A 28mm tyre at 15% drop means ~4.2mm of deflection; too firm and rolling resistance climbs (higher impedance on road imperfections), too soft and rolling resistance also climbs (hysteresis). SILCA's professional calculator refines this with rider weight, surface quality, and tyre model. On rough surfaces or gravel, higher drop values (17-20%) are often faster than 'correct' 15%. Tyre drop is the foundation of modern tyre pressure calculation.
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