The noise and vibration are directly related to the perceived quality of a
vehicle and it is crucial that the manufacturers focus their efforts to reduce
that. When an unusual noise appears, it is a great challenge to define an
approach for understanding the phenomenon, identifying the cause and then
defining a solution to reduce its effect. A “knocking noise” coming from the
brake rigid pipes is perceived while driving the vehicle in a cobbled pavement
at low speed and it coincides with the closure of brake system module inlet
valves. When a valve closes quickly, there is a sudden change in the flow
velocity, which generates a pressure transient in the brake fluid inducing
vibrations in the rigid pipes. The pressure transient can be minimized by
reducing the speed at which the pressure waves travel in the pipe. The bulk
modulus, the density of the fluid, the velocity of valve closing, the Young’s
modulus and the dimensions of the pipes, determine the wave speed. The objective
of this work is, first to correlate the phenomenon with the theory regarding the
transient flow by presenting the measurements of pressure change in the brake
lines during the brake unit valves actuation. Afterwards, evaluate the
experimental results of changing the variables that affect the wave speed, and
its influence in the noise perception. At the end, show how the application of a
hybrid pipe solution, which reduces the Young’s modulus of the brake line, can
reduce the pressure transient as well as the “knocking noise”.