New materials for suspension systems



Lightweighting of vehicles has become one of the key trends in the automotive industry, as OEMs endeavor to meet future emission targets. Lighter cars enable the downsizing of engines, better performance and better fuel efficiency, and every sector within manufacturing is under scrutiny for weight reduction.

Chassis and suspension systems are one such sector, and the use of lighter metals and composite materials for components is a growing trend among suppliers and manufacturers. We’ve seen a number of innovations in recent years in the use of carbon-fiber reinforced plastics (CFRP) and glass reinforced plastics (GRP) for lightweight suspension parts, and it’s a trend that the industry expects to continue. We’ve also seen the development of active and intelligent suspension systems, and in the future can expect to see electronically controlled and regenerative systems entering the market.

Lightweighting trends and innovations

Back in 2014 Audi introduced GFRP suspension springs in an effort to reduce weight, and the German automaker has unveiled a range of innovations since. The lightweight springs, constructed from a glass-fiber reinforced polymer composite enabled a weight saving of 2.5 kg per spring, and a 40 % reduction compared with steel springs. The springs are constructed of long glass fibers twisted together and impregnated with epoxy resin. This structure is then wrapped with more glass fibers at counter-poised 45 degree angles. As well as providing comparable strength to steel springs, the GFRP springs are unaffected by corrosion or chemicals and use less energy in production.

A similar advance was made by Volvo in 2015 when it introduced new leaf springs for the XC90, manufactured from a glass-fiber reinforced composite material. Behind the leaf springs were automotive suppliers Henkel and Benteler-SGL. Henkel (well-known for its adhesive technology), also produces solutions for automotive structural components using high pressure resin transfer molding (HP-RTM). The company’s Loctite MAX 2 twocomponent polyurethane composite matrix resin system enabled composites specialist Benteler-SGL to produce the RTM glass-fiber reinforced springs for Volvo. Used in place of traditional steel springs, these new composite components reduce weight by 4.5 kg. One key advantage in terms of production is that Henkel’s Loctite MAX 2 has a very low initial viscosity. This enables fast injection into the mold and excellent penetration between glass fibers, while the quick curing time could allow demold times as little as one minute per part.

The problem of cost of production is neatly highlighted by a concept at the other end of the scale - Ford’s 2016 GT, unveiled at its Palo Alto research and innovation center last year. The lightweight supercar uses a range of materials such as magnesium and carbon-fiber to significantly reduce weight, and according to Ford CEO Mark Fields at the time, it represents “a whole decade of work into areas around lightweighting”. At over $ 400,000, the new GT highlights the issue of cost, but as production processes become cheaper the technology will trickle down to mass volume models.

Audi suspension concepts

Earlier in 2016 Audi debuted the all-new A4 Allroad Quattro at the Detroit Motor Show, featuring optional suspension with damper control for the first time. Sensors measure the movement of all four wheels plus lateral and longitudinal acceleration, and the damper characteristic adapts accordingly to surface conditions and driving situation. An updated five-link suspension is used on the front axle, with Audi developers reducing weight by 5 kg compared to the previous version. In the rear a five-link construction replaces the trapezoidallink suspension previously used, with an intelligent mix of materials used to reduce the weight of axle components by 7 kg. Later this year Audi also unveiled the 2017 Q5, which also features the five-link suspension system, as well as optional adjustable air suspension.

Yet another innovation from Audi is seen in the ‘eROT’ prototype, in which electromechanical rotary dampers replace the conventional hydraulic dampers to enable a system which recuperates energy. The principle behind eROT is to use the energy lost to heat. Undulations in the road surface cause kinetic energy in the car, and conventional dampers absorb this energy. With the electromechanical system - which will rely on a 48 Volt electric architecture - the energy can be recouped. To do this a lever arm absorbs the motion of the wheel carrier. This arm transmits the force via a series of gears to an electric motor, which converts it into electricity. Audi says that initial tests are promising, and this could be a technology that features in a future 48 Volt line up.

Citroën hydraulic suspension

Citroën has used a successful hydropneumatic suspension for some sixty years, but last year confirmed it would discontinue the system. In June 2016 the automaker released details of an all-new innovative hydraulic suspension as part of its Advanced Comfort program.

Where conventional suspension systems use mechanical stops, Citroën’s new system includes two progressive hydraulic cushions - one for rebound and one for compression - at the top and bottom of each suspension unit. The configuration allows the suspension to act in one of two ways depending on road conditions:

  • In instances of slight rebound and compression the springs and shock absorbers work together to control vertical movement without the need for the cushions. However, the new cushions provide the vehicle with greater freedom to deliver what Citroën calls a ‘magic carpet’ ride.
  • When more significant rebound and compression forces are in play, the springs and shock absorbers work together with the hydraulic cushions at the ends of the suspension travel. The cushions gradually slow the movement rather than having abrupt stops at the extremes of compression and rebound. Unlike a mechanical stop, which absorbs and partially returns energy, the hydraulic cushions absorb and dissipate energy.

Lightweight composites to reduce weight

Although drive comfort and regeneration are important topics in suspension systems, the main motivator for using different materials is to reduce the overall weight of the vehicle without affecting performance. To this end one of the industry’s most prominent component suppliers, ZF, is continually developing new designs.

One study carried out by ZF demonstrates the use of GRP to reduce the number of components in a typical axle. Where a conventional chassis features a complex control arm and spring concept, the ZF engineers have used a single component - a wheel-guiding transverse spring - to assume the task of optimal roadholding. As well as reducing weight, the component reduces the complexity of the axle, making it easier to install.

Another innovation is the lightweight McPherson suspension strut with integrated wheel carrier, which is made of GRP and weighs just half as much as conventional steel suspension struts. The concept is based on use in mini and micro cars under a ton in weight. An upside-down design for the cylinder tube compensates for high bending stress, and the strut provides all the characteristics of a modern damper. ZF is working on applying this technology to larger vehicles.

Summary

Glass and carbon-fiber reinforced plastics are increasingly being used in all areas of automotive manufacturing. Production times and costs are falling, and new technologies are developing stronger, more flexible products than ever before. The use of such plastics in suspension systems is expected to increase as volume production processes improve, and more complex parts can be made in mass numbers.

Innovations in suspension design, such as those demonstrated by Audi and Citroën, show that there are several ways in which development can progress. The use of lighter metals and plastics can provide engineers with greater freedom of design as they develop the next generation of suspension systems.

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