ARVINMERITOR has announced launch plans for a new composite trailer spring, initially for use with 9-tonne ROR brand mechanical trailer suspensions and axles. All variants of the spring will be released for production in the first quarter of 2002, according to Martin Peaker, advanced engineering and test manager.
The European Transglide consortium has spent eight years perfecting the composite production process and has now licensed its technology to EM Fiberglas in Denmark. ArvinMeritor is the only heavy truck and trailer suspension supplier that has been part of the Transglide Consortium.
The ROR spring is a complete, fully precision moulded composite that contains no metallic elements. It consists simply of glass fibers contained within a polyester resin matrix. The challenge has been to control the manufacturing process accurately in order to achieve consistent performance. The spring marketed by ArvinMeritor uses a resin transfer moulding (RTM) technique whereby a spring-shaped pack of glass fiber, known as the preform, is low-pressure-injected with resin and then cured.
The preform is made up of layer upon layer of glass fiber tape of constant width. The construction of the tape is closely controlled, with the glass fibers being tensioned to maintain the correct finished width and to control fiber alignment. The tape is accurately cut to specified lengths to give the preform its familiar tapered shape. Finally, the preform is sheathed in a thermoplastic sock, making it easier to handle before curing and improving corrosion resistance of the finished leaf. Every preform is weighed, and if necessary adjusted, to ensure an accurate moulding and consistent spring rates and performance characteristics.
The only other additions to the leaf are: specially moulded spring ends, and a center pad — both designed to transfer the loads into the spring as evenly as possible. The spring wear ends, which contribute to the spring's ride quality, are made from a specially developed reinforced nylon material with extremely low friction characteristics, and the center pad is made from a tough thermoset, polyester polyurethane elastomer (TPPE). Both materials have excellent environmental and chemical-resistant properties, and TPPE in particular has been used successfully in suspension applications for many years.
Composite springs have a number of desirable attributes. They are very light, weighing about a quarter that of steel. Typically a three-leaf steel spring for a 9-tonne (20,000-lb) axle weighs 34 kg (75 lb), while an equivalent composite spring has one leaf and weighs less than 8 kg (16 lb). This translates to a total vehicle weight saving of 156 kg (344 lb) on a three-axle bogie.
Composite springs have a damping factor 200 times that of steel, which benefits ride and handling, and they also generate significantly less noise. They last between 1½ to five times as long as their steel equivalents. If failure occurs, it takes the form of gradual de-lamination rather than a sudden fracture, allowing the vehicle to reach its base.
The ROR composite spring installs exactly like a steel spring, but performs more like an air spring. Its ride characteristics are similar to an air suspension. The composite spring's low dynamic load coefficient — the factor that determines road damage — qualifies it as equivalent to an air suspension and therefore as “road friendly” as air. Currently, however, EEC “road friendly” legislation specifies air suspensions.
ArvinMeritor does not expect composite springs to displace most air suspensions, but argues that they provide an ideal alternative to steel springs in tough conditions. In this situation they save weight, improve ride and handling, last longer, are quiet, and are no more expensive to maintain. Compared with air suspensions, composite springs offer similar ride and handling, significant weight saving, and cost less to install and maintain. They cannot, however, offer ride height control that has become the norm for air suspensions.
The company has not released costs, but indicates that the price, like the performance, will fall somewhere between the established steel and air suspensions.