be obtained from the frequency of pulses from one of the sensors 16 17 it may be desired to have more information than is derived from a single marker on the periphery of the shaft fig3 illustrates an optical encoder 31 mounted on the shaft 11 two of which can be used in place of the single markers 14 15 of fig1 and 2 having a multiplicity of windows 32 giving rise to a train of pulses every revolution fig3 might as well illustrate a magnetic encoder in which the windows 31 are magnetic 34 marks 34 on a magnetic disc like a magnetic data store for a computer the optical read head 33 would then be a magnetic read head fig4 illustrates one arrangement in which the force measure derived from such sensors as are described with reference to fig1 to 3 is used to evaluate vehicle gross weight the force measure is input from the sensor arrangement 41 to a computer 42 together with a time signal from a speedometer arrangement 43 which gives pulses as the needle passes say the 5 mph 8 kph and 15 mph 24 kph marks 44 45 it is assumed here that a measurement will depend upon the vehicle being driven with constant accelertion between those two speeds and on level ground the force measure will be in the form of a time interval which will be proportional to force and can be represented as k 1 f where k 1 is a constant and f is the force the acceleration measure will be in the form of a time interval which will be inversely proportional to the acceleration and can be represented as k 2 a where k 2 is another constant.