Hollow MKV Rims from BBS

**mikinoz** · 27-05-2006, 04:30 PM

World novelty at the Paris Motor Show 2004: The VW GTI wheel marks the world’s first production order for hollow space wheel construction. BBS started supplying original VW wheels with the introduction of the Golf II GTI.
Significant weight reduction: The 18" GTI wheel weighs 1 kg less than the same design without hollow space technology. This significantly improves driving dynamics and comfort. The effective weight reduction for all four wheels adds up to more than 4 kg. In fact, total weight reduction amounts to 8 kg because a rotating mass effectively weighs twice as much because it has to be simultaneously rotated and set in forward motion. At the same time, this unsuspended weight has a more significant impact on driving comfort compared to the remaining vehicle weight.
Lower rotating masses for better driving dynamics: Weight has been reduced on the GTI wheel in precisely the areas where the laws of dynamics ensure the best results – at the wheel-rim shoulder. The further away the rotating mass is from the hub, the bigger the effect of weight reduction on the mass moment of inertia. This increases by the square of the radius. The mass moment of inertia of the GTI wheel is 14 percent less than the same wheel with solid shoulders. Reduced wheel inertia improves acceleration. This physical relationship is known as the pirouette effect: Bringing the arms closer to the body reduces the rotating mass at the extremities and results in acceleration. Reducing the rotating mass also shifts the threshold margins on corners and enables higher cornering speeds. This is a result of the reduced centrifugal effect or centrifugal precession. Precession is a phenomenon of rotating masses. This is something that most motorcyclists are familiar with. But aside from professional racing, it has received little attention in the automotive world. The decisive moment that determines the ideal line in a curve is when the vehicle steers into the curve. Motorcyclists typically steer slightly to the left when entering a right turn. This is because the rotating wheel has a tendency to counteract the steering force. Thus, a motorcycle virtually steers itself into the curve. The wheel suspension on a car essentially works the same way. The precession on the front wheels first tends to counteract the steering force. This is known as correcting torque, and is the product of the moment of inertia and angular acceleration. The precession effect is therefore dependent on the radius of the rotating masses. The hollow wheel-rim shoulder effectively counteracts this steering resistance. The same relationship can be derived from the spin of the wheels. It is calculated as the product of the mass moment of inertia and angular velocity. The rotation speed progressively increases in proportion to the mass moment of inertia. The lower the rotation speed, the easier the wheels are to steer. The greater the circumferential rotating mass, the bigger the flywheel effect. This explains why flywheels are designed to have as much mass as possible at the outer edge. Braking efficiency also increases when the circumferential rotating mass is reduced resulting in longer brake life.
Reducing unsuspended weight improves driving comfort: Lowering unsuspended weight improves road adhesion and steering precision while reducing torque steer. Lightweight wheels improve steering system and road surface response. Wear on suspension and shock absorbers is also reduced because there is less spring and shock absorber action. This results in less oscillation between suspended and unsuspended masses and a significantly smoother ride. Lowering the unsuspended weight makes it possible to use stiffer springs without sacrificing driving comfort.
Effective reduction of vehicle weight by 8 kg: A recent research project showed that replacing a steel Golf hood with a synthetic one reduces vehicle weight by only 5 kg. The BBS GTI wheel makes a significant contribution to lightweight construction. Larger wheel diameters and widths with run-flat tires can be implemented without increasing vehicle weight. Reducing weight also offers benefits in terms of performance, range, fuel economy, and CO2 emissions.
Opening the door to previously unmarketable designs: Hollow space technology makes it possible to create new wheel designs that would have otherwise been too expensive to manufacture. This means that attractive hollow space technology wheels will be available in the future.
Increased stability for run-flat tires and off-road use: Hollow space designs are used where high structural strength is required. From modern aircraft designs to human bone structures, nature and technology offer plenty of examples of how hollow structural components provide added strength. In wheel design, this technique further optimizes the wheel load/dead weight ratio. The BBS hollow wheel shoulder is ideal for handling the higher loads of run-flat tires and off-road driving conditions. Increased strength and expansion resistance are achieved by uncompromising process technologies based on rotation rolling, crimping, and laser welding. The key to structural stability is rim rolling or flow forming of the rim well and the resulting material density. This also makes the process more economical and environmentally compatible. After all, a GTI wheel is more than just a hollow casting.
Environmentally compatible powder coating: The award-winning BBS surface treatment is also used on GTI wheels. BBS was the first manufacturer to introduce chrome-free conversion coating for wheels. This process is based on the following steps: chrome-free conversion coating, solvent-free powder priming, water-soluble base color coating (introduced by BBS in Germany), diamond-polishing of the surface, sealing coat with neutral acrylic paint powder. This process offers significant advantages. The coating is twice as thick as the surface produced by horizontal painting. This significantly improves corrosion resistance and protection against damage. An even coat also improves wheel appearance.
Environmentally compatible dry processing: After casting, the casting gate and burrs are removed in a fully automated CNC machining station. Once the casting has been cleaned, the holes are drilled for the wheel bolts, tire valve and axle hub. BBS recently started handling these steps as a dry process, i.e. without the fluids that are normally required for cooling and lubrication in metal machining processes. All excess metal is fed directly back to the melting furnace. The complete machining station is hermetically sealed to prevent aluminum dust emissions. For the GTI wheel, pre-processing, drilling and diamond polishing are all dry processes.