Four Wheel Drive Transmission

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Four Wheel Drive Layout

Typically in a Four Wheel Drive layout, a transfer box is interposed between the gearbox and back axle.The function of the transfer box, as can be comprehended, is to transfer the drive to both the front and the rear axles.









In the transfer box is pinion A, see figure,driven by a coupling from the gearbox output shaft. The pinion, through an intermediate gear B, drives a third gear C, mounted on the cage of a differential gear assembly. From the differential gears, one shaft is taken forward to the front axle and the other rearwards to the back axle. Both the axles house their own differentials and final drive gears, but that at the front carries at its outer ends the universal joints D, which are necessary to allow the front wheels to be steered. The differential at C, in the transfer box, is necessary to distribute the drive equally between the front and rear axles and to allow for the fact that, when the vehicle is driven in a circle, the mean of the speeds of the front wheels is different from that of the rear wheels and therefore the speeds of the two propeller shafts must differ too. Other factors include different rolling radii of the tyres owing to, for example, manufacturing tolerances, different degrees of wear and, perhaps, different tyre pressures. Provision is usually made for locking this differential out of operation, to improve the performance and reliability of traction when the vehicle is driven on slippery ground.

Four-wheel-drive offers two main advantages. First, there is the increased traction obtainable from four driven wheels, which is especially useful on soft or slippery ground. Secondly, if the front wheels drop into a ditch they tend to climb out, whereas with rear-wheel drive they tend to be forced downwards, except when the vehicle is driven in reverse, in which case, of course, the disadvantage of the lower traction of two-wheel drive remains. The principal disadvantages are increased weight, bulk and cost.






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