Formula Used
The calculator uses two methods. If current pressure and measured sag are entered, it uses direct sag calibration.
New pressure = current pressure × measured sag ÷ target sag
Without measured sag, it estimates pressure from rider load, travel, sag target, style factor, and leverage ratio.
Target sag = travel × sag percent ÷ 100
Rear leverage ratio = rear travel ÷ shock stroke
Force = pressure × piston area
Temperature and altitude use absolute pressure. Gauge pressure is corrected from local air pressure and gas temperature.
How To Use This Calculator
- Select fork or rear shock.
- Enter rider weight and carried gear.
- Add travel, stroke, and target sag.
- Enter current pressure and measured sag if known.
- Add temperature and altitude when conditions differ.
- Press calculate and read the result above the form.
- Download the CSV or PDF file for your setup log.
Example Data Table
| Setup |
Rider Load |
Travel |
Sag Target |
Typical Start Pressure |
Ride Feel |
| Trail fork |
78 kg |
150 mm |
25% |
91 psi |
Balanced |
| Enduro fork |
86 kg |
170 mm |
23% |
118 psi |
Firm |
| Rear shock |
82 kg |
150 mm |
30% |
245 psi |
Plush trail |
RockShox Air Pressure Setup Guide
RockShox air suspension uses compressed air as a tunable spring. The pressure supports the rider, gear, and repeated trail impacts. A correct starting pressure makes the fork or rear shock sit in useful sag. Sag is the travel used while you stand or sit in riding position. It affects grip, comfort, steering, and bottom-out margin.
Why Sag Matters
Low pressure creates deep sag. The bike may feel soft and slow. It can dive during braking and use travel too early. High pressure reduces sag. The ride may feel harsh, tall, and nervous on loose ground. Balanced sag keeps the wheel active while preserving support for drops and turns.
Physics Behind The Estimate
This calculator treats the air chamber as a gas spring. Gauge pressure is converted to absolute pressure by adding local atmospheric pressure. Temperature and altitude are then applied with the ideal gas relation. The tool also estimates piston force from pressure and piston area. This gives a useful view of how much upward spring force the air chamber can create near the sag point.
Fork And Shock Differences
A fork normally uses lower pressure than a rear shock. The rear shock works through the frame leverage ratio. That ratio multiplies wheel movement at the shock. A higher ratio usually needs more shock pressure. The calculator uses travel and stroke to estimate that leverage. It then adjusts the pressure by sag target and ride style.
Practical Tuning Advice
Use the result as a starting point, not a final rule. Set pressure with the suspension fully extended. Cycle the suspension several times. Recheck pressure after equalizing the positive and negative chambers. Ride a familiar trail. Add pressure if it rides too deep. Remove pressure if grip feels weak or harsh. Change pressure in small steps. Record each change after every test ride.
Safety And Limits
Always check the pressure limit printed on the fork, shock, or service guide. Never exceed that limit during setup. Use a proper shock pump, not a workshop compressor at home. Air springs can lose pressure during pump removal. That small hiss is usually hose pressure. Service worn seals before chasing numbers. Smooth movement matters during rough mountain rides and landings.
FAQs
Is this an official RockShox pressure chart?
No. It is a physics-based setup estimator. Always compare the result with the product label, service guide, and real sag measurement before riding hard.
What sag should I use for a fork?
Many riders start near 20% to 25% fork sag. Use less sag for firm support. Use more sag for comfort and grip.
What sag should I use for a rear shock?
Many rear shocks start near 28% to 30% sag. Frame design matters, so confirm the number with your bike manual.
Why does altitude affect the result?
Altitude changes outside air pressure. A sealed air spring holds absolute pressure, while your pump reads gauge pressure. The calculator corrects that difference.
Why does temperature affect air pressure?
Warmer air increases pressure. Colder air lowers pressure. The calculator uses absolute temperature to estimate the pressure change.
Should I enter current pressure and measured sag?
Yes, when available. Measured sag gives a better personalized estimate than body weight alone, because it reflects your bike and riding position.
Do volume tokens change sag pressure?
Tokens mainly change end-stroke ramp. They have a smaller effect near sag. This calculator includes a light adjustment and reports effective air volume.
Can I exceed the calculated pressure?
Only stay within the limit printed on the product. Increase pressure in small steps. Stop if the ride becomes harsh or unsafe.