Rider weight, tire width, and surface in. Front and rear pressure out. The formula is printed below.
Monsoon Cycles, Colorado Springs · Updated August 21, 2026
Load splits 45/55 front to rear (road riding position). Each wheel's pressure is reference psi for your tire width × (your wheel load ÷ 46 kg) × surface × tubes. The reference line is what a 46 kg wheel load (a 165 lb rider plus bike) wants on smooth pavement with butyl tubes: 23mm 95, 25mm 85, 28mm 75, 32mm 60, 35mm 52, 38mm 45, 40mm 41, 45mm 35, 50mm 30, 55mm 28, 61mm 28, 70mm 26, 80mm 24, interpolated between. It follows the 15 percent casing-drop tradition that Frank Berto measured and Jan Heine's testing carried forward, and it flattens at mountain bike widths the way real pressures do.
Surface runs 0 to 10 and only ever lowers pressure: velodrome 1.15, fresh pavement 1.08, typical road 1.00, worn or chip-seal 0.95, rough road and cobbles 0.90, hardpack 0.86, gravel 0.82, chunky gravel 0.78, XC singletrack 0.75, rocky trail 0.72, downhill 0.70. Tubes: butyl 1.00, TPU 0.97, latex 0.96, tubeless 0.90. It is a starting point. Drop a few psi if your hands buzz, add a few if the tire squirms in corners.
Two limits. Below about 20 psi, rim width, casing, and inserts matter as much as any formula, so treat mountain bike outputs as a grip-first floor and add pressure if you feel rim strikes. And never exceed the maximum printed on your tire or rim.