Rear Shock Sizing: Eye-to-Eye, Stroke & Trunnion

Three numbers that all have to line up

A rear shock has to satisfy three separate physical specs simultaneously to fit a given full-suspension frame: its overall mounting length (eye-to-eye), its shaft travel (stroke), and its mount hardware style (standard eyelet vs. trunnion). Get any one wrong and the shock either won't physically install, won't move through its intended range, or will contact something it shouldn't.

Eye-to-eye: the shock's mounting length

Eye-to-eye is the distance between the centers of the shock's two mounting eyes — common enduro figures are numbers like 205mm, 210mm, or 230mm. This has to match what the frame's linkage was designed around; it's a fixed geometric fact about how the linkage swings, so a mismatched eye-to-eye length either won't bolt in at all or will bind partway through the travel.

Stroke: direction matters

Stroke is how far the shock's shaft actually moves — for example 60mm, 62.5mm, or 65mm — and it's what a frame's linkage converts into rear-wheel travel. Critically, the same shock body length often comes in multiple stroke lengths, so eye-to-eye matching alone doesn't guarantee the stroke is right too.

A stroke longer than what the frame is designed for is a genuine error: the shock would be asked to compress further than the linkage and frame have clearance for, risking bottom-out contact or over-rotating the linkage. A stroke shorter than spec, on the other hand — with the eye-to-eye and mount type both still matching — is treated as a warning rather than a blocker, because it's a workable choice some riders make deliberately to reduce travel.

Mount type: standard eyelet vs. trunnion

Shocks mount to frames one of two ways: a standard eyelet (a bushing through a hole, bolted at both ends) or a trunnion mount (stub axles on the shock body itself, bolted directly to the frame at the top). These are not interchangeable — a frame built for one mount style needs a shock built the same way, and a mismatch here is treated as an error just like an eye-to-eye mismatch.

Coil shocks and hardtails

Some frames' makers explicitly state their frame is not coil-shock compatible — usually a leverage-curve consideration for that specific linkage. Where a frame maker has published that statement, a coil shock on that frame produces a warning; this check stays dormant otherwise, since it's only meant to reflect the maker's own stated guidance rather than a guess about leverage curves. And naturally, a hardtail has no rear shock at all — picking a shock for a hardtail frame is cleanly rejected as an error.

How BuildMyMTB checks this — and what it deliberately doesn't compute

• Shock eye-to-eye is compared against the frame's spec — mismatch is an error.
• Shock mount type (eyelet vs. trunnion) is compared against the frame's spec — mismatch is an error.
• Shock stroke is compared against the frame's spec — longer than spec is an error; shorter is a warning that names the frame's designed travel and points you to the maker's list of supported strokes.

One thing worth understanding: when a shorter stroke triggers a warning, the tool deliberately does not try to calculate a specific reduced rear-wheel-travel number for you. On a progressive-rate linkage, the relationship between shaft stroke and actual wheel travel isn't linear, so a simple ratio calculation would produce a wrong number dressed up as a precise one. Instead it defers to the frame maker's own published list of supported strokes — a case of the tool staying silent rather than presenting a computed figure it can't stand behind.

Frame-and-shock bundles

Some frames are sold as a package with a specific shock, and swapping in a different shock on those frames produces a warning that you'll still be paying for the bundle regardless. A small number of shocks are OEM-only (built for specific frame partnerships rather than aftermarket sale) — picking one for the wrong frame is an error, and picking one before any frame is chosen shows as an informational note.