Towards Safer Powered Two- and Three-Wheeler Riders: Enhancing Human Body Models for Thoracic Injury Assessment

Licentiate thesis, 2024

Powered two- and three-wheelers (PTWs) make up the second largest motor vehicle fleet, with their users representing the most vulnerable group of road users. One of the most common crash scenarios for PTW riders is PTW front-to-passenger car side, which often results in serious head and thorax injuries. While helmets effectively reduce the risk for head injury, there is no similarly effective protection for the thorax available.

Finite Element Human Body Models (FE-HBMs) offer potential as tools for improving PTW rider safety; however, they must first be validated for the loading experiences by riders in PTW crash scenarios. This Licentiate thesis takes the first steps towards enhanced FE-HBMs, originally designed as car occupants, to be able to predict thoracic injury risk as PTW riders in a common crash scenario–PTW front-to-car side.

The steps taken in this thesis can be grouped into three parts:

1. Systematic Review: Existing literature was systematically reviewed to identify the most common thoracic loading conditions experienced by PTW riders in PTW front-to-car side crashes. From the synthesized data four key impact parameters—location, distribution, direction, and magnitude—were identified. These showed that the thorax often is impacted at the anterior and lateral aspect across its entire height, with the force direction varying from anterior-posterior to lateral, often accompanied by vertical components.

2. Model Validation: Four relevant Post-Mortem Human Surrogate (PMHS) test series (hub and bar impacts), that matched part of the identified thoracic loading, were selected to validate the SAFER HBM, which demonstrated fair kinetic biofidelity for 8 out of 10 impact conditions. However, loading to the superior-lateral anterior part of the thorax and vertical force components, identified in part 1, could not be matched with existing PMHS tests, highlighting the need for new tests to further validate FE-HBMs for thoracic injury prediction as PTW riders.

3. Posture Analysis: To support representative PTW rider positioning in e.g. safety system development, anatomical landmarks of 20 average male volunteers were measured and analyzed across three PTW types: naked, scooter, and touring. In addition to describing the average postures, a principal component analysis (PCA) identified seven components (PCs) explaining over 80% of the posture variability. These PCs encompassed changes in rider fore-aft position, extremity flexion-extension, pelvic tilt, spinal curvature, and head positioning. The results suggest substantial individual variability beyond what is determined by PTW handlebar, seat, and foot support configurations.

These steps address key gaps in PTW rider safety research, advancing FE-HBM validation for thoracic injury prediction and establishing a framework for modeling realistic rider posture variability to support future validation and safety systems development.