Above: Stretch limousine brake systems require drums and rotors the size of those drums used on trucks, yet with perfectly smooth hard metal finishes to eliminate unwanted noise. Photo: Kirk Weaver

When building luxury limos, silence is golden

March 2013 - You expect a certain experience when riding in a luxury limousine. A smooth ride, fun amenities and a worry-free trip to your destination—these are things you would take for granted. Squealing brakes at every stop sign and red light would be an unwelcome disruption.

A few years ago, Miller Industrial Products, Jackson, Mich., received an order to make brake drums for Cadillac stretch limousines—a job requiring attention to quality and performance. The heavyweights of passenger cars, these big limos required the support of hefty axles.

“These stretch limos were not only much larger than standard models in overall size, but also featured bulletproof windows and doors, plus a heavy-duty undercarriage,” says Bill Miller, owner of Miller Industrial Products. “Therefore the brake system required the size of brake drums used on trucks. But at the same time, since these were ultra-luxury vehicles, the brakes had to be quiet—and that was a new challenge.”

Asbestos-lined shoes previously used on drum-brake systems were inherently much quieter than new ones. Since asbestos has been outlawed, new brakes—whether drum or disk styles—tend to be noisy if the hard metal finishes are not perfectly smooth.

“[A smooth finish] is more difficult to accomplish than many people thought at the time,” Miller says. “Any irregularities in the surface of the drums would cause a squeal, hum or slapping noise that was unacceptable to the OEM, which was very understandable.”

Miller says a unique honing process was needed to give the brake drums an ultra-smooth finish. To accomplish this, he turned to Brush Research Manufacturing Co., Los Angeles, a manufacturer of power brushes, hones and other finishing tools. To achieve the needed smoothness, Brush Research manufactures a ball-style honing tool, the Flex-Hone. 

“Brush Research has a history of solving finishing problems,” Miller says. “So we looked at this honing tool and developed a special process to machine the brake drums and then hone them in such a way that they were completely silent. It worked so well that the engineers at Cadillac were amazed.”

Multifaceted tooling

The Flex-Hone is a highly versatile tool used throughout the industry where metal machining is involved. This ball-style hone is recognizable by the abrasive globules, sometimes called dingle berries, mounted to nylon filaments. The tool is used for deburring, plateau honing and deglazing. The precision-quality, ball-style hone not only deburrs bored metal but also easily removes microscopic metal shards and fragments.

Brush Research makes a special line of ball-type hones for use on brake system rotors to produce a finish free from harmonic vibrations. The same tool also can be used on flywheels to remove torn, cut and folded metal fragments. 

Miller says the ball-style hone has led to increased business since the Cadillac limo order.  “We’re in a very competitive industry and it’s always 9-1-1 on turnaround,” he says. “A lot of customers come to us because they hear we’ve got the ability to turn a project around quickly. So we’ve used the Brush Research tool and even developed new models with their engineers to modify and streamline processes that have really impressed our customers and have led to new ones.”

When Miller needed new rotors, he worked closely with Brush Research during the design and development process. He was able to build some machinery that expedited production by finishing both braking surfaces simultaneously. The collaborative relationship continued with other projects. “For this and various other projects, I kept letting [Brush Research] know what our needs were,” Miller says. “So we put our heads together and came up with custom solutions.”

Miller worked with Brush Research to produce a much larger hub and shorter nylon filaments on the Flex-Hone than were previously available. “The filaments still had a lot of flex, but were even stronger than the longer version. And we incorporated larger abrasive globules,” Miller says. “These design changes allowed us to apply much more power and pressure to the tool, which sped up production while still maintaining a perfectly smooth finish.” 

Avoiding shortcomings

Slight flaws can occur during the machining process and need finishing with special techniques to avoid noisy rotors. “When you turn a metal piece on a lathe, you always create a lead-in groove in the metal,” Miller says. “That is because you’re cutting it with a single-point tool. As the parts revolve and the tool feeds the metal in, it leaves a slight groove, or score, in the product you’re working on. Many brake specialists have found that this groove creates noise unless it is polished off by honing. You can try slowing the feed down and accelerating the rpm, [which might] minimize the tool lead. But, unless you eliminate the groove and create a surface that is very smooth and nondirectional, you are still likely to get noise, and also brake wear life may be lessened.”

The lightweight and portable Flex-Hone is adaptable to many processes and setups. The tool can be placed easily into a variable-speed electric drill motor or low-speed air drill. In the case of Miller Industrial Products, this flexibility has led to considerable added work contracts. “We’ve also showed our use of the hone to other companies and even built machines for them to use for their own projects,” he says. FFJ

Keeping Texas Quiet, Too

All Star Muffler & Brake, a full-service repair shop in Conroe, Texas, uses Brush Research Manufacturing’s Flex-Hone for brake rotors. “We use it to put a nondirectional surface finish on all the disk brake rotors we service,” says manager Jason Deming. “This finish prevents squeals and other noise in a brake system.”

If not for the ball-style hone, Deming says his shop would have to use a less efficient type of tool, such as sanding disks. “The Flex-Hone is easier to use, and puts a better finish on the rotor,” he says. “That finish also makes it last longer.”