Mercedes-Benz Light-Alloy Wheel Design Process

Whatever model of Mercedes you may drive, Mercedes-Benz and its subsidiary Mercedes-Benz Accessories GmbH have just the right light-alloy wheel to suit every taste. When ordering a vehicle, customers can currently choose their perfect wheel from a selection of around 20 different design variations per model series, which are available either as standard or as optional extras. In addition, almost as many wheel models are also available from Mercedes-Benz Accessories for the current model range.

All of the light-alloy wheels on offer have to meet the same high safety standards – there is no difference whatsoever in term of quality between the models available as standard or as optional extras, or even the versions available from Mercedes-Benz Accessories. The customer always receives a product which has been optimally tuned for Mercedes-Benz vehicles: it will provide exemplary ride comfort and high driving dynamics, meet demanding design standards, and will also be characterised by high performance combined with an excellent service life. These benefits are not only the result of close collaboration between vehicle development and specialists from wheel development. Wheels which are to bear the Mercedes star must also pass a comprehensive test programme which goes far beyond the statutory requirements laid down by the German road traffic type approval law. This includes actual test procedures which are based on the harshest of operating conditions in real driving situations, as well as inspections carried out using the latest x-ray systems and computed tomography scanners.

Mercedes-Benz Accessories: high-sheen two-tone wheels top the popularity charts in spring 2010

Topping the popularity charts this spring are Mercedes-Benz wheels featuring a two-tone finish. According to the Mercedes-Benz designers, however, they are more than just a passing fad and are set to enjoy increasing popularity in the future. These high-quality wheels enjoy additional appeal thanks to the interesting contrast created between the bare-look metal surface and the dark painted areas, thus giving the vehicle an even more exclusive appearance. Mercedes-Benz Accessories is currently offering the following wheels in a high-sheen two-tone finish (as at April 2010):

The fitting of wheels does not require any reports or Technical Inspection Association registrations to be carried out, since all wheels form an integral part of vehicle type approval.

Wheel design: bringing everything together

Obviously without wheels a vehicle is going nowhere – this simple observation not only describes the basic technical requirement for a properly functioning motor car, but also highlights the importance of the wheel as part of a powerful vehicle design. At Mercedes-Benz, high-quality wheels which match the bodywork and form a logical continuation of the design therefore play a significant role in the high design standards of the overall vehicle. At the Mercedes-Benz design centre, light-alloy wheels available either as standard or as optional extras are designed by specialists in the area which is also responsible for creating the entire exterior form of the vehicle.

Complex test programme forms part of product development

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.

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. This is why it is called ZWARP (biaxial).

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.

Another stress test is the rotary bending fatigue test, which subjects the wheel structure to 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 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.

Leading role in industrial computed tomography

An important part of the development process for new light-alloy wheels are examinations which look at the internal microstructure of the wheel, right down to atom level. While the examinations conducted on the various test systems and facilities give an indication of whether the performance of a new wheel is able to meet the high standards required by Mercedes-Benz, supplementary examinations using reflected-light or scanning electron microscopes, and both x-ray and computed tomography systems, provide an insight into why some wheel prototypes are more resilient than others.

While it is always necessary to destroy the test wheel in order to produce samples for examination under reflected-light or scanning electron microscopes, non-destructive testing on the other hand is also used to supplement the development phase and makes an important contribution to checking subsequent series production at suppliers. The wheels are inspected at the acceptance stage using x-ray systems and examined for cavities (air pockets) and imperfections larger than 0.3 of a millimetre. If even more detailed examinations are necessary then computed tomography (CT) is used. This is a process used in the medical field and was introduced by Daimler AG for industrial use – the first automotive company to do so – as early as 1995. The company now plays a leading role in this field. The Stuttgart-Untertürkheim plant currently has the most modern CT system for industrial use in the world.

Unlike x-ray examination, CT produces three-dimensional images. As a result, the position and size of the slightest weak spots caused during the casting process, such as cavities or gas pockets, can be accurately determined. Used in conjunction with the tests conducted on the test benches, these CT assessments provide evidence of where the smallest imperfections might have a big impact on performance. These component areas can then be enhanced during the technical casting and production processes. In addition, the knowledge gained can be carried over into future wheel development.

Special corrosion tests for high-sheen wheels

High-sheen light-alloy wheels require a very special paint structure which provides reliable protection against corrosion for the bare-look metal surfaces.  In developing new anti-corrosion paints for high-sheen wheels, Mercedes-Benz introduced the toughest of test methods – a series of ultimate tests for wheel coatings.

To begin with, staff at the corrosion test centre prepare individual wheel segments by scribing deep marks on high-sheen areas which penetrate down to the bare light alloy. These prepared wheel segments then undergo the so-called CASS test for 24 hours. In this chamber-like system, the paint finishes are permanently subjected to different, highly-corrosive salt sprays. The next stage for the test samples is the filiform chamber. Here the test samples – now partly coated in a crust of salt from the CASS test – must withstand a 28-day variable climate programme. As part of the final evaluation, the employees assess the filiform corrosion around the scribed marks. Approval is only granted for those anti-corrosion paints which reduce undercutting corrosion to a minimum and at the same time do not negatively impact on other properties.

