How do bumpers protect the vehicle and pedestrians in low-speed collisions? What is their energy absorption principle?

Using Flexibility to Overcome Hardness: Secrets of Double Protection and Energy Absorption of Buffer Plates in Low-Speed ​​Collisions
In daily traffic of city road, small collisions such as reverse scrapes and low speed rear-end collisions are common. Many people mistakenly think of bumpers as "shock resistant," but their core value lies in "intelligent attenuation" -scientific structural design and material selection that protects the body from structural damage while reducing the risk of pedestrian injury. Behind this dual protection lies the precise control of collision mechanics and the ingenious application of the principle of energy absorption in automotive engineering.

car bumper protection function always revolves around the core logic of ``avoiding hard bumps and providing soft cushioning '', forming two dimensions of body protection and pedestrian protection, closed-loop protection.
For the body, the main task of the bumper is to isolate low-speed impacts and prevent damage to the core structure of the vehicle. In a typical 10-20km/h low-speed crash, the "hard-hitting" logic of a conventional steel bumpers tends to transfer the impact directly to the chassis, longitudinal beams or rear panel, causing structural damage such as door deformation and trunk dents that can easily cost thousands of dollars to repair. Modern bumpers, however, are effectively cushioned by a three-layer system of "buffer-energy-absorption-dissipation" protection: the plastic shell disperses the force first through slight deformation, the middle buffer absorbs most of the energy and, finally, the energy absorber guides the remaining impact to the vehicle's longitudinal beams. At this point, the longitudinal beam can only bear a very small load, the main structure is basically unaffected. According to one garage, the proportion of body parts damaged in low-speed collisions has fallen from 60% to less than 10% per cent for cars with modern energy-absorbing bumpers, with average repair costs plummeting by 70%. More importantly, the design can prevent body structure deformation from affecting the stability of the vehicle and reduce subsequent safety hazards at source.

For pedestrians, bumpers are the first line of defense for lower limb safety. When pedestrians collide head-on with a vehicle, their lower limbs first come into contact with the vehicle, resulting in a high incidence of knee ligament rupture, lower leg fractures and other injuries. Modern bumpers are designed to have three main features: first, a soft surface treatment with a flexible plastic cover to avoid local stress concentration due to overly rigid materials; second, a highly attuned design that aligns the underside of the bumper with the center of a pedestrian's lower leg to reduce the risk of leg rollover; and third, a continuous stress band design that avoids sudden stiffness changes in locations like a mop or radar modules to prevent high intensity damage. Eligible bumpers must limit knee ligament elongation to 22mm and calf dynamic torque to no more than 340 N·m, according to national standard GB 24550-2024 Car Pedestrian Collision Protection. Real-world test data shows that optimised bumpers can reduce pedestrians' probabilities of leg injuries by more than 37%.

The core of bumper energy absorption is the transfer of collision kinetic energy to non-destructive kinetic energy through material deformation and structural collapse, rather than directly to the colliding object. This principle can be summed up as the dual effect of "material empowerment + structural synergy."

The choice of materials is the basis of energy absorption. New material, represented by EPP (expanded polypropylene, completely overturns the traditional view that hardness equals safety. EPP material is filled with individual chamber air bubbles, 0.1 to0.5 millimeters in diameter, like countless miniature springs. On impact, the air bubbles compress and deform to absorb 70%-80% of the impact, bounce back on impact and can be reused with simple maintenance. Compared to conventional steel bumpers (which have a density of approximately 7.8g/cm3) and conventional polypropylene plastic bumpers, EPP bumpers have a density of only 0.03-0.05g/cm3, enabling a lightweight design while increasing energy absorption efficiency several times. In addition to EPP (Energy Precipitation Panel), bumper system use different materials depending on the functional area: bumper beams are mainly made of steel or aluminum alloy and are 480MPa-980MPa in strength to ensure basic support; energy-absorbing blocks use materials such as polyurethane foam to further enhance the cushioning effect.

Structural design is key of energy absorption, and the bumper system gradual energy dissipation through multi-level coordination. The entire bumper system consists of four parts: the outer plate, cushioning layer, anti-impact beam and energy absorber, which form an orderly chain of energy absorbers. After impact, the outer plate first deforms flexibly, then disperses the impact force. The cushioning layer (EPP or foam) compresses, converting kinetic energy into material deformation energy energy. The anti-impact beam acts as a bridge, supporting the full deformation of the cushioning layer, transferring the dispersed impact force to the suction chain. In addition, the bolt connection between the suction box and the longitudinal beams is designed to facilitate maintenance and replacement after impact, avoiding concentrated transmission of impact of welded structures.

From early days of rigid steel bumpers to today's "soft yet strong" systems, the development of car bumpers is a history of evolving concepts of car safety. It proves that real safety lies not in "impregnability" but in "suction" -the perfect combination of materials science and structural engineering to protect vehicles and pedestrians in low-speed collisions. With the strict law of pedestrian protection regulations and the progress of material technology, the energy absorption efficiency and protection accuracy of bumper will be improved and become the important guardians of urban road safety. This advanced energy absorption system provides just the kind of quiet protection that "only needs painting and tinkering" after a minor crash in everyday driving.