Measuring vertical tyre stiffness of bicycle tyres

This contribution presents an analysis of the vertical tyre stiffness of 20” bicycle tyres as usually mounted on bicycle carriers for the transport of children. The current research contributes to the science on bicycle comfort with the focus on the next generation cyclists. Two different methods to measure vertical or radial tyre stiffness of bicycle tyres are presented – a dynamic approach on a dynamic press and a static approach. Parameters modified are tyre inflation pressure and vertical load in the static experiment. In the dynamic experiment additionally dynamic load and frequency are varied. The dynamic experiments are performed on two different tyres. The same tyres are also used for the static experiments and completed with a third tyre, which is a clincher version of the narrow foldable tyre. The tyres are made for 406mm rim diameter as usually for bicycle carriers since the comfort of children in bicycle transportation is the larger scope behind the experiments. The main findings are as follows: • The stiffness of the tyres is in a range of 31 N / mm to 147 N / mm. It must be considered that values below 50 N /mm are related to extremely low inflation pressure that probably do not work reliably because the rim will puncture the tube. • Tyre inflation pressure is the main factor that controls the vertical stiffness. • Type of tyre (balloon vs. narrow tyre) hardly affects the stiffness. • The dynamic stiffness at 1 Hz is slightly higher than the static stiffness. • With increasing excitation frequency the stiffness increases, however, this effect is non-linear and varies between 3.7% at high pressure in the narrow tyre and up to 20% at low pressure in the balloon tyre. • Similarly, there is a trend to higher stiffness with increasing vertical load in a magnitude of 20% increase.