Bogie (tandem axle) suspension

Multi-axle trucks with two driven rear axles, the 6x4 and 8x4 tippers and haulage trucks from Tata, Ashok Leyland and BharatBenz, carry them on a bogie suspension. On each side of the chassis an inverted leaf spring pivots at its centre on a trunnion shaft, a large pin held in a bracket hung from the frame midway between the axles. The ends of the spring rest in saddles on the two axle housings. Torque rods, above and below each axle, hold the axles square to the chassis and transmit drive and braking forces, because the spring ends are free to slide.

• The front axle of the pair rides over a hump and its end of the spring is pushed up.
• The spring rocks on the trunnion like a see-saw, pressing its other end, and the rear axle, down.
• Both tyres stay on the ground and the weight stays split between the axles.
• When both axles rise together the spring bends in the normal way and the trunnion bracket carries the load.
• Torque rods keep each axle in position while all this happens.

On a loaded tipper crossing a quarry floor the arrangement means neither axle loses traction or carries double its share, which protects tyres and the differential. Drivers notice a very firm ride when empty and a tendency for the trunnion bushes to wear and knock. Air-sprung bogies with separate bellows on each axle are the modern alternative on highway tractors; Indian tippers mostly keep the steel trunnion type for its toughness and cheap repairs.