Free, source-aware tool

MTB SUSPENSION CALCULATOR

Calculate fork and rear-shock sag in millimetres, get a manufacturer-backed starting pressure where the exact source supports one, and see when an official bike or serial-number lookup must take over.

THE IMPORTANT BIT

PSI is a starting input; measured sag is the setup target. This calculator never invents a universal rear-shock pressure, extrapolates beyond a published chart, or changes PSI automatically for riding style or volume spacers.

01 — RIDER

Body weight drives the FOX rear-shock starting method. Body plus kit is matched to the fork chart and is the weight you use when measuring sag.

Body weight

Enter body weight.

Kit + hydration

Unit

02 — FORK

Exact fork source profile

Uses the published 2026 FLOAT starting-pressure chart. Confirm the model year before using the number.

Fork travel (mm)

Published 2026 FOX 38 travel range: 130–180 mm.

Target fork sag (%)

FOX specifies 15–20% for the 2026 38. RockShox describes 10–20% as firmer and 20–30% as more sensitive; use the exact manual.

03 — REAR SUSPENSION

Exact rear-shock source profile

FOX says to begin at body weight in pounds, equalise the chambers and then adjust to 25–30% sag. Frame guidance still takes priority.

Shock stroke (mm)

Use shock stroke, not rear-wheel travel. Check the shock ID or bike manual.

Target rear sag (%)

FOX specifies 25–30% for 2026 FLOAT X/SL and approximately 30% for FLOAT X2.

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Quick answer

Enter body weight, riding-kit weight, fork travel, shock stroke and sag targets. Roadman calculates sag in millimetres and returns an official starting pressure only where the selected model-year source supports one. For bike-specific or serial-specific products, it sends you to the manufacturer lookup instead of inventing a universal PSI.

WHAT IT DOES

This is a source-aware setup assistant, not a universal pressure formula. It converts your chosen fork and rear sag percentages into millimetres, matches a dressed rider to the published 2026 FOX 38 chart, and applies FOX's published body-weight starting method to the selected 2026 FLOAT rear shocks. Those values begin the setup; measured sag decides it.

For RockShox, another FOX generation, another air shock or a coil shock, the tool deliberately withholds an unsupported number and routes you to the product, serial-number or bicycle-specific source. Tyre pressure has its own canonical calculator rather than a second hidden model on this page.

WHO IT'S FOR

→MTB riders who need a measurable fork and rear-sag target
→FOX 38 owners who can confirm the exact 2026 chassis variant
→FOX FLOAT X, FLOAT SL or FLOAT X2 owners checking the published 2026 starting method
→RockShox riders who need the correct path to Trailhead rather than a generic pressure table
→Air- or coil-shock owners who want to know where a calculator must stop

HOW IT WORKS

Sag in millimetres equals suspension travel or shock stroke multiplied by the chosen sag percentage. FOX 38 starting PSI is a direct band lookup from the official 2026 table—no interpolation, extrapolation, riding-style multiplier or volume-spacer multiplier. Selected 2026 FOX rear shocks use FOX's body-weight-in-pounds first-inflation method, subject to the stated maximum; rear sag then calibrates the bike-specific result. Other products require their official lookup.

01
Identify the exact products
Confirm fork and shock model, model year, travel or stroke, air-spring variant and every maximum stated by the bicycle and component manufacturers.
02
Enter body and kit weight separately
FOX's rear starting method refers to body weight in pounds. The fork chart and sag check use the weight of the dressed rider, so Roadman keeps those two values visible.
03
Choose a sag target from the exact manual
Enter the percentage specified for the product and bike. Roadman converts it to the millimetre distance the O-ring should move.
04
Inflate and equalise exactly as instructed
Open compression damping, use a shock pump, respect every maximum and follow the product-specific positive/negative chamber equalisation sequence.
05
Measure, adjust and record
Settle into normal riding position in full kit, measure sag without bouncing, make a small pressure adjustment, equalise again where required and repeat before touching rebound or compression.

EXAMPLE CALCULATIONS

2026 FOX 38 FLOAT, 80 kg dressed rider

· Body weight: 77 kg
· Kit + hydration: 3 kg
· Fork travel: 170 mm
· Target fork sag: 20%

The official 170–180 lb chart band starts at 93 PSI. The measurable target is 34 mm sag. Inflate, equalise and adjust to that target; do not add a riding-style or spacer multiplier.

