Rear-shock sag vs wheel sag: record the loaded reference, not a target

Sag describes a suspension reference under a specified load and condition. A useful technical record names which component was observed, its reference endpoints, the applicable load and support, and the state in which the observation was taken. The word loaded does not tell you whether the bicycle was seated, standing, supported, moving or settling after a disturbance. Those distinctions matter before two readings can be compared.

FOX’s sag discussion explicitly connects the observation to riding position and frame-specific context. Use that as evidence that condition belongs in the record, not as a universal percentage or an invitation to experiment with settings. This guide gives no pressure, spring, damping or sag recommendation. It is about preserving the meaning of an existing controlled observation so that a later geometry discussion does not confuse a change in conditions with a change in the component.

Take the fictional L0=210 mm, L=200 mm and usable stroke S=50 mm. The compression is 10 mm and the fraction is 10/50×100=20%. It is not 10/210×100≈4.8%, which would divide by a different quantity. It is also not proof of a recommended operating position. The example establishes what the numerator and denominator mean, not whether a real frame should use that loaded condition.

If the usable stroke is unknown, the observed 10 mm difference can remain recorded while its percentage stays unknown. If the extended reference is unknown, even that compression is not established; retain the 200 mm observation with its endpoints and condition. Do not substitute zero for either blank. This layered record avoids losing valid observations while refusing an unsupported calculation. It also makes clear which missing evidence would resolve the next question.

The wheel-displacement relationship belongs to the exact linkage and its declared convention. In the companion fictional models, both shocks compress 10 mm of a 50 mm stroke. A constant vertical mapping gives 30 mm of a 150 mm axle interval; the piecewise mapping gives 20 mm of that same interval. The shock fraction is 20% in both, while the vertical axle fractions are 20% and about 13.3%. Neither percentage establishes a real bicycle’s wheel sag.

If a manufacturer describes wheel sag along an axle path rather than vertically, keep that definition separate too. Path distance, direct endpoint separation and vertical change are not interchangeable. A total wheel-travel label divided by shock stroke only gives a whole-interval quotient for its defined quantities. It does not automatically supply the mapping at the observed loaded position. Leave wheel sag unknown until the relevant relationship and convention are established.

A compact reference note should retain exact frame and shock variants, source, mount-axis definition, extended reference, usable stroke, observed loaded distance or compression, units and the named condition. Keep instrument, method, date and repeat readings where they are available. Different readings under different loads are not a repeatability series. A spread under one controlled method is an observed spread, not an instrument-accuracy certificate or a tolerance for installation.

The free blank worksheet has supplementary reference and question spaces, not a dedicated shock-sag field. Use them with explicit names in your own controlled paper workflow, without relabelling saddle height or fork length. If you cannot establish the condition or denominator, mark it unknown and ask the responsible manufacturer or mechanic for the missing definition. Completing a record does not approve compatibility, a ride, handling, physical fit or medical outcomes; it simply keeps the evidence and its limits together.