Overslung Atkinsons
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• Overslung springs have been introduced on the rear bodies of Atkinson threeand four-axle heavy trucks and tractive units. A more flexible suspension with greater roll stiffness, and a weight saving of over 2cwt, results, it is claimed, from the change.
A spring base of 44in. width, which represents a 6in. improvement over previous underslung models, is achieved with the new design. And, because roll stiffness is proportional to the square of the spring base, over 25 per cent extra roll-stability has also been gained. Added to this, writes Ron Cater, must be the advantage that stems from having a roll centre about 12in. higher than with underslung springs, so that the height of the centre of gravity above the roll-centre has been reduced considerably in some cases by 35 per cent.
The inherently greater roll-stiffness achieved here has enabled more flexible springs to be used. The overslung springs give over 40 per cent more deflection than the underslung variety and, with fewer leaves, create less friction, a factor in ride harshness. Wind-up stresses in the main leaves have been reduced to the minimum by using a shallow spring and wrapping the second leaf around the spring eyes, and a negative camber brings the spring eyes closer to the axle centre line. The overslung springs are 3 ;in. wide and 45.75in. long. They are secured by shackle pins of 1.25in. diameter.
Maximum bogie load capacities of up to 20 tons can be provided but there are three main choices of spring-rating to give capacities of 14, 16 or 18.5 tons.