an accurate and high - speed measurement of the vehicle wheel alignment is achieved by an optoelectronic , non - contact measurement system . this system includes photoelectric sensors for detecting the distance between a reference vertical plane and a tire side surface of each wheel , and a stepping motor for adjusting the position of the sensors to follow - up the displacement of the wheel in a direction parallel to the longitudinal axis of the vehicle . operation of the stepping motor is controlled by a microcomputer on the basis of the updated distance data delivered from the sensor inputs . the microcomputer processes the input data to compute a toe - in or a camber of the wheel for the analog or digital display .