The rear bumper of the car - what is the lower part of the vehicle?
The rear bumper of the car is located at the bottom of the vehicle's rear end, where the lower edge of the tail lights connects with the rear body panel. It is the outermost protective structure at the rear of the vehicle. It is usually composed of a plastic shell, an energy-absorbing buffer layer, and an internal impact beam. During low-speed collisions, it deforms to absorb the impact force and protect the trunk and the cabin structure.
Structural composition
Outer panel (skin): Mostly made of polypropylene plastic, responsible for appearance and slight scratch rebound, lightweight and corrosion-resistant.
Buffer layer: The inner side's honeycomb foam or energy-absorbing material, for dealing with low-speed collisions.
Impact-resistant beam: Steel or aluminum alloy, the core component for resisting medium-high speed impacts.
Energy-absorbing box: A collapsible metal piece connecting the impact-resistant beam and the vehicle's longitudinal frame, which disperses energy through folding deformation.
Common names
Rear bumper diffuser / spoiler: The black part below the bumper, mainly functioning to reduce lift, lower wind resistance and fuel consumption, and enhance stability at high speeds, also protecting the bottom components.
Underbumper guard / lower trim panel: The guard covering the bottom of the vehicle, protecting against stone strikes and dust, some models also called rear bumper underguard.
A point that is often confused: The rear bumper itself is a covering component, belonging to the "face", responsible for styling and low-speed protection; the actual core component that withstands medium-high speed impacts is the hidden impact-resistant beam and the vehicle's longitudinal frame skeleton. When choosing a car, don't rely solely on knocking on the bumper to judge safety.
Rear bumper usually also integrates reverse radar sensors, fog lights or reflective markings, and some sports models have diffuser shapes on the bottom, used to optimize the airflow at the rear.
The lower part of the vehicle body of the rear bumper (usually called rear bumper underguard or diffuser) mainly fulfills three functions: protection, aerodynamic optimization and functional integration.
Protection function: Covers the bottom of the vehicle, preventing damage to the paint and vehicle structure from road debris, curb or minor collisions when driving at low speeds, and buffering the impact force during low-speed collisions to reduce maintenance costs.
Aerodynamic function: As a diffuser, it guides air flow at high speeds, reduces the lift at the bottom of the vehicle, prevents rear wheels from drifting, improves driving stability, and reduces wind resistance and fuel consumption.
Functional integration: Some models integrate reverse radar sensors, fog lights or reflective markings in this area, assisting in parking and night driving safety.
If the underguard is damaged, it is recommended to replace it in time - as it is not only a decorative component, without it, the lift at the bottom of the vehicle will increase when driving at high speeds, potentially causing steering instability.
The term "car rear bumper - lower body failure" is rather broad. I will first help you sort out the most common situations based on this, and you can match it with your vehicle condition.
These problems usually manifest as three types: cracking, loosening/falling, and scratches/scrapes. The handling methods are different.
Common faults and judgments
Cracking/crevasse: Mostly caused by external force collisions, low-temperature plastic brittleness or long-term aging. If it is only fine lines on the surface, it can be repaired; if the crack is deep, penetrating or affecting the brackets and radar, it is recommended to replace it.
Loosening/falling: Mostly caused by broken fixing clips, loose bolts, or long-term vibration and shaking. The inspection method is simple - gently push the sides and seams of the bumper in a quiet environment, if there is obvious shaking or abnormal noise, it is loose.
Scratches/scrapes/ paint damage: Most common when parking, meeting, reversing, caused by hitting obstacles, belonging to physical appearance damage, does not affect functionality.
Treatment suggestions
Minor cracking/surface fine lines: Can try professional plastic welding or special repair glue, before repairing, ensure the surface is clean and dry. You can also go to a repair shop for plastic welding reinforcement and then apply putty and paint.
Loosening/falling: Check the clips and bolts, if the clips are broken, replace with the original factory specifications, use a torque wrench to tighten the bolts according to the specified torque. If the fall is severe, check if the bottom brackets are damaged or rusted. Never forcibly press to reset, as it can easily crack the plastic part.
Severe cracking/structural damage: It is more reliable to replace directly. If the repair cost is close to the replacement cost, or the crack has affected the installation brackets, radar sensors, it is recommended to replace with the original factory parts.
If the rearview radar or camera is integrated inside the bumper, after disassembly, sensor calibration is required. The vehicle cannot be delivered directly after installation.
Precautions
Choose flat areas for parking. Avoid forcefully moving the vehicle in low-temperature conditions or getting close to obstacles.
Regularly check the condition of the clips and seams, especially on bumpy roads or after driving for over 50,000 kilometers.
When washing the car, do not use a high-pressure water gun to directly spray the seams of the bumper, and do not hang heavy objects on the rear bumper.
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