Pedestrian protection legform impact assessment measures the injury risk to a pedestrian's lower limbs when struck by the front end of a vehicle, using standardized leg impactors launched at 40 km/h against the bumper area. The evaluation verifies compliance with regulatory and rating requirements such as GB 24550, UN ECE Regulation No. 127 and the Euro NCAP pedestrian protection protocol, and is typically carried out by accredited third-party laboratories. This article explains the technical principles, sample requirements, measured parameters, test methods and typical applications of automotive legform impact testing.
Technical Overview
Pedestrian protection legform impact assessment is a standardized laboratory crash-test procedure that quantifies how a vehicle's front-end structure, primarily the bumper, lower fascia and front-end module, behaves when it strikes the lower limbs of a pedestrian. In vehicle-to-pedestrian collisions, injuries to the legs and knees are among the most frequent serious outcomes, and ruptures of the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL) or medial collateral ligament (MCL), together with tibia and fibula fractures, frequently result in long-term disability. The purpose of the assessment is therefore to verify that the bumper zone deforms in a controlled, energy-absorbing manner rather than transmitting excessive loading to the leg and knee joint.
The test is defined by a family of international and national requirements, the most widely applied being:
- GB 24550, the Chinese national standard specifying pedestrian protection performance requirements for passenger vehicles, including lower leg impactor to bumper tests.
- UN ECE Regulation No. 127 and the UN Global Technical Regulation No. 9 (GTR No. 9), which define legform-to-bumper test procedures and injury thresholds for type approval.
- Euro NCAP and C-NCAP pedestrian protection assessment protocols, which apply the same physical tests but with stricter, dual-level performance limits used for star ratings.
- ISO 6487, which governs measurement instrumentation, transducer calibration and data channel frequency classes used during impact data acquisition.
Three generations of lower leg impactors are used depending on the protocol and vehicle geometry: the rigid EEVC-style lower leg impactor of approximately 13.4 kg, the flexible pedestrian legform impactor (FlexPLI) of approximately 25 kg with a segmented, biofidelic tibia, and the advanced pedestrian legform impactor (aPLI), which additionally represents the femur for vehicles with high bumpers or SUV-style front ends.
Testing Samples
Complete Vehicles
Full-scale assessment is performed on production-representative vehicles in a defined ride attitude, with production bumper systems, grille structures, energy absorbers and underbody components fully installed. The vehicle is conditioned to a stable laboratory temperature, typically around 20 °C ± 4 °C, set to the specified tire pressures and laden condition, and rigidly supported so that the bumper test zone is presented in its designed position relative to the ground plane.
Front-End Subsystems and Components
During design and development, legform tests are frequently performed on subsystem rigs rather than complete vehicles. Typical samples include bumper beams, crush cans, polypropylene or foam energy absorbers, bumper skins, lower grilles and complete front-end modules mounted on a rigid fixture that reproduces the vehicle installation position and backing stiffness. Subsystem testing allows rapid iteration of absorber geometry and material selection before full-vehicle verification.
Materials and Development Specimens
Material-level characterization supports the impactor assessment, including energy-absorbing foams, expanded polyolefins, long-glass-fiber polypropylene brackets and other deformation elements. Quasi-static and dynamic compression tests on these specimens provide input data for CAE models that are later correlated against physical legform impact results.
Testing Items
During a legform impact, a defined set of biomechanical and kinematic quantities is recorded, and each result is compared against the performance limits of the applicable protocol. The principal measured parameters are:
- Tibia acceleration, the primary assessment quantity for the rigid lower leg impactor.
- Knee bending angle, indicating the risk of ligament rupture due to excessive knee joint rotation.
- Knee shear displacement, the relative forward displacement between the tibia and femur sections.
- Tibia bending moments measured at multiple instrumented cross-sections of the flexible legform impactor, indicating fracture risk along the tibia.
- MCL elongation, the stretch of the medial collateral ligament measured by the knee instrumentation.
- ACL and PCL elongation, the stretch of the cruciate ligaments within the knee joint.
- Femur bending moments and loads for the advanced legform impactor when a femur section is fitted.
- Test validation quantities, including impact velocity, impactor attitude, contact timing and high-speed video documentation of the deformation sequence.
Testing Methods
Test Area Definition and Point Selection
The legform test area is defined across the bumper between the corner reference locations established from the vehicle's outer contour. A grid of candidate impact points is laid out at intervals of approximately 100 mm, from which points are selected to cover structural hot spots such as bumper beam ends, crush can positions, towing attachments and grille stiffeners. Computer-aided engineering analysis is commonly used to preselect the most critical locations before physical testing.
