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140 
ern a teacher at a school that has recently purchased
Page 78
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a 15-year-old diesel-engined coach for private use. It is equipped with a four-speed gearbox and consideration is being given to (a) fitting it with a five-speed gearbox with overdrive top gear, and (b) retaining the existing box and equipping the vehicle with a two-speed axle. We seek increased flexibility in the hilly terrain in the area and an improvement in motorway performance. Which of the alternatives do you advise?
AEquipping your coach with a two-speed
axle would increase the number of ratios available from four to eight which would include a high-top or overdrive ratio. In the event, you would be well advised to arrange tuition of the driver (or drivers) in handling the vehicle to ensure that the best use is made of the gearbox in combination with the twospeed axle. It is known that many drivers fail to exploit fully the advantages provided.
In addition to the extra ratios that a twospeed asle affords, it may have a particular merit when the vehicle is being driven at high speed in overdrive. Step-up gearing is incorporated in the axle and operating in overdrive does not, therefore, increase the speed of the propeller shaft. This may be important because a propeller shaft has a critical "whirling speed" which produces severe vibration. If the overdrive is incorporated in the gearbox and overdrive is engaged, the speed of the propeller shaft is increased. And this may raise the rpm to tha whirling speed. Wear in the universal joints tends to reduce the speed at which whirling occurs.
Consideration should be given in each case to the suitability of the overdrive ratio. The standard final drive ratio is possibly the most suitable for normal operations and an overdrive would only be of benefit when the vehicle was travelling on a motorway, or the equivalent, with a light load or with a tail wind.
It could well be argued that a five-speed overdrive box would be the best bet because it would provide appropriately spaced ratios. Although a two-speed axle doubles the number of ratios available, the spacing of the ratios is often such that the "useful" number of ratios is less than the total number. It would be advisable to chart the ratios spacings provided by a two-speed axle before deciding on its use. Ideally, the ratios should be evenly spaced.