Bicycles are among the simplest and at the same time most fascinating vehicles. Despite their apparently basic design, their stability, dynamics, performance, and safety are still largely unknown. In a world where cities suffer from critical air pollution levels and are often congested by car traffic, bicycles can represent a cost-effective and ready solution to address part of the problem. However, the increasing use of bicycles enlarges the risk of bicycle-related accidents and injuries. Many of these accidents involve falls, highlighting the need of proper studies on bicycle-rider dynamics. While research has been conducted on the effect of bicycle parameters on the handling quality properties, we still do not know enough about the specific role of tyres on bicycle-rider dynamics. To better simulate what happens in the real world, existing bicycle mathematical models need to be integrated with robust tyre models that include side slip, as they can change both the lateral and longitudinal dynamics. Actually, the use of nonlinear tyre models allows taking into account the lateral force and self-aligning torque saturation. This is needed for instance to model the wobble as a non-divergent vibration modes, contrarily to what happens when using a simple linear tyre model. These features relate directly to predict critical situations for which we need a deep knowledge of bicycle tyres, thus proper test-rigs to measure their mechanical characteristics...

Tyre characteristics can strongly affect bicycle dynamics, therefore the overall bicycle performances. However, it may be hard to measure lateral characteristics with low uncertainty. Proper test-rigs are needed to obtain reliable tyre parameters, to be used then for modelling. The paper presents VeTyT, acronym of “Velo Tyre Testing”, a new test-rig specifically developed for bicycle tyres at the Department of Mechanical Engineering of Politecnico di Milano. It is the first test-rig for bicycle tyres in compliance with the standard ISO 9001-2015. We also present the results of an experimental campaign conducted on a road racing bicycle tyre. In particular, the impact of rim stiffness is relevant to tyre characteristics, leading to a 13% increase in cornering stiffness under the same test conditions.

The use of bicycles as a cheap and healthy way to travel the “last mile” is spreading widely in the cities. This new way of dealing with short trips, known as “micro-mobility”, is also fostered by the new awareness of the global impact of ICE vehicles and rising fuel costs. In recent years, also cargo bikes are knowing a large use, both for families with children and for delivery purposes. They are featured by a long frame that can carry loads usually placed in between the rider and the front wheel. This requires fairly skilled riders to deal with driving dynamics, different from the common bicycle we are used to (Miller, M., 2023). They can easily reach a speed of 25 km/h (according to the regulations in most EU countries) being usually pedal-assisted. Tyre characteristics may strongly affect bicycle dynamics (Bulsink, V., 2015). This applies even more for cargo bikes as they are featured by remarkable load variation (load/unload configuration), relatively high speed and torque applied to the tyres, both during acceleration and braking phases. In this context, it is important to have a good understanding of tyre characteristics. With the aim of designing safer and more performant bicycles, numerical models are required. Furthermore, existing mechanical models of bicycles mostly ignore tyre dynamics and need to be updated with realistic tyre models (Dell’Orto, G., 2022). Measurements were performed with VeTyT, an indoor test-rig specific for bicycle tyres, designed at the Department of Mechanical engineering of Politecnico di Milano (Figure 1) (Dell’Orto, G., 2022). It is the only test-rig for bicycle tyres complying to the standard ISO 9001-2015. We can measure lateral force and self-aligning torque, as tyre parameters vary. The tyre 20”x2,15 was mounted on a standard aluminum rim and tested on flat track. The specific dimensions of the cargo bicycle wheel forced us to update the test-rig, designing a new steel fork to ensure sufficient stiffness and new steel plates to carry the wheel on flat track (Figure 2). Inflation pressure was set to 400 kPa, as recommended by the manufacturer. Tests were performed applying a vertical load of 411 N and 526 N, according to the technical limits of the test-rig. The camber was set to 0 degree, as first stage of the study. The lateral force and self-aligning torque as function of the slip angle are shown in Figure 3 and Figure 4, respectively. It is clear the difference in outcomes adjusting the vertical load. As the vertical load increases, both the lateral force and the self-aligning torque increase in magnitude as well. As expected, the tyre can generate higher forces with higher vertical load. It is worth noticing that the peak value of lateral force will be reached for very large slip angles (> |6| degrees, as maximum value tested in this study). Tyres for cargo bicycles are designed to carry large loads, therefore we expect to reach saturation conditions for higher vertical forces or, conversely, large slip angles. The cornering stiffnesses are reported in Table 1: for vertical load 526 N it is 23% higher than that found at 411 N.

Bicycles are becoming always more popular as a cheap and healthy tool for urban travels. The concerns für crowded public transport means are changing the habits after the pandemic situation caused by Covid-19, encomaging people towards the use of bicycle. As stated in literature, tyres play a large role in the handling ofbicycles . This is why it is necessary to characterize tyres so as to derive useful parameters for modeling. To this purpose, proper experimental methods have been implemented for bicycle tyres. A deepen knowledge of the phenomena occurring at tyre/road contact patch is indeed fundamental to ensure proper adherence and safety conditions, especiatly for vehicles as bicycles or motorcycles working with high camber angles. This paper aims at enabling the future development of bicycle tyres, in order to improve the safety and the performances. Specifically, the focus is devoted to understand how the road temperature can impact on tyre performances, and therefore on bicycle handling. After a brief section describing the methods and instruments used für this research activity, the results of an experimental campaign carried out on road racing tyre are presented and discussed. The remarkable variation of temperature oftyre rolling surface can have multiple impacts on the performances. lt can affect the noise emissions as weil as rolling resistance, as noted in, where higher temperature was co.rrelated to lower rolling resistance coefficient. In the temperatu.re influence on car tyre lateral characteristics is investigated on a drum testrig. A They found a decrease in cornering stiffness as temperature increases, while no particular variations on relaxation length were observed. Despite the known influence of the temperature on tyre properties, there is a lack of studies regarding bicycle tyres. In a test on testrig of wintertype tyre revealed remarkable differences with respect to the mechanical characteristics of other tyres tested at room temperature. This may suggest the important role played by temperature on bicycle tyres characteristics, thus affecting the tyre/road interaction.