Audi revives UK development – 30 years later

Audi has revived a concept developed by GKN over 30 years ago – namely glass-fibre reinforced polymer springs for vehicles.

Audi’s new, lightweight suspension coil springs are made of glass fibre-reinforced polymer (GFRP), and will appear in an executive class Audi model before the end of the year. GKN tried to introduce GFRP leaf springs in light trucks in the 1980s.

Developed in collaboration with Italian supplier Sofegi SpA, Audi’s light green colouring immediately sets them apart from conventional steel springs, and they have thicker fibre strands and a slightly larger overall diameter with a lower number of coils. Most importantly, they are some 40 per cent lighter than their more ‘traditional’ counterparts.

In 2012, Tier 1 supplier Sogefi, announced it had developed what it claims were the industry's first glass fibre-reinforced plastic coil springs. Designed for passenger car and light commercial vehicle applications, the firm claims weight savings of up to 70%. Audi is its first published customer.

Whereas a steel spring for an executive class model weighs nearly 2.7kg, a GFRP spring with the same properties weighs approximately 1.6kg. Together the four GFRP springs therefore reduce the weight by roughly 4.4kg, half of which pertains to the unsprung mass, helping the suspension to react more quickly to changes in the road surface and to absorb imperfections more effectively.

“The GFRP springs save weight at a crucial location in the chassis system. We are therefore making driving more precise and enhancing vibrational comfort,” said Dr. Ulrich Hackenberg, member of the board of management for technical development at Audi AG.

The core of the springs consists of long glass fibres twisted together and impregnated with epoxy resin. A machine wraps additional fibres around this core — which is only a few millimetres in diameter — at alternating angles of plus and minus 45° to the longitudinal axis.

These tension and compression plies mutually support one another to optimally absorb the stresses acting on the component. In the last production step, the blank is cured in an oven at temperatures of over 100°C.

The GFRP springs can be precisely tuned to their respective task, and the material exhibits outstanding properties, according to Audi. It does not corrode, even after stone chipping, and is impervious to chemicals such as wheel cleaners. Last but not least, production requires far less energy than the production of steel springs.

                                          British development

However, it was in 1985 that GKN announced full production of a new and innovative development – composite commercial vehicle leaf springs. Production had begun ‘on target’ at the company's Telford, Shropshire, plant, although the first customer for the epoxy reinforced glass fibre units at that time remained a closely guarded secret.

In the early 1980s, GKN had entered into confidential talks with Freight Rover about the supply of composite leaf springs. The vehicle builder, part of British Leyland, wanted to launch a revised Sherpa for the 1986 model year.

Freight Rover needed to add a feature that would give the Sherpa a more high-tech image (as it compete against the Ford Transit) and issued GKN with a letter of intent to order springs starting in mid-1985. On this basis, GKN built a production line in a corner of the Sankey site in Telford.

GKN invested £1m in the development of the composite springs with the first units to be used on Sherpa light vehicles.

A pilot plant had already been operating at the GKN's Technical Centre in Wolverhampton producing prototype units using the same production techniques employed at Telford. GKN claimed production capacity at the Telford plant could be as many as 600,000 springs a year.

To make the idea of a composite soring feasible, however, engineers at the Wolverhampton Tech Centre devised a clever way of clamping the bush to the spring without having to drill it.

As to the Sherpa itself, before production mid-1985, all durability testing almost had been completed both on test rigs and on vehicles; vehicle ride and handling had been signed off.

GKN proudly discovered through road testing that its springs were more than a match for 1,000 miles of Pave at the Motor Industry Research Association (MIRA) test track at Nuneaton, UK

However, at the very last minute, during its own testing, Freight Rover, unearthed a brake-steer problem on composite-sprung Sherpas. In an emergency stop, a right-hand drive vehicle could veer markedly to the right. It also happened on steel sprung vehicles but the effect was about five-to-ten times worse with vehicles equipped with composite springs.

GKN worked day and night to find a solution. A special test rig was built at GKN’s Tech Centre using a complete Sherpa front suspension and steering assembly mounted on a steel frame. Hydraulic jacks simulated the braking loads. During a series of measurements it was noted that as the front beam axle underwent torsion under simulated heavy braking, the front axle bent backwards, causing the vehicle to veer off to the right. The front axle design was strengthened and the problem seemingly eliminated.

GKN carried out various projects to manufacture prototype springs for the larger version of the Sherpa but this did not reach production order status. Neither, unfortunately, did projects for Mercedes-Benz, Iveco or Chrysler, though Chrysler did order production tooling but did not issue a production order. The only other production vehicles using GKN Composite springs are thought to be London taxi cabs and a snow-mobile.

Interestingly, some of the last development projects GKN worked on were transverse leaf spring suspensions for Mercedes-Benz, Nissan and Volvo and these eventually saw production but using composite springs from other suppliers.

Even when plans for the introduction of GFRP springs were being planned with Freight Rover in 1985, GKN was discussing development orders with major European manufacturers, as well as heavy truck builders in the US and Japan.

And five years later even, in 1990, GKN Automotive announced it had won an important contract for the supply of lightweight composite leaf springs into the Japanese truck market. GKN Composites at Telford, Shropshire, planned to manufacture the leaf springs for use in the Mitsubishi Fighter, an 8-tonne truck. The composites were combined with steel leaves to form a hybrid leading to a weight saving of 40kg per vehicle and improved ride and handling.

GKN Composites however could not survive on such meagre production levels and shuttered the business in 1991.

In 1988, GKN Technology of Wolverhampton, West Midlands won a Queen's Award for the development of vehicle leaf springs made from glass fibre and epoxy resin (jointly with GKN Composites Ltd).

When it made its GFRP coil spring announcement in 2012, Sogefi claimed weight reductions – typically 4-6kg per vehicle – would reduce tailpipe CO2 emissions. Steel coil springs are relatively heavy, and require surface treatments to prevent corrosion and increase fatigue life; composite components do not.

Sogefi noted also that weighing up to 70% less, they contribute to a real reduction in fuel consumption compared with traditional steel coil springs. In addition the production is three-to-five times less energy-intensive.

It also pointed out that steel springs need painting or powder coating to prevent corrosion and shot peening to prevent fatigue. Its technology does not, and the firm claimed superior durability.

Other suppliers are developing composite chassis parts too: ZF is testing fibre reinforced plastic leaf springs for torsion beam rear suspensions.

In 1979, US firm Celanese Corporation patented carbon fibre (CF) composite coil springs, and more recently Hyundai Motor has developed a CF coil spring.