Mercedes-Benz guarantees the quality of its new wheel models with a comprehensive development programme. As part of this, the tests and inspections which are carried out go far beyond the statutory requirements. Even when it comes to wheel development, one basic principle is applied:
In the development and testing phases, Mercedes-Benz bases its work on the actual load profile of light-alloy wheels under real operating conditions and coordinates its programme accordingly. As a result, light-alloy wheels bearing the Mercedes star are among the safest, best performing and most durable products on the entire automotive market. At the same time, it is irrelevant whether we are talking about a wheel from the standard or optional ranges, or one from the wide range of accessories available from Mercedes-Benz Accessories – the process is always based on the same high standards.
Development dominated by the digital worlds
Early in the development phase of a new light-alloy wheel, engineers first specify the rough framework conditions within which to work: new wheel types for car models are determined in close cooperation with those responsible for the vehicle model series. At the same time, wheel specialists examine wheel market trends. These parameters then define the necessary wheel dimensions. The following basic principle is applied: data such as the gross axle weight, the size of the wheel well or any necessary brake clearance provide the installation space within which the designers have free space for their designs – all of course in accordance with exacting Mercedes-Benz design requirements. After a technical feasibility study, the final wheel design is determined.
A three-dimensional volume model is then produced using modern 3D computer programs. For visualisation and discussion purposes with the development team, developers can use this model to create three-dimensional drawings, or to specify component properties such as weight, machining options in the subsequent production process, material distribution or even natural resonances and inertia moments. On the basis of these data records, the wheel is optimised with virtual test data using finite element analysis (FE analysis for short). In the digital world, it is possible to simulate demanding mechanical or thermal operating conditions: for example, cornering at maximum wheel load, or driving over a pothole or the kerb, or the brake heat load generated on a long downhill stretch. As such, conclusions can also be drawn about the subsequent production process and how it can be improved: if the intended wheel model is gravity die-cast and the material solidifies as required, will it be possible to remove the cast wheel blank from the mould without any problems? After preparing the light-alloy wheel in this virtual world, a digital mock-up is created – a computer-aided wheel model – which serves as the basis for all subsequent steps.
Based on the mock-up, the wheel manufacturer responsible for production sets up the necessary moulds, production tools and processes, after which the first sample wheels are produced. In this “prototype phase”, the wheel manufacturer conducts detailed examinations in cooperation with Mercedes-Benz to assess the resulting wheels and the entire production process. The objective: to optimise production at a high quality level. If this objective is reached, a true ordeal awaits the new wheels, which will be taken from the finely-tuned large-scale production test run.
In cooperation with the vehicle development divisions, aerodynamic aspects can also be incorporated into the design process. Flow simulations have shown that aerodynamically optimised light-alloy wheels and tyres can improve the overall aerodynamics of the vehicle, which in real driving conditions helps to reduce fuel consumption and can lead to a reduction in CO2 of one gram per kilometre.
“ZWARP” replaces six weeks of test driving at the Hockenheimring
One of the most effective test methods for assessing a new light-alloy wheel is “ZWARP”, from the German “ZWei-Axiale Räder-Prüfstand” (biaxial wheel test bench). Unlike a conventional rolling test in which the wheels run straight on an external roller with a specific ground contact force, the ZWARP uses an oversized roller to subject the wheels to both ground contact and lateral forces generated as a result of additional transverse movement of the test system in two directions. This is why it is called ZWARP (biaxial).
First of all the wheels are fitted with the corresponding tyre size, and to make the test conditions tougher they are initially damaged on the inner wheel rim flange by simulating driving over a kerb at 2.5 times the normal wheel load. At the same time this initial damage is also used as an individual “inner rim flange impact” test, in which deformation is not to exceed a few millimetres. Once this starting requirement is met, the actual test run takes place, divided into 22 load blocks. The load blocks are based on the subsequent application profile of the vehicle – so according to whether the wheel is intended for a saloon, a roadster, an off-road vehicle or a people carrier. The test conditions are extreme: as part of the test run the wheels are subjected to a ground contact force of up to 35 kN over several thousand kilometres, which in real operating terms equates to the distance travelled over the entire service life of the vehicle. By applying steering movements, the wheel is also pressed against the side lip of the rotating drum with a lateral force of up to 25 kN, thus simulating the wheel loads generated during sharp cornering. The requirement imposed by Mercedes-Benz for this marathon is that despite initial mechanical damage, the test wheel should not show signs of any cracks over the test distance. If a wheel passes the ZWARP test, based on experience it will usually last the life of the vehicle several times over under normal operating conditions.
Looking back, we can now appreciate the significance of the ZWARP: previously these loads were applied to the vehicle in test drives on the race track in Hockenheim and took around six to eight weeks. At that time, together with the other tests, only around 10 light-alloy wheels could therefore be tested and finally approved in a year. The ZWARP, on the other hand, is significantly more consistent over just a few days since, unlike the practical test drives in Hockenheim which were influenced by changing weather conditions, it always runs under the same, defined conditions. Today it takes around four weeks for complete approval. In one year, therefore, the engineers and technicians can issue approvals for around 150 new light-alloy wheels. Currently, however, the test programme does not run completely without any practical tests: prototypes of new vehicle models are for the most part fitted with new wheel types for their trials and test drives. These test results are also incorporated into the assessment and approval of new types. And there is an interesting point to note here: you may not be aware but for the most part photos of prototypes therefore not only depict new vehicles, but also new wheels.
