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ZF is studying the possible use of composite materials for use in the construction of leaf springs and other chassis components.
ZF’s Multi Compliance Twist Beam Axle design can provide the handling performance of a multilink at the cost of a twist beam.
For years, chassis development teams looked at pretty conventional solutions to improve vehicle handling dynamics, whether it be adding structural members, reconfiguring axle designs or incorporating active suspension technologies. It seemed like this approach was pretty much set in stone—until the world changed. Increasing demand for smaller, affordable vehicles and new regulations designed to help improve fuel economy are forcing significant changes in the way chassis systems are going to be engineered in the future. In this world, two factors lead the way: cost and weight. Sure, dynamic performance will be critical, but the pressure to innovate on cost and weight will grow exponentially.
That innovation is likely to focus on new material applications. There's little doubt chassis engineers will default to increased use of aluminum components. The material has proven itself a reliable solution for weight reduction while delivering stellar results in dynamic control and crash performance—but the added cost of using the material is likely to limit its ability to permeate into high-volume small car applications. Magnesium is another alternative, but issues remain with corrosion performance.
The solution to reducing weight, while cutting costs, may lie with developing solutions that use composite materials. Engineers at ZF Friedrichshafen (zf.com) are already hard at work developing what they describe as a "best-cost axle" using a number of composite-based solutions. "You could have a sway bar made out of plastic and a leaf spring made out of a carbon fiber material," says Werner Kosak, who heads ZF's global chassis development group. His team is already working on conducting virtual testing of a suspension using a composite-based leaf spring, but the work is still in its infancy. He thinks it may be ready by 2015.
"We still have a lot of work to do, but our initial understanding shows this solution will work quite well—we haven't had any failures," Kosak says. ZF plans to build prototypes of the composite leaf spring solution and install it on a few mules to determine its real-world behavior, focusing on crash performance. "What is unknown to us is the behavior of the composite spring in a side impact; we don't know how those fibers are behaving inside and we need to see if there are any hidden damages we don't see," Kosak says.
The march toward composite-based components isn't only going to change the way engineers deliver weight-saving and cost-effective solutions, it's also going to require a fundamental change in the way chassis components are manufactured. "This is going to be a major issue where we have to break new ground," Kosak says, noting that engineering and manufacturing are going to have to be more closely interlinked as various parts of the suspension and chassis become interlinked deploying multiple materials that rely on each other to provide a holistic solution that doesn't degrade reliability and performance. New manufacturing technologies will be needed to marry the various materials into one part without added bonding materials or unnecessary welding. "We can achieve these types of solutions in the laboratory, but we need to do this cost-effectively in high volume, which is not possible today," Kosak says.
More Conventional Solutions
ZF is also looking at more conventional solutions designed to improve ride comfort while also reducing cost, including its Multi Compliance Twist Beam Axle, which uses the basic architecture of the cost-effective twist beam as the basis for a new design that features an additional wheel carrier on each side that produces a virtual pivot point that moves the wheel in toe-in under both lateral and longitudinal load forces. The result is a suspension that acts more like a multi-link than a beam. The secret is the use of three rubber mounts arranged in a way to provide improved stiffness in lateral loads, while softening during longitudinal forces, reducing understeer. Additionally, the use of decoupled wheel carriers—which reduces NVH—allows for stiffer configuration of the main bearings during longitudinal travel, increasing the axle's lateral stiffness. The decoupled design also allows for various suspension components to be used, depending on the vehicle price class. "You can use this solution in small vehicles, medium priced and higher priced vehicles because it is modular," Kosak says.
Weighing Battlefield Chassis Solutions
Even the U.S. Department of Defense is worried about fuel economy. Why? Consider this: the Army uses billions of gallons of fuel per year, accounting for up to 70% of annual logistical tonnage at a rate of up to $100 per gallon in shipping costs alone. This is why the Army has established a goal to cut its fuel consumption by 75% by 2020. So the military is turning to civilian contractors to devise creative solutions. One of those contractors, Ricardo, is devising new chassis systems that will carefully balance the needs of the fighting force against operational mandates. Not only are military vehicles expected to traverse difficult terrains at extreme axle load capacities, they are required to provide adequate protection for those who operate them, while having infinite life expectancy. “The military is very interesting in the way they look at vehicle development because to them 70% of the cost of the vehicle will be maintenance, while 30% is acquisition,” says Rob Ellis, chief engineer for military vehicle programs at Ricardo, whose top priorities are developing chassis-based solutions that maintain performance requirements while reducing weight and drag.
When it comes to weight savings, Ellis expects the military to increase its use of aluminum-based alloys in chassis construction, as well as advanced composite materials. He also expects to see rapid deployment of active suspension systems for height adjustment based on terrain. “We’re looking at things like magneto-relay and electro-hydraulic systems,” Ellis says.