The paper describes how internal pressure, vertical force, speed and rolling surface temperature may affect the mechanical characteristics of a road racing bicycle tyre. The results were obtained from an experimental test campaign performed with VeTyT, a test-rig specifically designed for measuring the mechanical characteristics of bicycle tyres. The static deflection of tyre for different inflation pressures and vertical loads was measured to determine the static tyre vertical stiffness. Results for tyre rolling on flat track or on a drum were compared. Dynamic analyses were focused on evaluating the effect of inflation pressure and vertical load, for two rims featured by different lateral stiffness. Then, the respective effects of speed and of temperature of the rolling surface on the lateral force were considered. Stiffer rims can ensure higher values of cornering stiffness. In addition, higher inflation pressure is recommended only for heavy vertical loads. For low vertical loads, too inflated tyre results to be less performant, i.e. to show lower values of cornering stiffness. The speed can affect the mechanical characteristics of bicycle tyres mainly for slip angles less than 1.5°, while the temperature of the rolling surface is the most affecting parameter for slip angles larger than 3°.

@en

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.@en

The paper presents the measurement of the relaxation length of road racing bicycle tyres. In the paper the relaxation length is given as the ratio between cornering stiffness and lateral stiffness of the tyre. Tests were performed with VeTyT, a test-rig specifically designed for measuring the lateral characteristics of bicycle tyres. The results from road racing bicycle tyres of two different brands are discussed. Then, a comparison is presented between of 26 mm and 28 mm wide racing tyres. The tyres were tested both on a flat track and on a drum of 2.6 m diameter. The relaxation length for tests on flat track resulted to be on average 15% higher than the ones on drum, for the same vertical load and inflation pressure. In addition, the large role played by inflation pressure on the relaxation length was discovered. Since the relaxation length may affect the bicycle dynamics, the study provides the first quantitative information for the further development of safer and more performant bicycles.

@en

In this paper, the twisting torque of bicycle tyres is studied. The twisting torque is just the self-aligning torque for null lateral slip and non-null camber. The relationship between twisting torque and contact patch area has been analyzed. As the latter increases, twisting torque increases as well. A theoretical model to evaluate twisting torque has been implemented. Since twisting torque is mainly due to longitudinal slip in cambered wheel, increasing the contact patch area the stresses are located at a larger distance from the median plane of the contact patch. The resulting torque will be higher in magnitude. Finally, the model has been validated comparing the outcomes with the experimental data, with a mean error less than 3% for camber angles less than 10°.

@en

The authors regret an error made in Table 4.3. The terms i_α and i_θ must be expressed in [deg], not [mm] as indicated. The authors would like to apologise for any inconvenience caused.

@en

Bicycles will be largely exploited in city mobility, as a smart and cheap tool to achieve the goals of reducing air pollution levels and increasing the liveability of cities. Their growing popularity requires more knowledge on bicycle dynamics to prevent instability. In this context, tyres may play a crucial role, as already stated in literature (European Environment Agency, 2019, 1). This is why proper test-rigs are needed to obtain reliable tyre parameters. 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. The development procedure is described step by step, enlightening the main goals achieved. Vetyt frame was reinforced to test tyres on flat track, with vertical loads up to 550 N. After that, a new air-cooling system to keep constant the temperature of the rolling surface was added. An uncertainty model was developed on theoretical basis, and a static verification of forces recorded by Vetyt was performed. At the end, the results coming from a very first experimental campaign are presented. Lateral forces and self-aligning moments are depicted for racing road bicycle tyre, for vertical loads of 400 N and 490 N. The variation of camber angle (tested up to ±10°) contributes to the increase in magnitude of the values of lateral force and self-aligning moment.

@en

Tyre characteristics can strongly affect bicycle dynamics. In light of this, proper test-rigs are required to accurately measure tyre parameters. The paper presents a review of the methods and devices developed for the experimental characterisation of bicycle tyres in the lateral direction. The main aspects of the known test devices are described, highlighting the respective features. The technology of test-rigs for bicycle tyre parameter measurement seems to be just beginning.

@en