Experienced bicyclists notice the effects of improper tire pressure and take action. Inexperienced riders feel the effects without understanding the cause. Extremely low pressure could damage the rims, while extremely high pressure causes a rough and painful ride. Between those extremes, the right pressure changes with terrain and with weather as well as with the style of bike and tire.
Significance
Tire pressure determines how much the tire flexes under load and how much tire surface actually contacts the ground or roadway. Greater surface contact improves traction, but too little pressure actually causes trouble. Low pressure allows obstacles, including rocks and curbs, to press the tire flat and batter the wheel rim. Low pressure also increases rolling resistance, causing the rider to expend more energy to move forward. High pressure stiffens tires, reducing traction. Extremely hard tires bounce over obstacles instead of deforming -- when tires leave the ground, control suffers. Hard tires also transfer road impacts to the rider instead of absorbing bumps.
Function
Two riders with different body weights would each find different optimum tire pressures even if using the same types of bicycles and tires. Heavier riders deform tires more than lighter riders and should inflate tires to higher pressures. Tire manufacturers stamp acceptable pressure ranges on the sidewalls of the tires but this only provides a rough guideline. Narrow tires require higher pressure than wide tires do, but with each type the correct pressure for optimum function allows a compression of 15 percent of the tire depth from ground to rim under load. Load weight includes bike, rider and all gear carried. Riders place more weight on the rear wheel, which typically carries 55 percent of the total load.
Types
The pressure requirements for road bikes depend on more factors than the size of the tire and the load on the bike. Traction requirements change with the terrain, allowing higher pressures in tires designed for smooth hard pavement. Mountain bikers lower pressure to gain traction on soft ground and rough areas. Dropping below the manufacturer's minimum inflation rating could damage the tires or rims. Punctures at low pressures sometimes result from pinching instead of piercing.
Misconceptions
Front and rear tires should not be inflated to the same pressure. Front tires need 20 percent less pressure than tires on the rear. Graphs which provide information about proper inflation pressure for different tire styles and widths under a range of load weights actually describe an average optimum pressure. Take 10 percent off that for the front tire and add 10 percent for the rear.
Expert Insight
Inflating tires with a small hand pump could be too much for some riders. Pump up a flat tire at home with a manual hand pump to see if it's truly practical as emergency gear. Pumps powered by CO2 canisters add weight to the rig if you carry spare CO2, but allow fast and accurate full inflation while on the road. Portable electric pumps with adapters for 12VDC car systems allow quick and easy tire maintenance at home.
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