Around 250 wheels per year undergo this test marathon. But it is not only new wheel models which are approved on the test bench. The specialists also carry out the comprehensive test programme when a wheel supplier wants to introduce new materials, such as a base coats, clear coats, functional coatings such as anti-corrosion paints, and also primers or pre-treatments. The changes only receive a “basic material approval” once all tests have been passed without any issues. At the same time, wheels from current production are also tested to ensure that they meet strict Mercedes-Benz quality criteria. The severity of the test series conducted in the climate chamber is demonstrated by the enormous amount of salt which is consumed at the corrosion test centre: the plant requires around 5 tons of salt each year. Used salt is recycled in the basement of the test facilities.

High quality from the best and most prestigious suppliers

When it comes to producing light-alloy wheels, Mercedes-Benz only trusts the world’s best and most prestigious suppliers. These companies have a high level of know-how and produce wheels in accordance with strict Mercedes specifications. Numerous test procedures, both on current production as well as in the laboratory, help to guarantee a high standard of quality. These procedures include monitoring the raw materials at the time of delivery and during subsequent processing, 100-percent x-ray examination, leakage tests, continual monitoring of strict geometric data and concentricity during machining, as well as monitoring the painting process.

The production of two-tone wheels involves a considerable amount of additional effort since special coatings are applied to provide reliable protection against corrosion. After the first coating the system removes these wheels from the production process and transports them to special turning machines fitted with diamond cutting tools which remove the coating layers in specifically defined areas of the wheel – mostly on the spokes or the outer wheel rim base. This so-called “high-sheen polishing” process creates metallic high-gloss areas which contrast against the remaining coating. Of course these areas must be resealed as part of a new coating process. This has a three-layer structure: after being pretreated once more, the wheel then receives a transparent anti-corrosion coating, specially developed for Mercedes-Benz wheels, followed by various clear coatings to build up the final glossy finish.

A smooth process – flowforming

In conjunction with cast, one-piece aluminium wheels, “flowforming” is a relatively recent machining method. Basically wheels which have been machined using the flowforming method offer the best of both worlds. Not only does the designer retain the high degree of freedom over the design which a cast wheel provides, but a flowformed wheel also enjoys the advantages afforded by forged wheels, which are expensive and complex to produce: high strength combined with optimised component weight.

As with the wheel production process described above, first of all a blank is cast which has a very narrow wheel rim base but with significantly greater wall thickness. After being heated up to around 350 degrees, this blank is then stretched on a cylinder which tapers slightly towards a cone at the top. The blank and cylinder are rotated while three rolling heads, which are also rotating, are pressed against the blank from the outside in a downward motion and at high pressure – around 120 tons. This forces the metal into the desired shape over the cylinder, hence “flowforming”, and at the same time compresses it. A wheel rim base created in this way has a similar structure to a forged wheel, offering the highest degree of stability with the lowest possible weight. Subsequent heat treatment of the flowformed wheel then gives it its final strength.

Care tips and methods

There is one thing which the best development programme, the most innovative production technology or a perfectly tuned test programme cannot affect, however: how customers treat their vehicles, and as a result their valuable light-alloy wheels. As always, how the wheels are treated has a significant influence on their value retention and also the safety of the vehicle. Here are some useful tips:

• Do not use abrasive cleaning agents. The products recommended by Mercedes-Benz will clean the wheels gently and effectively.

• Do not use rough or abrasive sponges for cleaning wheels.

• Observe the specified wheel tightening torques. Tightening the wheel bolts too much, or not enough, can damage the wheel over time.

• Do not oil or grease the wheel bolts, since the initial stress will be higher than the specified tightening torques and could damage both the wheel and the bolts.

• Use original Mercedes-Benz wheel bolts only.

• If the wheels are subjected to a heavy blow, e.g. after driving over deep potholes, check the wheels. If there is visible deformation you must have the damage checked by a dealer.

• Mechanical or thermal wheel repairs, such as welding, bending or grinding can reduce the stability of the wheel due to changing the microstructure of the metal and are therefore not permitted. Any wheels which are damaged should be exchanged!

• Only recoat scratched wheels with cold varnishes. Bake-on coatings can also change the microstructure of the metal and therefore reduce the strength of the wheel.

• Only fit tyres of the size permitted for the wheel.

• When driving with a heavy load and/or a trailer, adjust tyre pressure accordingly and observe the permitted wheel loads.

Legendary light-alloy wheels from Mercedes-Benz

Mercedes-Benz introduced the use of light-alloy wheels on volume produced vehicles at the start of the 1970’s, with forged aluminium wheels from well-known producer Fuchs. The “premiere wheel” was the first to be made available as an optional extra for the 280 S to 300 SEL 6.3 W 108 series Saloons, and was later christened the “baroque wheel” on account of its ornate design. After this initial foray into the world of light-alloy wheels the range continued to be expanded. For the 190 compact model series and models from the 200 to 300 E mid-size model series, a 20-spoke light-alloy wheel first appeared in the accessory catalogue from the mid-1980’s, which thanks to its characteristic wheel disc shape soon earned the nickname the “police star wheel”. The final triumphant move came in the same decade in the form of the legendary “manhole cover wheel” which – as its charming name already suggests – resembled a manhole cover.

Today, with a few exceptions, almost all of the series production vehicles from Mercedes-Benz run on light-alloy wheels. How successful they have been is reflected in the following comparison: around 4600 examples of the “baroque” model were sold each year. In 2009, the majority of new Mercedes-Benz vehicles were on “alloys”. Together with the sales figures from Mercedes-Benz Accessories, the number of alloy wheels brought into circulation has increased to around 4 million per year.