## 1 Introduction  
Bicycling, for recreational, transport and sport purposes, provides health benefits for the individual as well society [1]. However, due to different reasons, bicyclists can be subjected to crashes, which can cause injuries over the whole body. Of these, head injuries can lead to serious brain damage and, in extreme cases, death [2]. Head injuries can be prevented by wearing bicycle helmets, thereby increasing cycling safety [3]. However, while it is well known that bicycle helmets can be lifesaving in case of an accident, they are not worn frequently. A variety of barriers of social, psychological, cultural and biological origin have been reported [4]. One of the most common complaints associated with wearing bicycle helmets appears to be thermal comfort [5]. In a survey study by [PERSON] et al. [5], it appeared that \"uncomfortable\" and \"it's hot\" were two of the most important barriers for wearing a bicycle helmet in all three age categories (children (7-10), adolescents (11-19) and adults (>19)). Furthermore, [PERSON] et al. [6] concluded in their review study, which investigated the ergonomics of headgear, that unfavourable thermal sensation or thermal discomfort is frequently used as an argument for not wearing headgear. Wearing a bicycle helmet alters the local skin temperature and sweat rate, which can lead to thermal discomfort [6]. Moreover, under exertion, the human body dissipates a significant fraction of its excess heat through the head, which, during cycling, is placed in a strong air current. The helmet insulates the head, limiting the transfer of heat to the air and the evaporation of sweat [7]. Therefore, it is of utmost importance for bicycle helmets to be designed in a way that favours thermal comfort whilst meeting mechanical protection requirements. This dual goal of protection and comfort poses a great challenge because of the often-contradictory requirements of thermal comfort and impact protection [8]. Sensations concerning thermal comfort are the result of a cognitive process described by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standard 55 [9] as \"that condition of mind, which expresses satisfaction with the thermal environment\" (ASHRAE standard 55-66). Comfort is a recognisable state of feeling which is the result of the entire environment, including psychological and physiological variables. It is usually associated with conditions that are pleasant and compatible with health and happiness, whereas discomfort is associated with pain, which is unpleasant [10]. Skin temperature and sweat rate are examples of the body's mechanisms to keep body temperature quasi-constant. These mechanisms are controlled by a region in the brain called the hypothalamus. This regulation centre monitors body temperature and controls it directly by physiological processes and/or indirectly by behaviour (i.e., behavioural thermoregulation). Although a person's reported state of thermal comfort is purely perceptive, the body's thermoregulatory actions influence thermal comfort by its outcomes (e.g., sweat rate, skin temperature, etc.) [11; 12; 10]. An often-used method to accurately assess thermal sensation (TS) and comfort (\(T_{\text{C}}\)) is to ask individuals directly about their thermal sensation perception [13; 11]. Individuals express their opinion to rate their thermal sensation/comfort when they are exposed to given thermal conditions by using a scale from cold to hot that has a predefined number of points. Mathematical models of thermal sensation and comfort have been developed to overcome the difficulties of direct enquiry of subjects. The development of such models has mostly depended on statistical approaches that correlate experimental condition data (i.e., environmental and personal variables) with thermal sensation votes obtained from human subjects [11; 14]. Thermal comfort models, such as predicted mean vote (PMV), predict the state of thermal comfort from thermoregulatory actions such as skin temperature and sweat secretion.