on 4 interdependent variables that are non-linear among themselves.
The relationship between these variables is non-linear because air compresses according to the polytropic law (P·V^γ = constant, with γ calibrated empirically within the expected physical range for rapid compression with partial heat transfer). This means pressure doesn't scale linearly with weight: a 100kg rider doesn't need exactly twice the pressure of a 50kg rider. It also doesn't scale linearly with stroke.
The printed tables in Fox and RockShox manuals are valid approximations for the most common weight ranges but deviate significantly at the extremes. Our model reproduces those tables with a mean error of 1.66 psi and extends them to the entire operational range with honest confidence bands.
This is not an interpolation table. It is a complete physical dual-chamber model with polytropic compression, calibrated through inverse differential optimization against official data from Fox, RockShox, SR Suntour, and Öhlins. For those who want the full technical details —equations, calibrated parameters, Monte Carlo validation results, documented limitations— the Technical Full White Paper v3.0 is available.
What matters to use the calculator:
The calculator's result is your documented starting point, not the final tuning. Use the recommended pressure, physically measure the SAG, adjust within the indicated confidence range, and fine-tune by feel on the trail. Rebound and compression should be set after confirming the SAG.
Entry-level models (SR Suntour, RS Judy, RS Recon) have a ±15% confidence band due to the non-linearity of their Solo Air system — use the range, not the central number.
Repeat the measurement 3 times and average it. The first result has bias due to the direction of SAG entry (seal hysteresis produces different readings if you mount slowly vs. with a bounce). The average of 3 attempts using the same technique yields the real value.
For full-suspension bikes, there is a technical distinction that most configurators —including Fox and RockShox apps— omit: the SAG you measure on the rear shock is not the same as the real SAG of the rear wheel.
The rear shock is connected to the frame through a leverage system with a multiplication ratio called Leverage Ratio (LR). A shock with 25% SAG on a frame with LR = 3.0 produces a wheel SAG of 75% of the total frame travel, not 25%.
By specifying your frame's linkage type (Horst Link, VPP, DW-Link, Single Pivot, Switch Infinity, High Pivot), the calculator computes the real wheel SAG and shows it next to the shock SAG. Frame manufacturer recommendations are expressed in wheel SAG — now you can compare directly.
The calculator runs completely in your browser locally. No data is sent to any server. You can save the page for offline use.
The calculator delivers a central pressure value and a range. For example: "94 psi — Range: 89–99 psi — ±5%". This range is not an inaccuracy in the model: it is an honest representation of the real operational variability under which the model was validated. Temperature, manufacturing tolerances, measurement technique, and rider weight variations produce this dispersion in practice.
The rebound and compression values are starting points derived from the calculated stiffness ratio (force at the SAG point) and the selected discipline. They are reasonable values to start riding with, not final settings. The correct protocol to tune the rebound: in the workshop, completely compress the fork with your hands and let it go. If the wheel jumps off the ground → rebound is too fast, close it (more clicks). If it returns very slowly → rebound is slow, open it.
Tokens (volume spacers) reduce the positive chamber volume and increase progressivity without affecting initial pressure. More tokens = suspension is stiffer at the end of the stroke. The calculator recommends the number of tokens that keeps the progressivity ratio within the optimal range of your discipline. If you already have tokens installed, enter that number so the pressure calculation incorporates them correctly.
The pressure calculation for rear shocks depends heavily on the frame's leverage ratio. Without that data, uncertainty multiplies by 2–3×. Always specify your frame's linkage type to get useful results. If you can identify the specific frame, even better.
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For the complete technical methodology behind this calculator: