Rear-shock eye-to-eye vs stroke: two lengths, different endpoints
Decode a rear-shock size without confusing mount-axis distance, usable stroke, exposed shaft and rear-wheel travel.
Published 11 October 2026 · OpenBikeFit
Rear-shock reference · not to scale
Axis distance ≠ movement capacity.
Fictional extended axis distance L0=210 mm; applicable stroke S=50 mm.
Declared axial compression s=10 mm → axis distance L=200 mm; 10/50=20%.
Neither body outline, visible shaft nor rear-wheel travel is inferred. No target or replacement approval.
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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 toolRead the pair before reading the silhouette
A rear shock can have a long body, a visible shaft and a reservoir, yet none of those outlines alone defines its size. A familiar two-number label usually separates nominal extended mounting length from usable axial stroke. Keep the quantity names and units alongside the numbers. The first number is not a body measurement and the second is not automatically the distance travelled by the rear wheel. A useful reference begins with what each number connects, rather than whether two products look similar in a photograph.
Write the manufacturer, model, generation, exact size variant and source of the label before interpreting it. A frame specification, product marking and catalogue drawing are different evidence records. They may describe an original build, an available variant or the component currently installed. Do not silently treat one as the other. If they disagree, keep both statements with their origins and ask for clarification; a neat single number obtained by choosing the more convenient source would hide the unresolved identity.
Mounting axes define the extended length
For a conventional two-eye shock, eye-to-eye is the distance between the centres of the two mounting axes in the specified extended condition. It is not measured between the outside edges of the eyelets, the ends of the bolts or the end of the reservoir. Trunnion designs use a different upper mounting arrangement, so identify that family before interpreting the dimension. Manufacturer drawings are the right place to locate its upper axis; do not borrow an endpoint from the outline of a standard eyelet.
The SRAM fitment drawing labels length and stroke separately and distinguishes mount families. That helps identify the dimensions, not approve a particular frame. A projected length in an angled photograph is not the same as the direct mount-axis distance. Perspective, unknown orientation and hidden endpoints prevent a reliable conversion. Record an unclear endpoint as unknown, even when a ruler next to the picture appears to supply a plausible value.
Stroke is capacity, not exposed shaft
Stroke describes the applicable usable axial movement between the shock’s stated reference limits. The amount of shaft visible in one photograph is not sufficient evidence of that capacity. Body construction, the fitted variant and internal limits belong to the exact component specification. Do not infer that a long exposed surface can all enter the body, or that a travel marker has demonstrated every available millimetre. Those are different observations with different conditions.
The SR SUNTOUR glossary separates built-in length, stroke and mount family. Use that distinction to label a technical record, not as a home service procedure. This guide does not tell you to release pressure, remove a spring, dismantle a limiter or force the assembly to its end of travel. If the usable stroke cannot be established from the exact documentation and qualified inspection, retain it as unresolved. A guessed denominator can make every later percentage look precise while describing the wrong part.
A fictional 210 by 50 reference
Assume a purely axial model with extended mounting-axis distance L0=210 mm and applicable stroke S=50 mm. At a separately declared compression s=10 mm, the mounting-axis distance is L=L0−s=200 mm. The compression is 10/50×100=20% of the stated shock stroke. These are fictional quantities for understanding endpoints. Twenty percent is not a recommended sag, and 200 mm is not a new nominal replacement length for a component labelled 210 mm.
The same arithmetic would give a formal axis distance of 160 mm at s=50 mm in this ideal example. It does not establish real bottom-out geometry, bumper behaviour, reservoir clearance or a permitted compression procedure. Neither the 10 mm observation nor the 50 mm capacity provides the rear axle’s coordinates. A linkage connects these quantities through additional geometry. Keep the subtraction local to the named shock axes instead of applying it directly to wheel travel, BB height or saddle coordinates.
Similar labels do not approve a substitution
Two fictional declarations, 210×50 and 210×55 mm, share one nominal extended length but not the same stated stroke. That similarity does not establish identical mount hardware, reservoir shape, spring system, tune, frame permission or clearance through the full movement. A longer stroke is not a free extra-travel option. Even two declarations with the same pair may refer to different assemblies, so the pair should never be used as the entire compatibility test.
RockShox directs compatibility questions back to the exact frame requirements and separately identifies dimensions, tune and mounting hardware. Preserve those independent checks. A catalogue variant being listed for sale is not a frame approval. An old original-equipment specification is not proof that the presently installed part matches it. Do not create an adapter, stroke-change or replacement plan from this guide; unresolved constraints remain questions for the exact manufacturer and a qualified mechanic.
Make a non-invasive reference record
Begin with documents and an existing controlled technical record, not an experiment that moves the suspension to discover its limits. Name the upper and lower axes, mounting family, source-defined extended condition, nominal length, applicable usable stroke and units. Keep an observed loaded axis distance in a separate row with its load and support condition. Label whether a value is a manufacturer declaration, a marking, an observation or an assumption. Missing entries should say not established rather than zero.
A repeatable record also preserves the frame variant and configuration. A photograph can document which part was present, but it does not verify dimensions hidden inside it. A rounded catalogue number and an instrument reading do not automatically have the same precision or uncertainty. Keep their original values and methods. If a later record changes only the source or state, that is already a meaningful difference: do not erase it simply because both displayed numbers round to the same millimetre.
Carry the distinction into the geometry sheet
The blank worksheet has general reference and question spaces, not dedicated rear-shock fields. Add clearly named supplementary notes under your own controlled paper workflow. Do not write shock stroke into fork length, wheel radius or saddle height. The geometry comparator likewise uses declared dimensions and state; it does not obtain a rear-linkage model from a shock size. A complete label helps you ask a better question, but it does not supply all the information needed to answer it.
Continue with the mount guide to identify receiving interfaces, then the wheel-travel guide to separate linkage displacement from axial stroke. Finally record the loaded condition before comparing geometry. This sequence stops one convenient size pair from doing four different jobs. It is a discussion reference, not a purchasing decision, fit verdict, pressure setting or installation instruction. Exact physical permission and current measurements remain outside the mathematical example.
Practical questions
Frequently asked questions
Does 210×50 mean 210 mm of body and 50 mm of wheel travel?
No. Identify the source-defined mounting-axis length and shock stroke. Body outline and rear-wheel travel are separate quantities.
Can I call a loaded 200 mm shock a 200 mm replacement size?
No. A loaded axis distance is an observation under a condition, not the nominal extended length.
Is the visible shaft a reliable usable-stroke measurement?
Not by itself. Establish the exact variant and applicable usable stroke from its documentation and qualified inspection.
Does the fictional 20% prescribe my sag?
No. It only demonstrates 10 divided by 50. This guide supplies no target or adjustment instruction.