# Study on Inertia Load Resistance Analysis Method of Light Truck Door Latch
## 3 Theoretical Calculation of Inertia Load Resistance of Light Truck Door Latch
### Force Calculation of Parts of Light Truck Door Latch Outside Opening Kinematic Chain  
After the initial pulling force is obtained, each part of the outside opening kinematic chain of the latch was analyzed separately. The schematic diagrams of the force of the parts are shown in Figure 10, and the size parameters of the parts are shown in Table 5, where \(F_{Pull}\), \(F_{1}\), \(F_{2}\), and \(F_{3}\) are the interaction forces between the lever and the outside opening toggle arm, the outside opening toggle arm and the opening connect arm, the opening connect arm and the pawl connect arm, and the pawl respectively, \(\rm{F_{TS1}}\), \(\rm{F_{TS2}}\), and \(\rm{F_{TS3}}\) are the acting forces of the opening connect arm torsion spring, the lock return torsion spring, and the pawl torsion spring, respectively, and \(\rm{O_{1}}\), \(\rm{O_{2}}\), \(\rm{O_{3}}\), and \(\rm{O_{4}}\) are the rotation centers of each part.  
According to the force situation in Figure 10, the moment balance analysis was performed on each part, the force analysis on the pawl connect arm was performed, and the force and moment balance equations can be listed.  
\[\left(\begin{array}{cccccccc}F_{Pull}&F_{C_{1}}&F_{1}&F_{C_{2}}&F_{2}&F_{3}& F_{C_{3}}&F_{C_{4}}\end{array}\right)\left(\begin{array}{cccccccc}l_{0}, \theta_{1}&0&0&0&0&0\\ l_{0},l_{0},c_{1}\cos\theta_{1}&0&0&0&0&0&0\\ l_{0},l_{0},c_{1}\cos\theta_{4}&-l_{0},c_{2}\cos(90\arcmin\arcmin\arcmin\arcmin \arcmin\arcmin\arcmin\arcmin\arcmin\arcmin\arcmin\arcmin\arcmin\arcmin\arcmin \arcarcmin\arcmin\arcmin\arcarcmin\arcmin\arcarcmin\arcmin\arcarcmin\arcmin\arcmin \arcarcmin\arcarcmin\arcmin\arcmin\arcarcmin\arcarcmin\arcmin\arcarcmin\arcmin\where \(\theta_{1}\) represents the rotation angle of the outside open toggle arm affected by the inertial force; \(\theta_{2}\) represents the rotation angle of the open connect arm affected by the inertial force; \(\theta_{3}\) represents the rotation angle of the pawl connect arm affected by the inertial force. When \(\theta_{1}=\theta_{2}=\theta_{3}\), it means that the pawl does not rotate; when \(\theta_{1}\), \(\theta_{2}\), and \(\theta_{3}\) are not 0, it means that the pawl has rotated, and its specific value is related to the maximum torsion angle of the torsion spring. In the calculation of the resistance to inertia, half of the maximum stroke angle was taken.  
The results of the calculations are shown in Table 6. Under the side impact load of 60 g, the torsion force on the pawl along the opening direction was \(M_{T}=826.66\) N\(\cdot\)mm, which exceeded the torsion force of the pawl torsion spring by 139.55 N\(\cdot\)mm. In summary, the door lock will disengage under a 60 g side impact load.  
According to the above ideas, it can also be calculated that a minimum pulling force of \(F_{pull}=6.02\) N is required to unlock the latch in normal opening conditions. In the above calculation, \(F_{pull}=22.08\) N was generated by the outside handle to the latch under the side impact load of 60 g, indicating that the main reason that may lead to the unlocking of the door latch is that the structural design of the outside handle is unreasonable. If the structure of the outside handle is optimized, for example, by reducing the mass of the handle, increasing the mass of the counterweight, or increasing the torque of the outside handle torsion spring, etc., the F-pull of the outside handle to the latch due to inertia can  
Figure 10: The schematic diagram of the force of the parts of the outside opening kinematic chain of the latch. (**a**) Outside opening toggle arm, (**b**) opening connect arm, (**c**) pawl connect arm, (**d**) pawl.  
be reduced to less than 6.02 N so that the door latch system can meet the inertia load resistance requirements.