On the origin of the breakloose friction force

We discuss the origin of the static or breakloose friction force and describe a kinetic effect that can manifest itself as an (effective) breakloose friction force. The kinetic effect is important if the kinetic friction force Fk(v) has a maximum at low sliding speed, say for v = vc. If the driving speed v is higher than vc, the friction force at the onset of slip will first increase rapidly to approximately Fk(vc) and then rapidly decrease to Fk(v), resulting in an effective breakloose friction force larger than the kinetic friction force. We present experimental results for sliding friction of a racer rubber tread compound and a passenger car tread compound, which exhibit very different breakloose friction as a result of this kinetic effect. We show that pre-slip in the contact makes the breakloose friction be less than the product of the true contact area and static shear stress. For elastically soft materials, in the absence of the kinetic effect described above, this may result in a breakloose friction force equal to the kinetic friction force. We present experimental results for a passenger car tread compound, which illustrate this effect.