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Around the turn of this century, the automotive industry introduced a new type of technology to drive the gauges on a vehicle’s instrument cluster. The change was unannounced to the collision reconstruction world, but soon after, investigators observed a marked increase in crashed vehicles displaying frozen gauges at what often appeared to be correct readings. The new technology was the use of stepper motors which require power to return to the zero position. Hence if electrical power is lost, the gauges stop in position. There have been a number of previous papers covering the operation of the instruments and crash testing of cars and motorcycles to establish the ability of the instruments to withstand the forces on the instrument during a collision. This paper aims to compare the frozen instrument readings from real world collisions with the available EDR data from the crashed vehicles.

The behaviour of the instrument cluster of passenger cars during a collision is reasonably well understood, following a number of studies over the past decade. However, one of the frequent questions asked is if motorcycle instruments can be considered in the same way as passenger cars. Motorcycle instruments work in the same way as passenger car instruments. The older models use a Bowden cable to drive a spinning magnetic cup, which drags an instrument needle against a hair spring. Newer models (typically post 2000) use a stepper motor which requires power to drive the needle to any indicated position, including returning it to zero. Hence if power to the instrument is lost as a result of a collision, there is no electrical power to the motor to move the needle, and it should be left at its last position before the power was lost.

Various mechanisms have been used to drive speedometers and other instrument gauges. This paper reviews the mechanisms used; in particular investigates the ability of stepper motors which have become the most common instrument motor in the last decade to freeze at the apparent reading prior to impact. Stepper motors require power to drive the needle to any indicated position, including having to return it to zero. Hence if power to the instrument is lost as a result of a collision, there is no power to move the needle and it should be left at the reading shown at the moment the power was lost. However, not all stepper motor instruments are the same and before accepting the reading, a number of criteria need to be considered to give a level of confidence in the result. As part of recent ITAI (Institute of Traffic Accident Investigators) crash test events in the UK, a number of instrument clusters were installed in vehicles to simulate both frontal and side impacts.