- Brake FrictionWith the exception of antilock braking systems (ABS), brake technology hasn't changed much since we made the move away from asbestos pads and linings -- until recently, that is. In the last few years, a number of manufacturers have introduced new brake friction technology, or approaches to brake friction marketing.
Some involve measuring brake performance in accordance with new standards developed by an independent laboratory; some involve designing each brake pad stock keeping unit (SKU, or part number) differently, to suit the specific vehicle it will be used on; and some involve new integral-bonding technology that allows production fo a quieter brake pad for light trucks, vans, and sport utility vehicles (SUVs).
These changes are good for the brake business, good for the parts business, and good for the consumer. Virtually every major brake friction manufacturer has new products slated for the shelves. And that means that no matter what you do, you will probably have to learn some new brake data.
For our part, we'll leave the specific training in new skus and product attributes to the individual manufacturers. But to understand the new products, and the technology involved, you have to understand the basics of brake friction. And that's what we're going to cover here.
Brake Friction Technology
First, please note that making good friction products is technically more complicated than it might at first seem. Manufacturers do more than just throw a few ingredients into the brakealizer, turn the switch on, and watch a Charlie Chaplin-like parade of parts come rolling out a conveyor on the other end.
For instance, overall brake performance is not measured simply by how fast the vehicle can stop when the brake pedal is stomped on and held. While that's certainly a vital measurement, from the engineering standpoint, brake performance is measured against a much larger list of different (and sometimes conflicting) design criteria. Some primary criteria are:
- Friction coefficient (called mu by engineers, after the Greek letter used to symbolize it in equations) gives a general measure of the friction material's ability to stop the rotor from turning. Related terms are cold mu, which measures friction coefficient at low ambient temperatures, and hi speed mu, which measures friction coefficient when the brakes are applied at high vehicle (and rotor) speed.
- Fade resistance measures the ability of the friction material to continue to stop the rotor during repeated stops from speed.
- Pad wear rate denotes the rate at which the friction material is worn from the pad's surface by the stopping action.
- Rotor wear rate indicates the rate at which the friction material causes material to be worn from the surface of the rotor by the stopping action.
- Noise measures, in decibels, the sound generated by the friction material and related hardware during braking. It is often broken into components, like squeal and groan.
- Dust or debris generation characterizes how much, and, in some cases, what particle shapes and sizes are generated during normal braking operations. Less is better.
- Thermal conductivity measures the friction material's ability to conduct heat away from the surface in contact with the rotor.
While this list is lengthy, it still doesn't cover all of the nuances of brake friction performance. Pad surface roughness and consistency of material distribution throughout the pad (homogeneity) certainly affect the friction material's performance over the life of the pad, as do some other characteristics.
But knowing something about how the parameters we've defined affect brake friction will let you help your professional and your do-it-yourself (DIY) customers make the right friction purchasing decision.
What Materials In Friciton?
Not too long ago, asbestos was by far the most common material used to make brake friction. Then we learned what repeated exposure to asbestos dust can do to healthy lungs, and the entire country made a concerted effort to eliminate asbestos from most products with which humans have any contact. And that included brake friction.
While no brake friction manufacturer will tell you exactly what its proprietary friction fromulations are, current materials fall into three general categories. They are: nonasbestos organic (NAO), semimetallic, and low steel. Most current production uses one of these three families of friction.
The most common are the nonasbestos organic and the semimetallic, with the most popular choice at the original equipment (OE) level currently seeming to be the NAO.
CAUTION: In spite of the general attempt to eliminate asbestos from brake friction material, you can't assume that any friction material already on a vehicle is asbestos-free. If you look at a customer's vehicle, don't brush away brake dust with your bare hands, and don't blow it off with a compressed air hose or other means that would blow particles into the air where you might breathe them. Asbestos is nasty stuff.
Within any material family, performance compromises are typically necessary at the design stage. The wear characteristics and friction coefficient (mu) are often at odds. And producing a material with a very low pad wear rate can mean unacceptably high rotor wear. Consequently, determining the overall performance of brake friction material usually involves a series of laboratory and use tests.
Most manufacturers test their pads both on dynamometers, and on actual test fleet vehicles, so that they can characterize specific performance by pad part number.
One manufacturer has developed a line of application-specific friction products. Another has worked with an independent testing lab to develop a certification standard, called D3EA by the developing lab, to cerify its premium line is consistent with federal safety standards.
Make The Noise Stop
We Americans are a noise-sensitive bunch. We are not used to having our brakes make noise in new cars, and we don't like to hear them make noise after we've replaced the pads, either.
Because of our picky acoustical sense, brake manufactureres labor long and hard trying to find ways to dampen the vibrations that are natural in the application of disc pads to rotors.
You will see pads that have leading edge chamfers, trailing edge chamfers, and center grooves. And these modifications can indeed help in some cases.
But don't try this at home. The angle of the chamfer (often a composite angle) makes a real difference, as does the loctation. According to at least one reputable brake friction manufacturer, chamfering the pad at the wrong place, or at the wrong angle can actually increase its noise tendencies instead of diminishing them.
Another method of controlling noise is to put some kind of noise dampener -- a smooth metal shim, or a rubbery material -- on t he back of the pad backing plate to reduce the vibrations created when the caliper piston pushes hard on the backing plate as the brakes are applied, thus squeezing the friction material between the caliper and the spinning rotor.
One manufacturer has even figured out how to intergrally mold a dampening material into the base of the friction material and extrude it onto the backing plate. The formulation is engineered differently for specific applications, though so far this technology is only available on its light truck pads.
Interestingly, in Europe folks don't seem to be bothered by brake noise; in fact, they take it as a positive sgn that the brakes are working. We could all save some money if we adopted their attitude, but brake friction engineers would lose a major challenge. |
