Measuring a shock absorber means recording three numbers: extended length (fully open), compressed length (fully closed), and stroke (the difference between them — the usable travel). Where you take those measurements from depends on the mounting style at each end, and getting that reference point wrong is the single most common source of a shock that’s too long or too short for the application.
The Three Numbers You’re Actually Looking For
- Extended length — the distance between the two mounting points with the shock pulled fully open.
- Compressed length — the same distance with the shock pushed fully closed.
- Stroke (travel) — extended length minus compressed length. This is the total distance the shock can move, and it’s what determines how much suspension travel the shock can support before it either tops out (fully extended) or bottoms out (fully compressed).
A shock’s part number or spec sheet will usually list these three figures. If you’re measuring an existing shock to find a replacement, or sizing a shock for a custom or lifted/lowered suspension where no factory-length part exists, you need all three — not just an overall “length.”
Where You Actually Measure From Depends on the Mount Type
This is the step most guides gloss over, and it’s where measurement errors happen. A shock’s mounting ends generally come in two families, and the reference point for the measurement changes with each:
- Eyelet (loop) mounts — measure from the center of one mounting hole to the center of the other. This applies whether the eyelet takes a bolt directly or houses a rubber bushing or a rod-end/spherical bearing.
- Stud mounts — measure from the base of the stud (the shoulder or seating surface where the mount contacts the bracket), not the tip of the threaded stud. Measuring to the tip instead of the shoulder overstates the length.
Many shocks mix the two — an eyelet on one end, a stud on the other — so check each end independently rather than assuming both match. Mounting hardware suppliers (KYB and FOX among them) publish their own internal codes for these variations (KYB’s E1/E2/E3 eyelet types and S1 stem mounts; FOX’s SM/S/EB/ES/BP/C/SB/TP series). These codes aren’t a shared industry standard — each manufacturer defines its own — so treat them as a reminder that “eyelet” and “stud” each cover more than one physical design, not as something to memorize. When in doubt, check the mount against the supplier’s own diagram.
Two Ways to Get the Measurement
Off the vehicle (preferred when possible). Remove the shock, let it extend fully on its own or pull it to full extension, and measure. Then compress it fully (by hand or with a strap) and measure again. This gives you the shock’s true extended and compressed lengths without any influence from the vehicle’s other suspension components.
On the vehicle (when you can’t remove the shock, or you’re checking fitment before buying).
- Support the vehicle on jack stands and let the suspension hang free (wheel off the ground) to approximate full extension.
- Measure mount-to-mount at that position.
- Compress the suspension — by driving one side up a ramp or jacking under the control arm/axle — to approximate full compression, and measure again.
Safety note: Compressing a suspension by jacking under a wheel or control arm, or driving one side up a ramp, puts the vehicle in an unstable position. If you’re not experienced with jacking and securing a vehicle safely, have a qualified mechanic take these measurements — a vehicle that shifts or falls during this process can cause serious injury.
Checking That Your Ride Height Falls in the Right Part of the Stroke
Once you know a shock’s stroke, the next question is whether your vehicle’s ride height puts that shock in a usable position — ideally with the piston sitting roughly in the middle third of its travel, so there’s room to compress and rebound in both directions.
QA1’s technical documentation walks through this with a worked example: a vehicle with a 14.5-inch center-to-center ride height measurement pairs well with a 5-inch-stroke shock — compressed length 11-5/8 in., extended length 16-7/8 in. — because that puts the resting position near the stroke’s midpoint. A mismatched pairing shows up fast: QA1 also cites a common Mustang II-style front end where a 10.75-inch ride-height measurement paired with a standard 7.88 in./11.00 in. (compressed/extended) coilover shock leaves only about 1/4 inch of rebound travel — not enough — pushing the better choice to an 8.63 in./12.88 in. shock instead. These are QA1’s own product examples, not a universal lookup table; the point is the method (compare your ride-height measurement against a shock’s stroke and where it centers), not the specific inches.
Building in a Margin
Because “just barely enough” travel means a shock that occasionally slams into its own internal stops, several suppliers build in intentional margin rather than sizing to the exact measured length. Summit Racing’s technical knowledgebase gives a specific version of this as a rule of thumb: add roughly 1 inch to your calculated extended length, and subtract roughly 1 inch from your calculated collapsed length, using the distance from the bump stop to the axle/frame as the basis for the subtraction. This is one retailer’s stated guideline, not a universal formula — but the underlying reason (avoid topping out on droop, avoid bottoming the bump stop under load) applies generally.
What Wasn’t Found
None of the sources gathered for this article — retailer guides, manufacturer technical pages, or AI-engine answers — provided a general lookup table matching vehicle type or lift height to a specific stroke length across brands. Every source that addressed sizing pointed back to measuring your own vehicle rather than reading a number off a chart. If you’re shopping for a replacement or custom-length shock, treat your own measurement as the input, not a published spec sheet for “trucks with a 3-inch lift” or similar.
Related reading: none yet — Coilover Hardware is currently a single-question hub. No sibling question in this hub covers a closely related angle; revisit this section once more Coilover Hardware questions are written.
