Chapter 34 - The double-reduction axle

Abstract

This chapter discusses the double-reduction axle. In this type of axle, the permanent reduction of speed between the engine and road wheels is obtained in two separate steps. Double-reduction axles are used chiefly on heavy lorries and buses because such vehicles run at low speeds in comparison with passenger cars; thus, they also have larger diameter wheels. If these large reductions were obtained in a single step, using, say, bevel gearing, then either the bevel pinion would have to be made very small with few teeth, when it would be both weak and inefficient, or the crown wheel would have to be made very large, which would result in a heavy and expensive axle, and would reduce the clearance between the axle and the ground too much. Similar conditions are found, to a lesser degree, with worm final drives, and so the double reduction axle is adopted when the final drive ratio has to be large. On some vehicles they are used to enable a very low body position to be obtained.