Composite And Floating Rotors

Composite And Floating Rotors: Going Big And Going Light

When you hear the words "floating rotor," you might think of a racecar brake rotor with Allen head bolts holding the hat to the outer ring. But, more economical and robust "semi-floating" rotors have made it onto Audi, BMW and Mercedes-Benz daily drivers.

mercedes benz brake job featured

When you hear the words “floating rotor,” you might think of a racecar brake rotor with Allen head bolts holding the hat to the outer ring. But, more economical and robust “semi-floating” rotors have made it onto Audi, BMW and Mercedes-Benz daily drivers.

A semi-floating, dual-cast or composite rotor has two advantages over a fully cast rotor. First, these rotors weigh less. The weight savings increase fuel economy, while improving braking and handling performance since the unsprung mass is reduced.

Second, this rotor design allows the outer ring to expand and contract at a different rate than the hat, which can help prevent cracking on rotors that have diameters larger than 12 inches. This is why the BMW M5 and M6 can have rotors that are more than 15 inches in diameter in the front and rear.

Types of Rotors

Current composite or floating rotor designs use a cast-iron ring mated to a forged aluminum hat. Since it is almost impossible to weld cast iron to aluminum, two methods to mechanically fasten the rotor to the hat are being used by Audi, BMW and Mercedes-Benz.

Audi and BMW attach the hat to the outer ring using pins that go through the side of the hat and into the rotor. These are pressed in with extreme force. The heads of the pin can be seen through the wheels.

BMW TSB SI B34 05 15 discusses customer complaints of a ticking noise coming from the front wheels while cornering at slow speeds and during parking. This condition is caused by movement between the inner contact surfaces of the brake rotor and the flange. The solution is P/N 31 10 8 053 073 — a “friction washer” that fits between the flange and rotor.

On some mid-line Mercedes-Benz AMG vehicles, the company opted to use a design from a supplier that makes composite rotors for Maserati, GM and other manufacturers. This design utilizes an aluminum hat section; the hat has projections that go into the rotor around which the iron forms while the casting is spun.

bmw rotor machining
On this BMW rotor, the machining equipment is pressing in the pins that hold the forged aluminum hat to the outer ring.

Serviceability

Veteran technicians will remember the problems some domestic and import vehicles with composite rotors had in the late 1990s. These rotors had hats made of stamped steel that were cast into the iron rotor. They were prone to lateral runout due to installation errors.

The same is true for the new generation of composite rotors. If a technician tightens the lug nuts unevenly or with too much torque, runout could be induced. This runout typically results in pedal pulsation in 6,000 miles or less.

The bad news is that some of these rotors can’t be machined because of their design. On some applications, the front rotors allow for only 2 mm of wear before they are below the discard measurement. It is critical to check your service information for recommendations to measure and machine these types of rotors.

You May Also Like

Adjusting Parking Brakes

You should never estimate when it comes to parking brake adjustments.

The parking or emergency brake has to perform two distinct jobs. First, it must be able to hold the vehicle on an incline. Second, it must be able to stop a vehicle under a specific distance if the hydraulic brakes have failed. These standards are set by the Department of Transportation (DOT), and the kicker is these standards must be met with the force applied by a little old lady. But, over time, the cables can stretch and the friction materials change their length and thickness, which is why adjustments to the system are required.

Charging More for Brake Jobs

Here’s why charging more for brake jobs keeps customers coming back.

Servicing Multi-Piston Brake Calipers

Caliper replacement may be necessary in high-mileage vehicles because of fluid leaks or because the calipers are sticking.

Wheel Bearings

Higher quality wheel bearings use higher quality steel that is properly heat treated.

EV Brake Jobs

With these vehicles, the major aggravation for shops is brake noise and uneven brake pad wear.

Other Posts

Brake Fluid 101: What Does Brake Fluid Do? What is the Difference Between DOT 3 vs. DOT 4?

When you think about the braking system of a vehicle, you probably think about the major parts you see when removing the wheel: brake pads, rotors, calipers and maybe hydraulics. However, one of the most important parts of the system isn’t visible: brake fluid. Brake fluid comes in several formulations for different applications and plays

BMW Wheel Bearings

Two technologies are being used that are allowing engineers to put better bearings in smaller packages.

Brake Pad Diagnostics

Worn brake pads can tell you a lot about the entire brake system and keep new components from suffering the same fate.

Understanding Passive Wheel Speed Sensor Operation

Passive types of wheel speed sensors are still used in many applications so understanding their operation is important.