What is unsprung mass and why is it important?
The unsprung mass is approximately defined as the mass between the rail / road and the suspension. This mass affects the natural frequency of the suspension and therefore the ride quality. Undulations in rail or road surface are impacted by a force proportional to the unsprung mass, whereas the sprung mass is isolated by the suspension. The impact causes rail / road forces that result in surface wear. This is particularly important at rail joints. On rail systems these track forces have to be minimised to prevent excessive track wear rates. These forces are vertical, lateral and longitudinal. The dynamic vertical force, or P2 force, is often used in assessment of design quality and acceptability.
