You step on the gas of your Maserati Quattroporte Sport GT-S (play along) and are barely through the intersection, when a pedestrian runs through the crosswalk to beat traffic.

As much as you love the performance of your Italian sports car, you’re only going 10 miles per hour when you have to stop. You ease onto the brake and avoid striking the hasty pedestrian without having to work too hard. Under the fenders of your automobile, your front brake disc hasn’t had to work too hard either. The friction levels involved are low enough at that speed that little heat has built up, and there is no chance the disc will distort.

At higher speeds, the conditions are different. When you’re on the autobahn in your Maserati, the friction built up when you stop at the last second to grab a gelato creates heat sufficient to warp the metal that comprises the brake disc.

But thanks to brake systems specialist Brembo, Curno, Italy, your Maserati has a brake disc designed to handle both situations. To produce the part, the company co-casts two components in a proprietary process.

The Braking Point

To stop an automobile, friction is applied to the outer ring of a brake disc by a caliper. The disc is attached to the car’s axle (or in some cases axle half-shafts) at its center, so the reduced speed of rotation is transferred through the brake disc into the wheel, and the car stops.

According to the calculations of Brembo engineers, a 6,600-lb. car traveling 185 miles per hour requires a single front brake disc to impart as much as 800 kW of power to achieve a 1-g deceleration stop. That’s the kind of energy that, when converted to heat and combined with high levels of torque, can cause metal components to distort. But when stopping a car going 10 mph, the rotor is asked to impart far fewer units of energy, so Brembo engineers decided to integrate the best properties of two materials to create the perfect brake disc.

Iron brake discs are highly heat resistant, making the material ideal for braking in extreme conditions. But they’re also heavy and can make braking feel rough to the driver. Aluminum discs lack the same heat resistance, but they’re lightweight, which induces a smooth stopping pattern while contributing to overall automobile performance. To bring the properties together, creating a rotor that allows for minimal thermal distortion while providing a comfortable ride, the challenge was to create a disc composed of an iron friction ring around a central aluminum bell.

“As [a brake] disc gets hot, it wants to expand and grow,” said Brian Burke, technical driector for Brembo’s North American operations. “But since these materials have different expansion rates, the disc grows dynamically as you hit the brakes. It grows where it wants to grow, which reduces the thermo-mechanical stresses in the rotor. Picture an L-shape. What it wants to do is go back to a straight line, but what we have here is a symmetrical design that grows radially. It grows in the direction it wants to, so it gives zero axial deformation.”

According to a spokesperson for Cadillac, which utilizes the co-cast rotor on its high performance CTS-V model, the engineers developed two methods of bringing together the iron and aluminum parts. One, the dual-material rotor was created as two pieces and then machined and assembled with aluminum fasteners. Two, the iron and aluminum were melded in a two step casting process.
“They were seeking to avoid the complexity and intricacy of that assembly with the co-casting,” said Cadillac communications manager David Caldwell. “It’s intricate, difficult, specialized work that they’d love to use casting technology to eliminate.”

A Marriage Through Casting

The new brake disc is part of a larger braking system, first developed for European cars like the Maserati, that has recently traveled internationally to be integrated in the Cadillac CTS-V.

“It’s one of the world’s fastest sedans,” Caldwell said. “It blends the attributes of a sedan and a four-door luxury car. The power to weight ratio is tremendous, so that requires some big league braking to make it as civilized and well-mannered as possible.”

The full braking system contains four-piston, aluminum calipers and 14.4-in. (365 mm) one-piece rotors on its rear wheels—nothing out of the ordinary for Brembo. But on the front wheels, the system incorporates aluminum, six-piston monobloc calipers with 14.6-in. (370 mm) dual-cast rotors. It is the dual-cast rotors that required Brembo to step outside its traditional knowledge of the permanent mold casting process to produce something unique.

To bring the aluminum and iron together in one component, Brembo’s metalcasters moved from the outside in. They began by producing the cast iron outer ring in the low pressure permanent mold process. That incomplete component was then pre-machined to prepare its mating surfaces to be paired with the aluminum ring. The inner aluminum hat then was cast in the same permanent mold procedure, but with the iron piece fitted around it on the casting die.

“The challenges were to find the optimized process parameters (for example the temperature, filling pressure and time) to guarantee that the teeth were perfectly embedded for the tangential torque transfer from the cast iron brake ring to the aluminum bell,” said a Brembo engineer.

The final, dual material component was then finish machined to close tolerances with thicknesses that have been reduced from comparable brake discs. This reduced thickness contributes to the advantages offered by the new rotor, including:

• 15-20% weight reduction;
• reduced machining and assembly cost;
• greater driving comfort;
• less corrosion;
• wear and resistance to fade;
• better braking performance.

Company Policy

While other automotive suppliers divest their metalcasting operations, Brembo’s co-casting process marks the latest step for a brake system design company that makes metalcasting a part of its culture. Included in its 25 plants in 12 countries are several captive casting operations.

The company is currently adding a new metalcasting facility next to its existing plant in Dabrowa Gornicza, Poland. The expansion will give the company 75,000 additional tons of brake disc casting capacity. Brembo also purchased the brake discs division of metalcasting company Hayes Lemmerz in November of last year.

Today, Brembo manufactures in-house more than 32 million brake rotors a year for use on the brake systems it supplies to both OEMs and aftermarket purchasers. With 2006 sales of $1 billion, it serves systems to vehicles from approximately 30 different OEMs, including Aston Martin, Audi, BMW, Daimler, Chrysler, General Motors, Ford, Ferrari, Lamborghini, Mercedes and Porsche. And of course, your prized Maserati. ECS