Rear-shock stroke vs wheel travel: why one ratio is not a linkage
Separate axial shock movement, rear-axle path and vertical displacement; test why equal total travel does not determine loaded geometry.
Published 11 October 2026 · OpenBikeFit
Rear-shock reference · not to scale
Same totals. Different intermediate motion.
Fictional main-frame-relative vertical quantity W; not axle-path length or manufacturer kinematics.
A: W=3s. B: W=2s to s=20; then W=40+(11/3)(s−20). Both end at s=50 mm, W=150 mm.
At s=10 mm: A gives 30 mm, B gives 20 mm. Full-interval mean 3 is not a local ratio; B has an artificial corner.
On this page
- 01Name the moving points and the reference frame
- 02Arc length, chord and vertical displacement differ
- 03A total ratio is a full-interval average
- 04Two fictional models with identical totals
- 05Keep frame-specific configuration evidence
- 06More stroke does not prove more permitted travel
- 07Return to geometry with explicit unknowns
Free blank worksheet
Keep a named technical reference
Use supplementary reference and question spaces. There are no dedicated rear-shock or sag fields; do not relabel other dimensions.
Open toolName the moving points and the reference frame
Shock stroke and rear-wheel travel are measurements of different motions. The shock changes its mounting-axis separation along its declared working axis. The rear axle moves according to the frame linkage. Before comparing either value, identify the exact configuration and the coordinate system in which the axle motion is described. A number called travel without a definition may conceal whether it refers to a path distance, a projection or another manufacturer convention.
Keep the main frame reference distinct from the ground. A rear axle position relative to a fixed main-frame reference describes linkage motion; a complete bicycle can also rotate or translate relative to the ground. Those are additional motions. A side-view drawing may help illustrate a convention, but an unscaled photograph cannot supply all the pivot coordinates, link lengths and angles required to reconstruct a real linkage. This guide explains the distinctions without claiming to recover that geometry.
Arc length, chord and vertical displacement differ
A curved axle path has a distance along the curve, a straight chord between two endpoints and coordinate changes between those endpoints. These need not be equal. Vertical displacement is only one component of an endpoint difference. Do not treat a published wheel-travel label as a universal vertical change unless its source establishes that convention. An endpoint height also does not reveal the path that connected it to the starting point.
In a separate fictional planar example, an axle endpoint change of 30 mm rearward and 40 mm upward has a direct chord of 50 mm. Its vertical component is 40 mm. If the intervening path curves, the arc length is not determined by these two coordinates alone. None of those quantities is shock stroke unless a further model connects them. The arithmetic demonstrates why a ruler reading in one direction cannot silently substitute for all three descriptions of axle movement.
A total ratio is a full-interval average
Suppose a fictional frame-relative vertical axle quantity W rises from 0 to 150 mm while shock compression s rises from 0 to 50 mm. The total ratio 150/50=3 describes that complete interval for that specific vertical quantity. It does not establish that every millimetre of shock compression changes W by 3 mm. To predict an intermediate position, you need the applicable relationship over the interval, not only its endpoints.
Terminology matters when reading a leverage graph. A ratio may be defined as wheel movement divided by shock movement, or the inverse, and the wheel quantity may use a particular direction or path convention. Preserve the axes, units and ratio direction before comparing graphs. An instantaneous local ratio and a whole-interval quotient answer different questions. Neither becomes a spring-force, damping or handling model merely because the dimensionless number looks familiar.
Two fictional models with identical totals
Model A declares W=3s throughout 0≤s≤50 mm. Model B declares W=2s for 0≤s≤20 mm, then W=40+(11/3)(s−20) for 20≤s≤50 mm. Both start at W=0 and end at W=150 mm when s=50 mm. Model B is deliberately piecewise, continuous at s=20 and not a realistic smooth linkage. It is a counterexample showing that identical total labels cannot determine the intermediate relationship.
At s=10 mm, Model A gives W=30 mm and Model B gives W=20 mm. The shock fraction is 10/50=20% in both. The corresponding vertical fractions are 30/150=20% and 20/150≈13.3%. Thus a stated shock percentage is not automatically the same percentage of the declared wheel quantity. These fictional percentages are not sag targets, and the graph is not manufacturer kinematics. No change to an actual bicycle follows from selecting either curve.
Keep frame-specific configuration evidence
The Trek Fuel+ Gen 2 FAQ ties wheel-travel and leverage descriptions to particular rocker-link, lower-mount, wheel and shock configurations. It is a concrete reminder that a shock size cannot stand in for the whole assembly. Its frame-specific permissions and values must remain attached to that frame and generation. This guide does not transfer them to another bicycle or turn the FAQ into a generic longer-stroke conversion rule.
Record the applicable frame size, configuration, source and wheel-travel definition independently of the shock variant. If a source gives only total travel, keep the intermediate axle relationship unknown. Do not derive it by assuming a constant ratio. If two documents use different conventions, preserve the distinction and ask for the missing definition rather than combining the numbers into a single apparently comparable curve.
More stroke does not prove more permitted travel
A shock variant with a larger stroke can change the possible interval of motion, but the actual receiving system and its constraints determine whether that variant is permitted at all. Nominal extended length, mount family, tune, body envelope and complete-motion clearance remain independent. Do not extrapolate a published curve beyond its supported range. Nor does a total ratio justify multiplying extra shock millimetres into approved additional wheel travel.
RockShox’s compatibility guidance keeps the frame requirements, dimensions, tune and hardware separate. It supplies no universal equation for accepting a replacement. The guide does not direct pressure release, spring removal, cycling the linkage or bottoming a shock to obtain a new curve. A responsible controlled service process may establish appropriate evidence, but a mathematical illustration cannot authorise that process or certify its outcome.
Return to geometry with explicit unknowns
A shock compression does not by itself specify ground-relative head angle, BB height, wheelbase or rider contact coordinates. You also need the applicable linkage, front suspension condition, wheel support dimensions and complete frame pose. The existing BB and wheel-radius guides explain those ground-reference distinctions. Reuse them rather than importing the local shock subtraction directly into their quantities.
The geometry comparator requires its declared state and actual frame dimensions; it does not solve rear suspension kinematics. Write the source and unresolved axle relationship in supplementary paper notes where necessary. A useful hand-off might say: shock compression established under this condition; wheel-displacement convention known; intermediate mapping not established. That is more informative than a fabricated precise wheel sag. Continue to the loaded-state guide before interpreting any percentage as a geometry baseline or setup instruction.
Practical questions
Frequently asked questions
Does 50 mm shock stroke mean 50 mm rear-wheel travel?
No. The axle moves through the linkage, with its own motion convention and mapping.
Can I multiply every compression by total wheel travel divided by stroke?
Only if that constant relationship is separately established for the same quantity and interval. Total endpoints alone do not establish it.
Why can 20% shock compression correspond to 13.3% vertical axle displacement?
The fictional piecewise model gives 20 mm of a 150 mm vertical interval at 10 mm of a 50 mm shock interval. It is a mathematical counterexample, not a real bike specification.
Does a leverage ratio predict my BB height or spring pressure?
No. Those require additional independent geometry or force-model information not supplied here.