With globalization, vehicles are sold across the world throughout different
markets and their automotive brake systems must function across a range of
environmental conditions. Currently, there is no current standardized test that
analyzes brake pads’ robustness against severe cold and humid environmental
conditions. The purpose of this proposed test method is to validate brake system
performance under severe cold conditions, comparing the results with ambient
conditions to evaluate varying lining materials’ functional robustness. The goal
of this paper is to aid in setting a standardized process and procedure for the
testing of automotive brakes’ environmental robustness.
Seven candidate friction materials were selected for analysis. The friction
materials are kept confidential. Design of experiment (DOE) techniques were used
to create a full-factorial test plan that covered all combinations of
parameters. The test script involves brake applications at 5, 10, 15, and 20
bar, at both ambient/non-humid and cold/humid conditions. Each brake application
collects the stop time and coefficient of friction (COF) values throughout the
stop. Failure modes are subjectively long braking times and failed brakes.
The test results verify that brake pad effectiveness is dependent on friction
lining, braking pressure, and environmental conditions. Other than at the lowest
tested braking pressure, the COFs appear to be consistent across the tested
braking pressures. Each material was evaluated for robustness against cold
conditions by calculating their signal-to-noise (S/N) ratio, a common method
used during design for six sigma (DFSS) robust optimization analysis. The
braking time S/N is calculated using smaller the better (STB) analysis, whereas
the COF S/N is calculated using the larger the better (LTB) analysis. Using the
S/N ratio, it can easily be determined which brake pad friction lining material
is the most robust against environmental conditions.
Friction designation A was consistently calculated to be the most robust friction
material against the cold environmental conditions. All friction linings had
extended stopping times in cold conditions when compared to ambient conditions.
In some cases, the lining materials reached critical failure in severe cold
environments. Additionally, the collected friction values gave insight into
potential extreme pad wear rates.