## 1 Introduction  
Comfort is a decisive factor for motorcycle riders when selecting a helmet for use in warm climates [1; 2]. Given the lack of appropriate headgear, riders may dispense with a helmet entirely. For example, in a recent study of novice motorcycle riders in New South Wales (NSW), Australia, it was found that 10% of the subjects did not wear a helmet while riding in traffic [3]. Moreover, self-reported helmet use among teenage motorcyclists in Taiwan was found to be just 55% in summer months [4]. Accordingly, when designing helmets for motorcycle use, it is imperative that a compromise is reached between user safety and user comfort. In general, South East (SE) Asia provides an ideal location for studying the efficacy of helmets, since motorcycles and scooters represent the most common form of transportation [5]. Various helmet designs are available, including full-face, open-face, and half-face models. The choice of helmet depends on the particular preferences of the user, regarding comfort and safety. Due to the hot and humid conditions which are prevalent throughout SE Asia, open-face and half-face helmets tend to be the most frequently worn. However, wearers of open-face or half-face helmets suffer head injuries more than twice as often as wearers of full-face helmets [6]. Of the two types of helmet, open-face helmets provide a better user safety level, but at the expense of comfort. Thus, it is necessary to improve the ventilation efficiency of open-face helmets, without sacrificing their impact performance. Many different safety standards have been proposed for motorcycle helmets, including ECE R22.05, Snell M2010, and DOT FMVSS 218 [7]. In general, these standards present the requirements for the performance of a helmet, taking into consideration impacts to the top of the helmet at various energies and speeds [8]. However, while the strict test conditions prescribed in these standards are necessary to facilitate standardized testing, the collected data are only of limited use in analyzing actual crash scenarios, in which motorcycle helmets face widely varying impact angles, impact surfaces, and impact speeds. Accordingly, [PERSON] et al. [6] proposed a new test protocol for examining the crashworthiness performance of helmets under impacts at oblique angles. The impact performance of ECE-approved helmets has been investigated at various speeds in the range of 0.5 to 10.0 m/s [9]. However, few studies have investigated the impact performance of motorcycle helmets under realistic surface conditions (e.g., asphalt or vehicle bodies), or have examined the performance of common Asian helmets at typical impact speeds. Experimental investigations provide a detailed insight into the mechanical response of motorcycle helmets under impact. However, experimental tests are time-consuming and expensive. Moreover, it is impossible to accurately recreate all of the crash scenarios which may occur in real life. Consequently, the use of finite element analysis (FEA) simulations to evaluate the conformance of different helmet designs to safety standards, has attracted an increasing amount of attention in recent years. [PERSON] et al. [10; 11] performed LS-DYNA simulations of the impact response of several common helmet designs, based on the material mechanical properties obtained from experimental quasi-static impact tests. In addition, [PERSON] et al. [11] used a numerical modeling approach to evaluate the head injury criterion and brain stress for open-face motorcycle helmets, in which the conventional thermoplastic outer shell was replaced with a lightweight metal foam. Previous studies on helmet safety generally focus on just one particular helmet design (e.g., full-face or open-face), and investigate the performance of the helmet with regard to specific safety regulations.