Suspension fork sag measurement: keep millimetres, denominator and loaded condition together
Read an existing front-sag observation without confusing settled compression, exposed tube, a ride marker or an unsupported percentage target.
Published 14 October 2026 · OpenBikeFit
Same compression. Different denominator.
- Fictional axial compression c = 20 mm; applicable travel T = 140 mm → about 14.3%.
- Dividing the same 20 mm by a separate exposed interval E = 155 mm instead gives about 12.9% — a different denominator, not a changed fork.
- A later marker at 28 mm does not replace a settled 20 mm observation. Unknown travel leaves the fraction unknown. Neither percentage is a target.
Original schematic; not to scale. No physical measurement, sag target, pressure, part permission, adjustment or installation is supplied.
On this page
- 01Name the load and observation condition
- 02Establish the reference and denominator separately
- 03Work one fictional ratio with its units
- 04Preserve marker history rather than guessing
- 05Repeat the whole observation, not the display
- 06Keep front and rear states in different columns
- 07Distinguish a source recommendation from your observation
- 08Carry a readable condition ledger forward
Free blank worksheet
Keep supplementary paper references
General reference and question spaces, not dedicated fork-travel or sag fields or installation approval.
Open toolA fork-sag percentage is a compact description, not a complete measurement record. It can lose the source of the travel value, the loading condition, the reference state and the way the marker was observed. Two people can write the same percentage while describing different events. The first task is to preserve those differences, not adjust a fork until its number resembles someone else’s.
This guide interprets an existing observation made under the exact manufacturer’s applicable protocol. It does not supply a sag target, pressure table, spring recommendation, damping setting or home test sequence. If the method or support is unresolved, keep the record incomplete and seek appropriate model-specific help rather than trying a physical experiment from the fictional arithmetic.
Name the load and observation condition
Sag refers to compression under a specified resting load condition. SR SUNTOUR’s support definition includes riding gear, illustrating that the condition is part of the quantity. Retain the posture, carried equipment, support arrangement and observation protocol in a technical description. Different front/rear load distribution can change the observation even when the installed parts have not changed.
A resting measurement and a dynamic riding event answer different questions. Do not label a mark left after a ride as settled sag simply because the distance falls near a familiar percentage. A photograph of a rider on the bicycle also does not establish that the suspension has settled or that the observation followed an appropriate method.
Establish the reference and denominator separately
Keep the extended reference, the named loaded observation and the applicable travel T together. In an explicitly axial telescoping convention, their matched separation gives compression c. The percentage is 100 × c / T only when T is positive and established for the same configuration and movement convention. An unknown denominator leaves the percentage unknown, not zero.
Do not substitute exposed upper-tube length, axle-to-crown length or an offered family maximum for the denominator. Nor does an unconfirmed travel sticker become certain because a calculated percentage looks plausible. The preceding travel guide explains why a visible length belongs in another row; the existing fork-length guide owns its separate crown/axle endpoints.
Work one fictional ratio with its units
For independently declared fictional compression c = 20 mm and applicable travel T = 140 mm, 100 × 20 / 140 ≈ 14.3%. The same record may retain the unrounded fraction 0.142857 for arithmetic, but that does not make the observation accurate to many decimal places. Twenty millimetres is the raw declared compression; 14.3% is a derived description.
If somebody instead divided the same 20 mm by a separately observed exposed interval E = 155 mm, the answer would be about 12.9%. No part moved between these two calculations: the denominator changed. Neither value is a recommended sag, and choosing the smaller percentage does not improve the fork. Keep the applicable T beside the result so that another reader can audit the relationship.
Preserve marker history rather than guessing
A marker can move during an event after the intended settled observation. The FOX 2027 36 mm manual explicitly distinguishes the observation from movement while leaving the bicycle. That scoped idea explains why the condition and sequence must be known; it does not establish that a later mark is a valid resting measurement or authorize a universal procedure.
Consider a fictional settled observation of 20 mm followed by another movement that leaves the marker at 28 mm. The later marker position is 28 mm for its event history, not a correction of the settled 20 mm. If the earlier value was never observed, you cannot recover it by subtracting an assumed bounce or choosing a familiar percentage. Record the missing history as unresolved.
Repeat the whole observation, not the display
Independent observations require re-establishing the same declared condition and reference according to the applicable protocol. Re-reading an unchanged marker merely repeats a display. Keep every original result, the observation method and the condition notes. A changed posture, gear load or protocol should not be silently combined with the first series as if it were an unchanged-condition repeat.
Fictional matched observations of 19, 20 and 22 mm have mean about 20.3 mm and span 3 mm. With an independently established 140 mm denominator, their individual fractions are about 13.6%, 14.3% and 15.7%. The mean and span describe these observations only; they are not instrument accuracy, a safe band, a confidence interval or proof that an adjustment occurred.
Keep front and rear states in different columns
Front compression does not identify a rear shock’s compression or a rear axle’s path. Even an identical percentage would use different movement quantities. The existing rear-shock sag guide explains its stroke denominator and linkage limits. Retain both front and rear load observations from the same declared condition rather than using one as a substitute for the other.
An ideal rigid-rear example can illustrate how front shortening changes frame orientation, but a fully suspended bicycle needs its exact rear geometry and wheel support state as well. If these are missing, ground-relative BB or cockpit changes remain unknown. A fork-sag record is not a shortcut to a real whole-bike pose, handling verdict or contact-position instruction.
Distinguish a source recommendation from your observation
A source may describe setup guidance for a specific product. Preserve its scope and identity separately from what was observed. This page intentionally supplies no target: its fictional 14.3% teaches division, not an applicable recommendation. Do not combine percentages from different manufacturers, generations or load protocols into a universal setting.
A complete reference still does not establish permission to change pressure, spring parts, travel or damping, or prove that the mechanism is serviceable. If a reading is unexpected, the useful next question is which source, condition or quantity is unresolved. Numerical agreement alone cannot replace an appropriate physical inspection or exact manufacturer instructions.
Carry a readable condition ledger forward
Record exact fork/configuration, source for travel, reference state and endpoints, load description, method, all independent raw observations, marker history and any unknowns. Keep personal details out of supplementary technical notes where possible; free text is not automatically anonymous. The blank paper preview has no dedicated sag slot, so use clearly named supplementary references rather than relabelling a fixed bicycle quantity.
Continue through the existing length guide to the conditional geometry article. The free front geometry workspace retains declared dimensions and state but does not run that sag rotation. Reading or following these fixed links creates no saved bicycle, adjusts no suspension and imports no observation. Any external paper or downloaded copy remains in your own controlled workflow.
Practical questions
Frequently asked questions
Is the fictional 14.3% a recommended fork sag?
No. It is 20 divided by an independently declared 140, expressed as a percentage. This guide gives no suspension setting.
Can I divide compression by exposed stanchion length?
Not as a substitute for the applicable travel denominator. The fictional 155 mm exposed interval answers another endpoint question; using it changes the result without moving a part.
Does a 28 mm ride marker replace a settled 20 mm reading?
No. If it came from a later movement it describes that event history. Missing settled observations cannot be recovered by guessing how far the marker moved afterwards.
Do the mean and span prove that my measurement is accurate?
No. They only describe the retained observations under their stated conditions. They are not accuracy, a confidence interval or a fit result.