Brake Tech Series: Friction Without Noise

According to Bendix Brake Answer man Jay Buckley at Honeywell Friction Materials and Wally Marciniak, head of Technical Training at Affinia Group's Raybestos Brakes, the top two complaints arising from routine disc brake servicing are noise and incorrect pedal feel, especially pulsation. Also moving up quickly as a complaint is dust ruining the appearance of customers' wheels. The good news is that you're not powerless in fending off these complaints.

Note that some of these problems are self-inflicted – resulting from taking shortcuts or slipping into old habits, techniques that worked once but not with today's pads and rotors. Some conditions also can be controlled by informed choices of friction material and related parts.

NEW FRICTION

If you're an old hand at brake work, your experience – not your imagination – tells you that modern brakes are fussier than their counterparts a generation ago. For most vehicle classes, components are smaller and lighter. This resulted from the need to cut vehicle weight to meet fuel efficiency targets. It forced manufacturers to re-engineer parts that look similar to older pieces but behave differently.

Today's cars have powerful, smooth, clean, fade-resistant brakes because rotors didn't shrink in a vacuum. The pads they work with – though appearing similar to old pads – saw changes, too. Once, friction material choices were "cheap," "decent" and "darn good." Those choices still apply, but they're spread over three or four categories of friction formulation.

SEMIMETALLIC PADS helped make lighter rotors a reality. Friction material makers are understandably reluctant to share proprietary recipes, but semimetallic pads, by definition, contain a noticeable amount of iron and/or steel. It could be iron powder, ground-up steel wool or something else – manufacturer's choice.

Semimetallic pads provide good resistance against fade as the rotors heat up. For one thing, their metallic content lets them draw some heat away from rotors. In some applications, they tend to be noisy and/or they may create dark dust – some of which is powdered metal from the rotor itself. Several friction suppliers have lowered the metallic content in semimetallic pads to help make them quieter and easier on rotors and some now refer to "low-metallic" as a separate category.

NON-ASBESTOS ORGANIC (NAO) pads re place what used to be conventional pads, with mostly unnamed ingredients that take the place of asbestos. "Organic" is one of those words that doesn't quite fit, but it's become common in use. Standard, asbestos-bearing pads were called organic even though they weren't grown in pesticide-free gardens, either. Yes, some formulations may include, among other things, ground-up nutshells, and maybe "organic" fits in their case. But the name has stuck across-the-board for both the old-time pads and their asbestos-free replacements.

NAO pads are used where semimetallics' heroic fade-resistance isn't needed. This is not to say that semimetallics go in all performance and heavy-duty vehicles and everything else gets NAOs. Car and truck makers can vary specs, such as rotor mass and cooling-fin design, pad shape and size, and caliper piston diameter (and count) vs. master-cylinder bore size, and this has considerable impact on what type of pads may be used in a given application.

CERAMIC PADS are considered a subset of NAO. Clay dust is one ceramic component in some ceramic pads, but it's oversimplifying to say ceramics are just regular NAO pads with clay added.

Ceramic pads tend to be easy on rotors, create less dust and the dust from the pads themselves is often lighter-colored and less noticeable. As a group, ceramics tend to remove the least heat from rotors and are least suited for applications requiring high fade resistance.

FINDING THE RIGHT FIT

These descriptions might make you think that when a customer needs new brake pads you only have to find out about their driving habits and then choose: Is the customer a performance car owner with a lead foot or a pickup owner who loads his truck heavily for work? These might seem like (and could be) candidates for semimetallics. Is the customer an easy driver but fussy about appearance? Ceramics may seem like the way to go.

But you can get into trouble selecting pads based solely on customers' driving habits and tolerance for wheel dust. As mentioned, choices made by engineers designing braking systems affect what characteristics are needed in brake pads to deliver satisfactory performance.

Anyone can look up a vehicle's recommended pads, see what numerals are in the part number, then vary prefixes and suffixes to determine which pads of differing composition might physically fit. But it's not a good path to follow. A safer bet is to follow suppliers' and manufacturers' recommendations. Vehicle makers and aftermarket providers with a reputation to protect will only recommend pad types their engineers have determined work satisfactorily.

One controversial idea in the friction industry is "hybrid" pad combinations. Marciniak describes this as a pairing of a pad of one type with a different type of pad, at each wheel; the idea is to get a combination of the characteristics of each. Ceramics outboard teamed with semimetallics inboard are one hybrid example.

Marciniak says it's too early yet to know if these combinations will lead to complications, such as a problematic difference in wear between inner and outer pads. According to Buckley, no production vehicles today use these combinations.

If you do have a choice of approved pad types, you may be able to change pad formulation and better satisfy the customer. But you're going out on a limb if you don't use a type recommended by the vehicle manufacturer or a reputable aftermarket supplier. Check those recommendations often, though. They can sometimes change.