In addition to new lightweight vehicle bodies, a new weight saving design is available for commercial vehicles. This time it is the turn of the suspension unit, the weight of which is reduced through a spring design that optimises the use of the properties of the steel from which it is made.

Parabolic tapered leaf springs are designed to offer a significant and cost effective weight reduction in previously very high weight components (figure 1). They provide an alternative to traditional laminated leaf springs, offering:

· Approximately 40% weight reduction

· Optimised use of material properties

· Minimum inter-leaf friction and contact, and so the ability to use more durable corrosion protection and improved ride characteristics.

Other benefits include improved fuel economy or load carrying capacity, and reduced road damage for an equivalent axle weight.

In conventional uniform thickness leaf springs, the mid length is the highest stressed region and the thickness of the leaves is designed to accommodate this maximum stress. However, to either side the stress decreases with distance from the centre and the strength of the steel is progressively under used.

Parabolic tapered leaf springs, on the other hand, are designed with a reducing thickness from the centre outwards. The regions to either side operate at a uniform stress level, similar to that at the centre. This allows for maximum use of the material properties, and reducing the section thickness minimises the weight of the spring.

Parabolic tapered leaf springs can be fitted in place of standard laminated leaf springs with very little change to the design of vehicle attachment fixtures. They are produced using conventional metallurgical techniques and spring steels, including 0.6%C-Cr (SAE 5160, BS970:525A58) for thinner sections and 0.6%C-CrMo (SAE 4161, BS970:705A60) for thicker sections. The leaves are produced by hot rolling the tapered section, and then after roll forming they are hardened and tempered to a hardness of approximately 450HB.

For maximum fatigue resistance the leaves are stress peened to develop a high level of beneficial compressive residual stress in the surface. This involves deflecting individual leaves and shot peening while they are in the strained condition. Then they are assembled into the final component, which may comprise a number of parabolic leaves, figure 1. Typical weight savings per spring are shown in table 1.

Table 1. Typical weight savings using parabolic tapered leaf springs.

Application	Weight (kg) Parallel multi leaf	Weight (kg) Parabolic	Weight Saving (%)
Van	7.8	4.4	44
Articulated Vehicle -
Front Axle	103	71	38
Rear Axle	162	90	44

A further development, currently under evaluation to introduce additional weight saving, involves the use of a lower carbon steel that is hardened and tempered to a higher strength level. This lower carbon steel offers a higher heat treated strength combined with the retention of good ductility and toughness. The objective is to use these features to provide improved fatigue resistance and a possible additional 15% weight saving. Increased fatigue resistance is obtained from a combination of higher strength and an improved sub surface compressive stress profile following shot peening.

The hardenability of the new steel is adjusted to ensure a fully martensitic microstructure on hardening, thus producing a ductility level at least equal to that of a traditional spring steel, despite the higher heat treated strength. The spring can be produced with a similar form to that of a standard parabolic tapered leaf spring. Typical heat treated properties, compared with those of a conventional spring, are shown in table 2. In addition, a Weibull analysis of laboratory results shows an improvement it bending fatigue properties obtained from the development grade, compared with a 0.6%C-Cr steel (SAE 5160).

Table 2. Mechanical properties of steel for reduced weight springs.

Steel	0.2% P.S. (N mm-2)	Tensile Str (N mm-2)	Elong (%)	R of A (%)	Charpy Impact 2mm V
SAE 5150	1290	1430	11	37	10
New Grade	1530	1750	11	41	10

Parabolic tapered leaf springs are produced by Tinsley Bridge of Sheffield (UK), under the name of Taperlite. The latest development described above has the code name Extralite. Both types of spring highlight the potential for component improvement when the use of steel is optimised for a particular application.