ThyssenKrupp Automotive Systems has developed an all-new support structure for independent suspensions in big trucks.
Whereas independent suspension has been available for the steered wheels of buses and light trucks for some time now, the transfer of this principle to heavy duty trucks has posed a serious challenge.
On the one hand the system has to deal with high axle loads, and on the other there is little room for its installation as a result of the front-mounted drive unit. However, the advantages associated with independent suspension (suspension comfort, improved steering behaviour, modular design) generally justify the development of new support frame concepts.
Ever since trucks have been manufactured, their support structure has primarily consisted of two side members (most commonly with a C-shaped profile). Although such a ladder frame principle leaves sufficient room for adding various types of vehicle bodies and any form of rigid axle arrangement, it is not an optimal solution for load bearing and requires additional structural components for independent suspensions.
To solve this problem, ThyssenKrupp Automotive Systems has come up with an innovative concept whose starting point is an optimised topology within the vehicle’s front end packaging space. Another factor that influenced the endeavour to find optimal load paths for different load levels was the desire to improve partner protection during crashes (front underrun protection).
While the Chassis project unit at Automotive Systems was responsible for system integration, the project actually involved cross-company and cross-segment cooperation. For example, the Forming Technology unit provided the knowhow for sheet metal forming. Meanwhile, ThyssenKrupp Presta contributed its decades of experience in the development of truck steering systems, and Bilstein Suspension supplied its expertise in suspension and damping components. Finally, the innovative use of new steel grades was the contribution of ThyssenKrupp Steel.
Requirements for future truck chassis
Future generations of commercial vehicles will have to meet several requirements that will affect their design to various degrees. Among the objectives that commercial vehicle manufacturers continuously strive to achieve are the improvement of handling behaviour and the enhancement of ride comfort. Major aims in this regard are the reduction of body roll and pitch. Another challenge is to create more room for the engine and the drivetrain, which is particularly important for enabling diesel engines to meet future emission requirements.
The main features of an efficient design are a compact arrangement, modular construction, and preassembled modules. An additional aspect that may have to be considered is the optional use of alternative steering systems. Steering requirements to be met by the vehicles include a correspondingly large steering angle and a small turning circle.
Advantages of a double wishbone axle
ThyssenKrupp Automotive Systems uses double wishbones for its new steering axle with independent suspension. The wheel location is governed by the wheel knuckle, which is connected to the vehicle frame by means of an upper and lower control arm. In this design, spring travel was limited to a range extending from +100mm to –100mm, which is sufficient for meeting the needs of a mass-market semitrailer truck. The advantages of this type of axle design are obvious, since it allows kinematic properties to be optimally set and makes it possible to transmit forces between the wheels and the frame along favorable paths.
Multi-body simulations were performed during the design of the system’s kinematics. The main focus here was on improving various aspects of driving safety and cost of ownership, such as a reduction of tyre wear. It was decided to separate the suspension and damping elements in order to make more effective use of space and improve the load distribution. Because such axles generally have ball joint connections to the wheel knuckle, the number of components can be reduced. A particular design aim in this system is to keep the control arms as short as possible in order to provide more room for the drivetrain and reduce vehicle weight.
Requirements of the support system
It is assumed that the front axle investigated for this article has an axle load of 8 tonnes. This results in the following conditions for the arrangement of the components: the static wheel load is around 40 kN. If the dynamic magnification factor is set at 2.5, the corresponding vertical load is 100 kN. The forces from transverse impacts are similar in magnitude, at about 80 kN. And if a braking torque of 30 kNm is to be achieved, the longitudinal force at the wheel contact area of a tyre with a radius of 0.5 m will be 60 kN in the direction of travel. Naturally, this has to have a corresponding friction coefficient in relation to the wheel load in order to transfer this horizontal force from the tyres to the road surface.
