Some cars have variable-ratio steering, which uses a rack-and-pinion gear set that has a different tooth pitch (number of teeth per inch) in the center than it has on the outside.
Most of the third molar embryo had been formed and their tooth crowns had been calcified, the occurrence ratio was 84 0 % in maxilla and 85 2 % in mandible.
After getting the formula about the normal backlash and the stiffness of the tooth pairs of the gear, the contact ratio and the load correction factor could be obtained.