The Two Forces at Play
When you ride, two main forces try to slow you down. The goal of optimal tire pressure is to find the perfect balance between them.
1. Casing Losses (Heat)
As your tire rolls, it flexes where it touches the ground. This flexing creates heat, which takes energy away from your forward momentum. Rigid tires or tires with tubes create more friction.
The Traditional Wisdom: Higher pressure reduces flexing, so it reduces heat/casing losses. This is why we traditionally pumped tires very hard.
2. Impedance Losses (Vibration)
The road isn't perfectly smooth. Every tiny bump forces the bike and rider to lift up slightly. This vibration wastes a tremendous amount of energy.
The New Science: Lower pressure allows the tire to absorb these bumps like a suspension spring, keeping the bike level and forward-moving. This reduces "Impedance Loss."
The "Breakpoint"
There is a specific pressure where the Casing Losses (decreasing with pressure) and Impedance Losses (increasing with pressure) cross paths.
This intersection is the Breakpoint—the fastest possible pressure for that specific road surface.
The Golden Rule
Extensive testing has shown a critical asymmetry in these losses:
- If you are 5 psi too Low: You only lose a tiny bit of watt efficiency (casing drag), but you gain comfort.
- If you are 5 psi too High: Impedance losses skyrocket. You vibrate more, lose traction, and waste significant wattage.
Conclusion: It is always better to be slightly under-inflated than over-inflated.