Residual “Non-Zero” Speedometer Readings In Crash Tests Using a HYGE Crash Simulation System

The trend among analog speedometer and tachometer instruments in recent decades has been toward stepper motor drives. If power is interrupted during a crash, such gauges often do not return to a zero reading. Speedometers and tachometers displaying residual readings after a crash have been observed with increasing frequency in recent years. In conducting a crash reconstruction, a question often arises as to whether such a residual reading corresponds to the indicated vehicle speed at the time of impact in the crash. Prior publications in this area have included a variety of crash tests under a wide range of relatively uncontrolled conditions. The present investigation evaluated a total of nine instrument clusters with a range of static torque required to move the needles when unpowered. The clusters were mounted on a HYGE crash simulation sled and subjected to consistent impulses at orientations representing frontal, rear, left and right lateral, and left and right oblique impacts. Prior crash tests in this area used unpowered instruments with the needles manually set to particular readings prior to impact. In the present investigation, the instrument clusters were powered and supplied with signals on their CAN buses, causing them to indicate specific vehicle and engine speeds at the moment of impact. Electrical power to the clusters was interrupted during the crash impulse, simulating loss of power during a real-world crash event. The residual readings of each instrument were recorded. They varied considerably. Factors found to affect residual readings were the orientation of principal direction of force, the static torque required to move the needles when unpowered, and when during the crash impulse power was interrupted. Consistent with prior findings in this area, residual speedometer and tachometer readings should be addressed in the context of a complete crash reconstruction.