The vibration environment at the lower front of 4 cyl Lycomings seems
to be quite brutal. On top of that the alternator mount on our 172M
was "scientifically" measured to be resonant at 80 Hz (hand blow with a
hand held accelerometer & a scope). Eighty Hz is also the torsional
firing frequency (@2400 RPM) and there is also a second harmonic (of
course that is also 80 Hz at 2400 RPM) yaw rocking couple in the engine
due to its mechanical configuration

Maybe that,s why so many of alternator brackets become cracked. Does
the newest Piper bracket address this resonance problem?