Rear Shock Standards Explained: Metric vs Imperial Sizing, Trunnion Mounts & Hardware | BikeQuiver Cyclopedia

Also called: Metric sizing, Standard mount metric

Eye-to-eye in 20 mm steps: 190, 210, 230, 250 mm; strokes in 5 mm steps (2.5 mm with internal spacers). Common sizes: 190x40, 190x45, 210x50, 210x55, 230x60, 230x65, 250x75

The modern shock size family agreed on by the suspension industry in 2016. Both ends of the shock have a conventional eyelet — a round hole fitted with a bushing and sizing hardware that a frame bolt passes through. The size is written eye-to-eye x stroke in millimeters, e.g. 210x55, and is almost always printed on the shock body or air can.

Why it exists: The old imperial catalog had ballooned past 80 sizes, and popular sizes like 7.875x2.25 in left too little internal room for modern damper circuits. Consolidating to a short list of metric sizes gave designers longer bushing overlap (less binding, longer service life), more internal volume, and drastically fewer hardware SKUs.

Typically found on: Nearly all full-suspension MTBs and e-MTBs from about 2017–2018 onward that don't use a trunnion mount — trail, enduro and DH bikes from virtually every brand

Also called: Trunnion, TR sizing

Eye-to-eye in 20 mm steps: 165, 185, 205, 225 mm; common sizes 165x45, 185x50, 185x55, 205x60, 205x65, 225x75. A trunnion size is 25 mm shorter eye-to-eye than the equivalent standard shock

Introduced alongside metric sizing. Instead of an upper eyelet, the shock body itself has two threaded holes in its sides and bolts (typically M6) fasten it directly to the frame's upper mount, with the eye-to-eye measured from that mounting axis to the center of the lower eyelet. The lower end is a normal eyelet with hardware.

Why it exists: Mounting through the shock body saves about 25 mm of length for the same stroke, letting designers fit longer-stroke shocks into compact front triangles, lower the standover height, and free up room for water bottles — especially valuable on e-MTBs with large down-tube batteries.

Typically found on: Many modern trail/enduro bikes and e-MTBs — e.g. numerous Trek, Specialized, Giant, Norco, Canyon and Santa Cruz models from 2017 onward. Never interchangeable with standard-eyelet shocks

Also called: Shock hardware, Mount kit, Bushing kit, Reducers

Eyelet bushing bore 12.7 mm (older 12 mm); bolt holes M6, M8 (most common) or M10; overall widths from about 15.8 to 80 mm in fine steps (e.g. M8x22.2, M8x30)

The small parts that adapt a shock's eyelet to a specific frame: a low-friction bushing (metal-backed 'DU' type on RockShox, polymer/IGUS type on Fox and Marzocchi) pressed into the eyelet, plus a pin/sleeve and spacers sized to the frame mount's width and bolt diameter. Hardware is named bolt-diameter x width, e.g. M8x22.2 means an 8 mm bolt hole and 22.2 mm overall width.

Why it exists: One shock model must fit hundreds of frame designs with different mount widths and bolt sizes; separating the wear surface and sizing spacers from the shock lets a single shock be adapted cheaply and lets the bushing be replaced when it wears.

Typically found on: Every eyelet end of every rear shock — both ends of standard shocks, the lower end of trunnion shocks. The two ends of one bike frequently need different widths

Also called: Bearing mount kit, Roller-bearing eyelet

Replaces the DU bushing in a standard 12.7 mm eyelet with a small cartridge bearing assembly; same frame bolt sizes

An upgrade/OEM variant where a needle or cartridge bearing sits in the shock eyelet instead of a plain bushing, reducing friction where the shock rotates a lot as the linkage moves. Sold as kits by RockShox, Fox and Cane Creek for specific frames.

Why it exists: On frames whose lower shock mount rotates through a large angle (many modern short-link and flex-stay designs), plain bushings add friction and wear quickly; a bearing keeps small-bump response smooth.

Typically found on: Specified by some frame makers (e.g. certain Santa Cruz, Yeti, Specialized and Norco models) and available aftermarket; only worth noting if your bike came with one

By bike type, brand & year

Only full-suspension bikes have a rear shock — hardtails and almost all road/gravel bikes do not. If your bike is from 2018 or later, it is almost certainly metric; look at the upper mount: bolts threading into the sides of the shock body mean trunnion, a normal bolt through an eyelet at both ends means standard eyelet. Bikes from 2016 or earlier are almost certainly imperial. Your bike brand's website or archive usually lists the exact shock size (e.g. '210x55 trunnion') under the model-year specs.

By measuring

With the shock fully extended, measure center-to-center between the two mounting bolts — that is eye-to-eye (on a trunnion shock, measure from the center of the trunnion bolts to the center of the lower eyelet). For stroke, measure the exposed polished shaft between the seal head and the bottom-out bumper, or subtract the fully compressed length from the extended length. A result near a tidy metric size (210, 230, 185...) with a 5 mm-step stroke means metric; odd numbers like 200x57 or 216x63 indicate an imperial shock. For hardware, measure the pin width across each eyelet and the bolt diameter (usually 8 mm) with calipers.

By stamped markings / stickers

Almost all shocks have the size printed on them. Look for a small sticker or laser etching on the shock body or air can, e.g. '210x52.5', '230X60', '7.875x2.0', sometimes with a code like 'TR' for trunnion. Fox shocks carry a 4-digit ID code on the body that can be looked up on Fox's tech site for full spec; RockShox prints size on the air can and lists it under the serial number. Also note the model name (e.g. Fox Float X, RockShox Super Deluxe Coil) — it tells you air vs coil and inline vs piggyback.