Rigid axle (live axle)

A rigid axle, also called a solid, beam or live axle, is a single steel housing that runs from one wheel to the other. If the axle is driven, the differential and the driveshafts sit inside it, which is why it is then called a live axle. The axle is suspended under the chassis either on leaf springs, which also locate it, or on coil springs with a set of control arms and a lateral rod to hold it in place. Every Indian truck has rigid axles, and so do off-roaders and body-on-frame SUVs at the rear: the Mahindra Thar, Toyota Fortuner, Force Gurkha and, at both ends, the Maruti Jimny.

• One wheel rides over a bump and that end of the axle rises.
• Because the housing is rigid, the other wheel tilts too; the whole axle rocks about the opposite tyre contact patch.
• The springs on both sides share the movement and the dampers slow it down.
• Under acceleration the axle tries to rotate with the wheels, so the springs or control arms must resist this wind-up.

The appeal is strength and simplicity: there are no CV joints or ball joints, the ground clearance under the differential does not change as the suspension moves, and the tyres stay planted on rocks because when one wheel drops the other pushes up. Drivers notice a firmer, more jittery ride on broken tarmac, a shudder through the rear over mid-corner bumps and heavier steering if the front axle is solid. The large unsprung mass also makes the wheels slower to follow fast ripples in the road.