Shock Length Calculator

Rear shocks are sold as two printed numbers, like 230×62.5mm, and pairing the wrong shock with your frame costs you travel, geometry, or both. This calculator takes your frame’s wheel travel and leverage ratio plus your shock’s eye-to-eye and stroke, then tells you the stroke the frame actually needs, whether your shock delivers full travel, and the extended and compressed mount-to-mount lengths for clearance checks. It works in millimeters or inches.

Quick Answer

How to use the shock length calculator

Take the two numbers off your shock and the two numbers off your frame chart, then read the verdict. The results stay hidden until you click Calculate.

1. Enter the wheel travel your frame is designed for, from the geometry chart (165 mm is pre-filled as the worked example).
2. Enter the leverage ratio, the frame's average (travel divided by stroke). Common trail and enduro frames sit between 2.4 and 3.2.
3. Enter your shock's eye-to-eye and stroke, the two numbers printed on the shock, like 230 and 62.5 for a 230x62.5 mm shock.
4. Pick millimeters or inches and enter everything in the same unit. Read the stroke verdict badge, then the extended and compressed lengths for your clearance check.

The sizing math: stroke, travel, and the two mount lengths

The whole tool is four lines of arithmetic. Leverage ratio is defined as wheel travel divided by shock stroke, so solving for stroke gives the requirement, and the mount lengths follow from halving the stroke around the nominal eye-to-eye.

The extended and compressed lengths are the mount-to-mount distances with the shock fully out and fully compressed. That is the number you measure against your frame when you consider a different shock: if the compressed length closes the mounts further than your frame allows, something hits something. RockShox's fitment guidance is blunt about it: a shock that is too long or too short can cause irreparable damage to the frame, and changing to a longer stroke than intended can void the warranty and cause serious injury.

Worked example: 165 mm travel, 2.64 ratio, 230x62.5 mm shock

The calculator is pre-filled with this example. Required stroke is 165 / 2.64 = 62.5 mm, exactly what the shock offers, so the badge reads green: full travel is delivered, and the shock's 62.5 x 2.64 = 165.0 mm confirms it.

Change the stroke input to 60 mm and the verdict flips: the shock gives 158.4 mm of travel against a 165 mm target, with 2.5 mm more stroke needed. That is the tool's real job, telling you exactly how far short you are, not just that you are short.

Standard shock sizes, so you can shop the verdict

Once the calculator tells you the required stroke, you need a real size to buy. Metric sizing is now the standard, with each eye-to-eye length offering several stroke options; imperial sizes still circulate on older frames. The table below lists the common combinations from Fox's shock size listings.

Eye-to-eye must match your frame exactly: only the stroke has options, and only within the range your frame tolerates. Metric and imperial are not interchangeable: a 190x51 mm shock is not a 7.5x2.0 in shock in disguise (that converts to 190.5x50.8 mm), and close is not good enough on mounting hardware.

Leverage ratio is an average, not a constant

Real frames are progressive: the ratio changes as the suspension moves through its travel, so the single number you enter is an average. It is exact enough for sizing a shock and not a kinematics analysis. A higher ratio means each millimeter of stroke produces more wheel travel, which makes the suspension feel plusher but easier to bottom out; a lower ratio feels firmer and more supportive. If your shock's stroke is shorter than required, you get less travel than the frame was designed for, and the sag point and progression sit in the wrong place, which is why the calculator reports the achieved travel, not just a pass or fail.

Suspension works as a system with the tires, so once the shock is sized, set pressures with the bike tire pressure calculator: the two interact, and a well-sized shock with wrong tire pressure still rides badly.

Can you fit a longer-stroke shock for free travel?

Sometimes, and it is called a long-shock mod, but the eye-to-eye must match and the frame must have clearance at full compression. The compressed length in your results is the check: measure whether your tire, seat tube, and linkage clear at that mount distance. Many shocks can also be converted between stroke options with internal spacers (for example a 230 mm shock across 57.5, 60, 62.5, and 65 mm), but that is a damper rebuild, not a trailside adjustment. Frame engineers size the smallest shock that covers the target ratio range, as Trek's Dylan Howes explains: weight and packaging decide, with clearance for bottles and standover as hard constraints. Deviating from the spec is your risk to manage, not a free upgrade.

If you are shopping rather than modifying, run the frame's numbers first and buy used with confidence: the bicycle value calculator helps price the bike, and this tool confirms the shock on it is the right size. And once the bike is dialed, the bike cost per ride calculator keeps the economics honest. Sizing a bike is the same kind of fit math as sizing the rider: see the ski length calculator for the winter equivalent.

What competing calculators miss

Shock sizing knowledge lives in forum threads and a couple of thin tools, and each leaves something on the table:

• Two-input estimators (Bravecalculator's shock length page): travel and ratio in, then a "recommended eye-to-eye" picked from a pairing chart, like 55.5 mm of stroke "pairs well with" 200 mm eye-to-eye. But eye-to-eye and stroke are independent dimensions: a 230 mm eye-to-eye shock comes in 57.5, 60, 62.5, and 65 mm strokes. Recommending a pairing the shock industry does not enforce is guessing dressed as math.
• Ratio-only tools (leverage ratio calculators): they compute the ratio from travel and stroke and stop. Knowing the ratio is 2.64 tells you nothing about whether the shock on your frame is the right size or how far the mounts close up.
• Sag and spring calculators: useful once the shock is sized, useless for choosing the size. They answer pressure and spring rate, not fit.
• The gap this page fills: it starts from YOUR shock's printed size instead of inventing one, verdicts stroke adequacy with the exact shortfall in millimeters or inches, and gives the extended and compressed mount-to-mount lengths for the clearance check that actually keeps frames alive. One tool, four numbers, no invented pairings.

Frequently asked questions

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