The reasons for the improper control arm may include the following:

1. Size mismatch: the size of the control arm does not match the suspension system of the vehicle, which may lead to inaccurate installation position and affect the geometric structure and performance of the suspension system.

2. Inconsistent material: The selected material of the control arm does not meet the use requirements or specifications of the vehicle, which may lead to insufficient strength or fragility of the control arm and affect the stability and durability of the suspension system.

3. Incorrect installation angle: Incorrect installation angle of the control arm may lead to inaccurate wheel positioning and affect the driving stability and handling performance of the vehicle.

4. Insufficient or excessive torque: insufficient or excessive torque of the bolt during the installation of the control arm may lead to unstable installation of the control arm, loosening or fracture of the bolt, and affect the safety performance of the suspension system.

5. Accessories mismatch: The use of accessories that do not meet the vehicle model and specification requirements may lead to poor coordination between the control arm and other suspension parts, which will affect the working effect of the whole suspension system.

6. Bad installation environment: When installing the control arm, the working environment is unclean or there are sundries, which may lead to pollution or damage of the control arm and affect the normal operation of the suspension system.

Therefore, when selecting and installing the control arm, we should pay attention to the above reasons, ensure the matching and applicability of the control arm and the vehicle suspension system, avoid inappropriate situations, and ensure the normal operation of the suspension system and the safety and stability of the vehicle. In case of uncertainty, it is recommended to seek the help of professional technicians or automobile maintenance personnel for selection and installation.