Load Sensitivity

Deep dive

Paul Haney explains in The Racing and High-Performance Tire: "Almost all racecar setup adjustments exploit the load sensitivity of tires. An adjustable anti-roll bar on a racecar manipulates lateral force transfer across the front and rear tire pairs, making an extremely useful and sensitive adjustment of the relative grip of the front and rear tires, universally referred to as balance." He provides the physical mechanism: as increasing load presses rubber into surface asperities, "the asperities have penetrated into the slider and further penetration slows as elastic forces in the rubber resist movement." The result: "Fdef, the deformation component of rubber friction, rises with increasing load but at a decreasing rate. Increasing from light load to higher load results in smaller incremental increases in Fdef." Guiggiani confirms in The Science of Vehicle Dynamics that the global friction coefficient "typically slightly decreases as the vertical load grows." Milliken's Race Car Vehicle Dynamics Problems states it plainly: "Friction coefficient decreases with increasing normal load, reflecting the tire load sensitivity inherent to all tires."