Besides this basic concept for dealing with load requirements, the vehicle structure must also meet certain stiffness requirements to ensure that internal deformation of the support frame has no negative effects on the vehicle’s handling. Common influencing variables in this case are the degree of transverse and torsional stiffness. To determine these relative values, the frame is fixed at the rear, while the front is loaded with standard loads.
In addition, since 2003 the EU has required that newly registered trucks have front underrun protection. This protective system is designed to prevent trucks from rolling over passenger cars following a frontal collision. The regulation applies to vehicles over 7.5 tonnes GVW. In addition, more than 15 percent of heavy duty trucks are subject to a special exemption for off-road vehicles, allowing them to dispense with any protective measures for road traffic.
To achieve this protective standard the following requirements must be met: a horizontal force of up to 160 kN is applied at predefined points at a height of up to 445mm above the road surface. When this force is applied, the penetration must be limited to a depth of 400mm, which means that only a geo- metrical limit is defined.
In addition to meeting these minimum legal requirements, it is desirable that the front of the truck is equipped with energy-absorbing structural sections. To keep costs down, these deformation elements should be easily replaceable and feature a defined interface to the main structure, which must remain damage-free up to a predefined crash level. At its base in Bielefeld-Brackwede, the ThyssenKrupp Umformtechnik company has gained considerable expertise in the development of so-called crash boxes, which were easy to incorporate into the new axle system concept.
Optimised topology of packaging space
The design measures are restricted to the front end of the vehicle, and do not extend to the semitrailer truck’s rear frame surrounding the kingpin and the rear drive axle. The first task under these conditions was to create a model of the available packaging space, while taking the geometrical situation into account. This three-dimensional model was meshed by means of the finite element method and subjected to an optimisation process using special software. In a number of steps, this process reduced the structure to the regions required for transmission of the forces. Non-load-bearing areas were defined by means of a notional density graduation. The result was a skeletal residual structure. Transfer to a sheet metal design
Large-scale downstream operations and interpretations of the results are required to transfer the optimised 3D model into a design suitable for manufacturing from steel profiles. There are limits to how far the meshing process can be carried, even with the computer capacities available today. It is therefore unavoidable that the result will have a substantial layer thickness even in the minimalised structural areas. This is in contrast to stamped structures, in which the thickness of the surface elements is negligible compared to the length of the sides.
As soon as a welded or bolted design has been created that is suitable for manufacturing, it can be fine-tuned through the parameter-controlled optimisation of individual sheet thicknesses. It is during this phase that the special strengths of ThyssenKrupp Steel come into play. Whereas a traditional truck ladder frame is made of conventional steel grades, the use of new high-strength grades makes it possible to further reduce the weight of the support structure.
An evaluation of the final sheet structures of several passenger car axle projects shows that the results are 90 percent in agreement with those of the three-dimensional structures considered optimal. The concept created in this manner features several innovations that set the support structure apart from previous truck frame designs. The lower structural section is a major part of the overall system. The specified height at which force is applied for passenger car crashes results in a load path that must be suitably transferred to the structure of the main frame.
For this reason the pronounced substructure is connected to the main structure at several points. This results in a distribution of the force to be transferred and makes it possible to reduce the weight of the individual connecting elements.
The double wishbone principle of this all-new support structure combines all of the advantages required for a steerable front axle with independent suspension. A computer-based topology optimisation method was employed for the design of the support frame. This generated new possibilities for the load paths and consequently for the arrangement of the components.
The system’s modular design and the reduction of the number of variants and components create potential savings that make the concept interesting for commercial vehicle manufacturers. Considerable business opportunities are also created by the foreseeable tightening of the emission regulations for commercial vehicles and the associated requirements with respect to packaging space.
Volume-produced heavy-duty vehicles with independent suspension are currently not available on the market. However, for the reasons given above, it is only a question of time until such systems are at least introduced for front axles.
A lifesize model of the structure was first presented at the International Commercial Vehicle Show in Hanover in 2006, where it met with a great response from trade visitors.
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