From bending to breaking: the rotary bending fatigue test
Another stress test is the rotary bending fatigue test. For this, employees clamp wheels with the inner rim side locked positively in a jig, and fix the wheel disc to a hub using the normal holes for the wheel bolts, as when fitting a wheel to a vehicle normally. This stresses the wheel structure with load cycles through oscillating movements, simulating maximum cornering in which bending moments of between 1900 and 11,000 Nm are applied. This test is conducted in parallel on several wheels and under different load conditions:
• 4 wheels successfully complete 200,000 load cycles with a 100% bending moment
• 4 wheels successfully complete 800,000 load cycles with a 75% bending moment – this is four times the statutory requirement
All wheels have to survive under these conditions without any cracks forming. However, the test is continued until such time as initial cracks start to appear. This has shown that light-alloy wheels from Mercedes-Benz are able to withstand up to several million load cycles without suffering any damage – which means they are able to last for quite a few vehicle lifetimes under normal operating conditions.
Convincing proof of stability
In addition to the inner wheel rim flange impact test forming part of the ZWARP test, light-alloy wheels aspiring to bear the Mercedes star must also withstand two further attacks. This is where, figuratively speaking, the tests turn medieval, since the wheels are placed under a type of guillotine. In the so-called impact test, which simulates driving over an obstacle such as a kerb at an angle, the wheel is fixed horizontally at a slight tilting angle under a blunt guillotine. The guillotine blade is then dropped at a weight which is calculated according to the permissible wheel load (0.6 times the wheel load plus 180 in kilograms), and from a specified height, onto the outer wheel rim flange. This not only results in an ear-deafening impact, but also in deformation of the impacted area of the wheel rim. This deformation should not exceed a specified level, and there should also be no chips or leaks. The tyre which is fitted must retain its pressure after the impact, in order to enable the vehicle to continue to be driven if this were to happen in reality.
The second guillotine process is similar, only this time the wheel is stood up vertically and the tyre tread is struck by the guillotine blade with greater force so that it goes through to the wheel rim flanges. Here too the damage must not lead to the complete failure of the wheel/tyre system.
Often underestimated: the wheel bolt connection
The importance of secure wheel bolt connection requires no further explanation. Achieving a secure connection, however, is not as simple as is generally assumed. The bolt connection system – comprising the wheel bolts, the new light-alloy wheel and the vehicle wheel hub – is therefore checked and if necessary optimised using a special test facility. Of key importance for correct wheel bolt connection is the specified torque for tightening the wheel bolts. There is a defined prestress for this, which is responsible for connecting the wheel to the vehicle. The engineers at the Mercedes Technology Centre (MTC) carry out extensive fine-tuning in this area in order to ensure that the bolt connection guarantees the highest possible level of safety. This is because prestress is influenced by a number of factors: the frictional force of the bolt thread in the wheel hub thread, the frictional force of the bolt head in the wheel crown, and the contact areas between wheel, brake disc and wheel hub.
If the frictional force of the bolt connection is too low, the wheel nut could overstretch at the specified tightening torque due to the prestress being too high. Although it sounds ironic, these circumstances could lead to the bolt connection becoming loose. The wheel bolts should also therefore not be greased when being fitted as this could reduce the frictional force. If, in the opposite case, the frictional force is too high, the bolt connection would not have the necessary prestress at the specified tightening torque, and the wheel could also work loose. It is therefore absolutely vital that only original Mercedes-Benz wheel bolts are used, since they provide the optimum frictional force and therefore guarantee a secure bolt connection. Mercedes-Benz relies on the safest type of bolt connection by using wheel bolts which have a spherical crown behind the bolt head so that they fit precisely into the bolt holes of the wheel, which are also spherical. The correct prestress also supports the slightly concave contact area with which the wheel fits against the vehicle wheel hub.
Final OK after 3D measuring machine and visual inspection
The approval process for new light-alloy wheels also includes checking the geometric data with a 3D measuring machine using a fully automatic process. After the test wheel has been clamped in, the highly-precise system checks 20 main dimensions at 150 different points down to micro-level accuracy, and compares them against stored CAD data. Only when several wheels have also successfully passed this test within the very narrow tolerances, like all the other tests, is the Mercedes-Benz wheel development team able to carry out a final test for granting the final approval. Despite all of the technical options, these “human factor” assessments are also important. In particular, the team assesses:
• Paint quality: in addition to the tests conducted in the corrosion test centre, the paint finish is examined for its colour scheme, layer thickness, inclusions or pores. In addition, contact areas and wheel holes must be free of any paint.
• Quality of the casting: no porosity or surface cavities.
• Machining: clean deburring.
• Specified weight check.
• Correct designation.
• Trouble-free fitting of the valve and tyre pressure monitoring system sensor.
• Valve for the tyre pressure monitoring system is accessible using air refill systems normally available at filling stations.
• Hub cover fits correctly.
• Runout check.
Source: Daimler AG