Impactor Propulsion and Alignment
The selected impactor is accelerated in free flight by a pneumatic or hydraulic propulsion system to the specified velocity of 11.1 m/s (40 km/h) with a tolerance of typically ±0.2 m/s. The impactor travels parallel to the vehicle longitudinal axis, is oriented vertically at first contact, and, for flexible legforms, is positioned with its lower edge approximately 25 mm above the ground plane. Impact accuracy, velocity stability and angular deviation at contact are verified for every launch.
Instrumentation and Data Processing
The impactor is instrumented with accelerometers, strain gauges and displacement transducers located in the tibia sections and the knee joint. All channels are acquired and filtered in accordance with ISO 6487 using appropriate channel frequency classes. Each test is documented with high-speed video from at least two orthogonal directions, contact sensors, and photogrammetric tracking of impactor kinematics, allowing full traceability of every measurement.
Evaluation and Scoring
For the rigid legform under UN ECE Regulation No. 127 Phase 1 / GTR No. 9, typical compliance thresholds are tibia acceleration of about 170 g, knee bending angle of about 19 degrees and knee shear displacement of about 6 mm. For the flexible legform, regulatory limits are in the order of a maximum tibia bending moment of 340 N·m, MCL elongation of 22 mm, and ACL and PCL elongation of 10 mm. Rating protocols apply dual performance limits: for example, under Euro NCAP, full points are awarded when tibia bending moment remains below approximately 282 N·m and MCL elongation below 19 mm, while scores reduce progressively to zero beyond about 340 N·m and 22 mm, with ACL and PCL elongation required to stay within 10 mm. Results from all grid points are aggregated into an overall leg protection score used in the vehicle safety rating.
Applications
Legform impact assessment supports vehicle safety engineering and market access throughout the product lifecycle:
- Regulatory type approval and homologation of new passenger vehicle models against national pedestrian protection requirements such as GB 24550.
- Consumer safety rating programs, including Euro NCAP and C-NCAP star ratings, where legform results contribute directly to the overall pedestrian protection score.
- Front-end design optimization, including energy absorber packaging space, bumper beam position, foam density selection and lower grille stiffness tuning.
- Supplier component validation for bumper systems, front-end modules and pedestrian protection airbag integration.
- Correlation and validation of CAE models, enabling virtual legform simulation to reduce the number of physical prototypes required.
- Competitive benchmarking of competitor front-end architectures and documentation of design compliance during development audits.
FAQ
At what speed is the legform impact conducted?
The standard test speed is 40 km/h (11.1 m/s), representing a typical pedestrian collision speed in urban traffic. The tolerance on velocity is normally ±0.2 m/s, and every launch is verified with calibrated speed measurement immediately before contact.
What is the difference between the rigid legform and the flexible legform impactor?
The rigid lower leg impactor consists of foam-covered rigid femur and tibia segments with a mechanical knee, and is evaluated through tibia acceleration, knee bending angle and knee shear displacement. The FlexPLI uses a segmented, flexible tibia that more faithfully reproduces the bending response of a human leg, so injury is assessed through tibia bending moments and direct measurement of ligament elongation. The aPLI extends this approach with a femur section for high-front vehicles.
Can the test be performed on components instead of a complete vehicle?
Yes. Front-end modules, bumper beams and energy absorbers can be evaluated on rigid fixtures that reproduce the vehicle installation geometry. Component-level results are widely used during development, although final homologation and rating tests require the complete vehicle in production condition.
How do legform results affect a vehicle's safety rating?
In rating protocols, each bumper grid point receives a score based on where the measured values fall between the higher and lower performance limits. The point scores are aggregated into a leg protection result that contributes a defined share of the overall pedestrian protection score and therefore influences the vehicle's star rating.
What deliverables are provided after the assessment?
Accredited third-party laboratories typically provide a full test report including injury parameter time histories, filtered peak values against the applicable limits, point-by-point scoring, high-speed video, impactor calibration status and a compliance summary mapped to the requirements of the selected standard or rating protocol.
How can a design be improved if limits are exceeded?
Common countermeasures include increasing the deformation space between the bumper skin and the beam, adopting lower-density energy-absorbing foams, relocating or softening stiff structures such as towing brackets and grille reinforcements, and lowering the effective bumper stiffness in the tibia contact zone. Each modification is normally verified first by simulation and then by repeated physical impact tests.
