Hot Stopping

Squealing, squeaking, howling, humming. Canadian drivers describe brake noise differently to their technicians, but however they describe it, it means a dissatisfied customer. Reducing brake noise and excessive heat to acceptable levels is always within the grasp of technicians, thanks mainly to the efforts of friction engineers who design materials that address the “trade-offs” that define the segment: Performance, Heat, Noise and Cost.

Heat is the enemy of street-driven automotive braking systems, and dissipating or tolerating its effects are a major part of friction engineering. According to Walter Britland, director of aftermarket engineering for Federal Mogul Friction Products: “You can get compounds that are better for heat, in terms of thermal resistance, and conversely you can get compounds that don’t resist heat as well but have other attributes. There are plusses and minuses depending on the way you go. When we look at the different compounds and the effect of heat on them, the first thing we look at is stopping performance.”

“Stopping performance” is a little more complex than might be imagined, especially with the current crop of heavy, powerful street vehicles like the Lincoln Navigator or BMW 7-Series. Heat is a consequence of deceleration, which is what friction is all about, says Britland: “We don’t necessarily look at heat at the outset, we start with how the product performs in the service for which the vehicle is intended, and we work from there. In the hierarchy, stopping performance is paramount. Then we juggle between noise and wear life. Heat can be an issue on the performance side; when pads get too hot they may lose their effectiveness, depending on the pad. The second enemy of heat is wear life. As pads get hot they tend to wear faster.”

Stopping is about clamping friction onto a rotating element, and at the front, where the majority of braking force is distributed, rotors can have a decisive influence on friction performance. Rotors turned to minimum thickness have a reduced capacity to act as a “heat sink” driving temperatures at the pad much higher during repeated heavy braking. Surface finish is also important because of its effect on the “interface” between pad and the opposing rotor surface. Optimum stopping performance occurs when the area of contact between pad and rotor is 100 percent of the available friction area, but in the real world, roughness on the surface of a freshly machined rotor will not allow perfect contact. Federal Mogul’s Britland relates: “Too rough a surface will give poor effectiveness; it will reduce the contact area. In terms of friction material, you’re looking at flatness, and the (ability) to conform to the opposing surface. You want the opposing surface to be as smooth as possible. If you look at a rotor under 1000X magnification, it looks like the Rocky Mountains. You want to knock down the peaks of those mountains to get more contact area.”

Install pads on a rotor with an excessively rough surface, and the result can range from poor effectiveness to judder due to friction material transfer, or at best an excessively long break-in period. Imperfect rotor machining is enough of an issue that quality friction manufacturers often engineer compounds with additives or coatings designed to burnish rotors during the break-in period.

Noise is another consequence of squeezing a spinning disc with a friction material, and is another area where high-tech materials need competent installation practices for best performance. The root cause of brake noise is similar to a phenomenon familiar to everyone, says Wally Marciniak, director of technical services, Dana Brake and Chassis (Raybestos): “Remember when you were in school, and the teacher would drag a new piece of chalk across the blackboard? The teacher would either radius it, or change its position to eliminate the noise. And that’s what brake noise is, a high pitched vibration between the brake rotor and the surface of the friction material.” Dealing with that vibration is as much about moving the vibrations as dampening them. “Many different things control those harmonics such as heat, speed, pressure, and materials”, declares Marciniak, who adds, “We try to change the pitch. We move the harmonics.”

Peter Murnen, group manager, Federal Mogul Friction Products, agrees: “When you get into noise, and the fixes for noise, it becomes very application specific. It’s a lot of art in addition to heavy science. With each new platform it’s a different challenge. We’re working with different windows of heat, speed, working with different formulations, shims, slots and chamfers to try to negate some of the audible vibration.” Murnen notes that as important as the quality of the pad material is, the installer still needs to “sweat the details”: “The friction material and insulators only go so far. If the technician is not doing the rest of the brake job properly, you still stand to get noise. Friction is not a cure all. Rotor surfaces, condition of the caliper and caliper slides, proper lubrication, condition of the hardware, all go into the noise factor.”

Walter Marciniak states it simply: “It’s important that the pad is secured, installed and lubricated properly”, adding that “the dampening factor of the rotor is a factor. It’s not so much the mass of the rotor as the materials. If there’s not enough nickel or iron, the rotor may look good, but have a poor dampening effect. The quality of the rotor makes a big difference. The surface finish, when a technician refinishes a rotor, is also very important. If the surface is very rough, the pad bounces, generating more vibrations, which equates to noise.”

Surprisingly, noise isn’t necessarily a bad thing, depending on the preferences of the car owner. Colin Philip, manager of technical services for Bendix, relates: “The materials that we use in North America are different than the materials used in Europe. A material sold for a Mercedes Benz here in Canada, for example, must be quiet; it can’t make any grinding or squealing noise at all. To achieve that, you’re looking at a softer material. You can probably bring that vehicle to a stop under a severe deceleration twice or three times. Consider that same vehicle produced for the European market and you can probably bring it down (from the same speed) ten times. Noise is not an issue over there. If they don’t here a scraping or squealing they think someone is pumping WD-40 on their rotors. In Europe they want to feel their brakes. In North America, they don’t want to hear a thing.”

Softer materials are one method of dealing with noise, but performance and durability issues prevent their universal use, especially where vehicle manufacturers install harder compounds on the assembly line. OEM’s further complicate the issue with brake system designs which may not give the longevity or performance needed by owners operating at the extremes of the vehicle’s performance envelope. An example noted by Colin Philip is a manufacturer who switched rear brake systems on a pickup truck model to a non-servo system, based on consumer demographics that suggest that few drivers load their vehicles to maximum GVW. For those that do, however, premature front brake wear may emerge.

Ultimately, technicians will recommend and install the friction material that best suits their customers’ needs, and to make an informed choice, they need to be aware of both the properties of the material they’re installing, and the driving habits of the end user. A general rule is to install quality products, regardless of the composition. According to Philip, “The understanding out there is that at higher temperatures you’re going to see fade, more dusting of wheels and wear, but with higher quality material you can have high temperatures and good wear characteristics without destroying rotors.” Dana’s Wally Marciniak agrees: “‘White box’ may physically look the same, but performance-wise it won’t be. If you take, for example, a Ford Excursion and they say “semi-mets” and you put an organic pad on, in severe driving you could cause a br
aking condition where you can’t stop the vehicle. You do get what you pay for.” SSGM