2012 Lightweighting Special: The framework of a revolution

At the 11th international conference of the VDI (the Association of German Engineers) in Steyr, Austria, in May 2011 a young engineer at Magna Powertrain’s Engineering CenterSteyr (ECS) presented the first breakthrough in heavy duty truck frame design in over a century.

The C-profile steel frame with cross members goes all the way back to the ladder-type chassis in Gottlieb Daimler’s Phoenix, which hit the roads in October 1896. Built sturdy to handle increasing loads over the decades, any weight reduction potential this design might have offered has long been exhausted with the use of high-strength steels.

At least one global OEM is known to have investigated an all-aluminium ladder frame, and some researchers have even explored exotic designs that use carbon fibre, but none of these concepts proved viable when it came to a consideration of cost and durability.

Gregor Schwarz’s frame for a 3,600mm-wheelbase 4×2 semitrailer tractor combines modular and monocoque design concepts and is built almost entirely of S355 structural sheet steel. At 595 kg it weighs a whopping 30 percent less than the standard ladder frame, and allows the existing powertrain and suspension – leaf springs up front and air springs at the rear – of this largest-selling truck type in Europe to be retained as is.

Schwarz first came up with the idea a little over three years ago, when he was a student at the Graz University of Technology. His work at ECS to develop the first prototype was the subject of his diploma thesis, and the new frame broke cover at the VDI conference only a couple of months before this reporter was given a look.

The design consists of four modules — a cast front-end module, a double-walled front axle module with 2.5mm outer and 3mm inner shells, a single-wall monocoque middle module, and a rear axle module with 2.5mm outer and 6mm inner shells. The monocoque, which accounts for the “dramatic” reduction in weight, has a wall thickness of only 1.5mm, compared to the 8–10mm-thick C-profile of a standard ladder frame.

The choice of steel was to keep costs low, and because the joining techniques were well known. For the most part, the frame is made up of pressings and profiles that are straightforward to produce. The only exceptions, because of their complex geometries, are the front-end module, which was cast from GJS 600 spheroidal-graphite iron, and four deep-drawn sheet steel parts that connect the front and rear axle modules to the middle module.

The individual modules were assembled using spot welds, and then joined into a complete frame using a combination of spot welds and MAG welding. Only the cover of the middle module and the front end, parts that typically have to be removed and replaced during maintenance, are bolted to the rest of the frame. The individual torques of the cover bolts are a critical factor in the rigidity of the monocoque, which lends the overall structure a torsional stiffness “10 times” that of a ladder frame.

Connecting the front and rear modules to the extremely rigid middle module was particularly challenging, Schwarz admits. “Because we used carry-over parts [from the standard powertrain and suspension configuration], it was not possible for us to redesign the front and rear ends. Consequently many design and calculation loops were necessary to get an optimised geometry.” The modularity of the concept, though, already allows for a variety of configurations because each module can be varied independently of the others. With a longer middle module, for example, the wheelbase can be extended to 3,900mm. Adding another rear axle module, moreover, allows for a 6×2 or 6×4 wheelplan. And the front axle module can be adjusted to accommodate ECS’s single-leaf suspension by simply swapping spring mounts.

This innovative suspension delivers comparable driving comfort to more complex – and more expensive – air-spring suspensions, but weighs 60 kg less than a configuration with three-leaf parabolic springs on both sides. A future target is a module for an independent front suspension, which Schwarz reckons could cut another 50 kg.

The front-end module is dimensioned to accommodate a 1.2m2 radiator necessary for Euro VI, and though the prototype was built around an MAN D2676 6-cylinder inline engine and ZF 16-speed transmission as “placeholders in our package”, he says a V8 engine could equally be slotted into the front axle module.

The monocoque not only lends unprecedented structural rigidity to the frame, hence improving the driving behaviour of the truck; it also allows for the best possible space utilisation, Schwarz points out — two tanks within the “box” on either side of the driveline can hold up to 900 litres of fuel, and with additional tanks strapped onto the outside, the target is to achieve a fuel capacity of 1,500 litres plus 80 litres of urea solution (for the SCR exhaust aftertreatment). The closed understructure of the monocoque lowers drag, improving the aerodynamic efficiency and fuel economy of the vehicle.

While the prototype was designed for super-wide (super single) tyres on the rear axle, Schwarz says potential customers – MAN, Mercedes-Benz, and DAF were the first three to express interest – tend to prefer twin-tyred axles, which will require a redesign of the rear-axle module. The latest packaging concept includes a Euro VI aftertreatment plant and air tanks with a capacity of 100 litres.



Unrivalled capability

The development of the frame, as indicated above, involved exhaustive finite-element analysis for stress and strain using both standard load cases employed for ladder frames and also some specially developed ones like a “street repair” scenario that simulates a torsion in the frame of a truck parked on an uneven road for an examination of the drivetrain, to check whether every bolt can be tightened correctly when the cover is replaced.