2026 FOX FLOAT X with 55 mm stroke

· Body weight: 77 kg / about 170 lb
· Target rear sag: 30%
· Shock stroke: 55 mm

FOX's first-inflation method starts at about 170 PSI. The target is 16.5 mm sag. The final pressure can differ with the frame, so measure and adjust rather than treating 170 PSI as the answer.

LIMITATIONS

Roadman cannot identify a component from appearance, read its serial number, see the bicycle's leverage curve or know its OEM tune, service condition, temperature, permitted spacers or manufacturer limits. The numeric fork chart is intentionally limited to four 2026 FOX 38 variants and the published weight range. The FOX rear value is explicitly a first-inflation method, not a bike-specific final pressure. No generic coil spring rate, rebound click count or automatic style/spacer adjustment is produced.

When to get mechanical help

Suspension faults and compatibility are mechanical questions, not coaching questions. Stop riding and use a qualified suspension technician or the bicycle/component manufacturer if you cannot identify the product, a calculated starting method exceeds a limit, the unit is stuck down, loses pressure, leaks, tops out harshly or still behaves abnormally after correct sag setup.

SOURCES & TRUST

Author

Anthony Walsh — Cycling Coach & Podcast Host

Evidence sources

FOX 2026 38 mm Owner’s Manual — published pressure bands, 15–20% sag, maximum pressure and spacer boundaries
FOX 2026 FLOAT SL and FLOAT X Owner’s Manual — body-weight starting method, 25–30% sag, equalisation process and maximum pressures
FOX 2026 FLOAT X2 Owner’s Manual — body-weight starting method, approximately 30% sag, equalisation process and 350 PSI maximum
SRAM/RockShox Suspension Manual — product-specific pressure lookup, sag method and damping sequence
RockShox Trailhead — official product lookup for starting pressure, rebound, service and tuning data

Reviewed by Anthony Walsh — source-to-calculation and rider-facing method review · Last reviewed 26 August 2026

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FREQUENTLY ASKED QUESTIONS

What is the correct MTB suspension pressure for my weight?+

There is no universal rider-weight-to-PSI answer. Fork pressure depends on the exact chassis, air spring, travel and model year. Rear pressure also depends heavily on the bicycle's leverage curve and OEM tune. Use the matching manufacturer starting point, then adjust until measured sag matches the bike and component manuals.

How does the FOX suspension calculator work?+

For the four listed 2026 FOX 38 variants, Roadman selects the published weight band without interpolating or extrapolating. For the listed 2026 FLOAT rear shocks, it applies FOX's body-weight-in-pounds first-inflation method and stated maximum. In both cases the rider must then equalise the air chambers and tune to measured sag.

Why does the RockShox suspension calculator send me to Trailhead?+

RockShox directs riders to the pressure decal or Trailhead for the product-specific starting pressure and rebound setting. Model name alone is not enough because generations, travel and air-spring configurations differ. Roadman still calculates the sag distance, but it does not copy one RockShox table across incompatible products.

What is sag and how do I calculate it?+

Sag is how far the suspension compresses under the dressed rider in normal position. Multiply fork travel or shock stroke—not rear-wheel travel—by the target percentage. A 170 mm fork at 20% is 34 mm. A 55 mm shock at 30% is 16.5 mm.

Do volume spacers mean I should add more PSI?+

Not through a universal percentage. FOX describes spacers as a way to change mid-stroke and bottom-out resistance after sag is set, while RockShox describes tokens as increasing progression. Follow the permitted configuration for the exact product and repeat the complete sag process after a change.

Can rider weight alone calculate a coil spring rate?+

No. A defensible spring-rate calculation also needs frame motion ratio or leverage curve, rear travel, shock stroke, weight distribution and the manufacturer's preload limits. Use the bicycle maker or shock maker's bike-specific spring calculator; Roadman will not disguise a rider-weight shortcut as engineering.

Is Roadman affiliated with FOX or RockShox?+

No. Roadman Cycling is an independent publisher and calculator. FOX, SRAM and RockShox names identify the official source profile selected by the rider. Their manuals, bike-specific instructions, warnings and maximum limits always override this page.