But that wasn’t enough for Schwarz and his “lightweight concepts and development tools” group — they not only studied the response of the frame to vibration modes that it is likely to experience in an actual vehicle, such as lateral and torsional bending, but also ran the structure through a durability analysis on Magna Powertrain’s own Femfat fatigue life prediction software to validate the robustness of every single joint.

Because of the monocoque construction and use of sheet metal all over, the frame is “ready for automated manufacturing and assembly” not unlike a passenger car body-in-white. And if it comes to that, it can be “industrialised” together with Cosma International, the body and chassis specialist within the Magna group that entered the heavy duty truck market earlier this year with cab-in-white stampings and assemblies it designed and developed for Kenworth’s new T680 and Peterbilt’s Model 579.



So far, Cosma’s assistance has been limited to projections of production costs, but it could very likely become ECS’s manufacturing partner for the frame if, as Magna Powertrain’s director of business development for engineering services Werner Dantendorfer hopes, the European OEMs’ increasing inclination to de-integrate their own frame manufacture translates into decisive action in the near future.

“Today most OEMs produce their frames themselves, but they are also seriously considering outsourcing. Especially after the last crisis, the CV industry wants to outsource more in some areas to limit its risks. We are marketing our concept as a combination of engineering service and system supply, but if the customer decides it wants to produce the frame by itself, that’s also possible,” he says.

But not probable — because contracting the entire development and supply to Magna would clearly be a more attractive alternative to investing a large amount of capital in the separate plant that would be necessary. In any case, the new frame will only realistically fit into a new vehicle generation. This could open up new opportunities for improvement in the design — a close collaboration with an OEM in a full-vehicle development environment will remove some of the constraints to maximum optimisation of the front- and rear-axle modules.

Mercedes-Benz launched the latest generation of its heavy-duty Actros only a year ago, which leaves MAN and DAF as the most likely prospects at the moment. “We are looking to get details of their engine and suspension so we can modify the frame for them,” Schwarz says, suggesting that some progress has been made towards establishing a “joint venture” with at least one of the two.

The production cost might be a little bit higher than for a standard frame, he concedes, but the saving of 300 kg in material alone is going to have a “very positive” impact on the total cost. The final price of the frame, of course, will also depend on the number of units produced. But in the 10 months since this writer’s visit to ECS the design has been fine-tuned to reduce manufacturing costs, he says.

“We remodelled the frame for manufacturability, changing some parts to improve accessibility for the welding gun and modifying the bending system we had originally envisaged to simplify manufacture of the monocoque. But we still have to ensure that the design meets the overall tolerance requirements in final assembly — the tolerances in the mid-section, for example, are critical for the parallelism of the front and rear axles.”

The team also evaluated a number of corrosion-protection systems, finally settling on Henkel’s water-based Aquenceautophoretic (A-coat) process, a simpler, more energy-efficient, and more cost-effective alternative to cathodicelectrodeposition (E-coating). They then used ECS’s proprietary Alsim software to simulate the dip-coating process for the entire frame and individual parts, adding drain ports and holes to allow entrapped gas bubbles to escape and ensure uniform coating of the entire surface, including the hardest-to-reach areas.

At the moment they are investigating the feasibility of replacing the steel cover of the monocoque with one made of plastic or a carbon- or glass-fibre composite to further enhance its stiffness and reduce noise. A composite cover would also be easier to manufacture, Schwarz points out. And clinching is being studied as a lower-cost alternative to spot welding.

Beyond the frame itself, Schwarz’s concept has all along envisaged weight-reduced components and aggregates, like the single-leaf suspension mentioned earlier, and a lightweight front underrun protection beam made of “highest-strength” steel. He reveals now that he’s presently in discussions with Jost for a lightweight fifth-wheel that mounts directly onto the frame. This, he says, will reduce the weight of the tractor by another 80 kg.

In its endeavour to get the most real-world of Magna International’s recent concepts into, well, the real world, ECS is willing to do more than engineer and supply the frame alone — if an OEM so wants, Dantendorfer says, it could also build in custom-designed tank systems from Magna Steyr, which is now the primary supplier of fuel and compressed air tanks to MAN and Mercedes-Benz following its 2010 acquisition of Erhard GmbH.

This gives Magna a design integration and delivery capability that’s unrivalled in the commercial vehicle supplier industry. Schwarz is convinced that full weight-optimisation of the frame and frame-mounted subsystems, as he has described it to me above, can knock 700 kg off the kerb weight of the vehicle. To put that in perspective, he’s talking of a saving of more than 9 percent on MAN’s 7420kg TGX 18.440 EfficientLine, touted as one of Europe’s most energy-efficient semitrailer tractors.

In an industry that’s just about hit the limits of what it believes is technologically possible to achieve in its continual quest for the holy grail of ultimate transport efficiency, Gregor Schwarz’s lightweight truck frame is all set to spark off a new revolution.

ELIOT LOBO