What’s the best kind of brake pad?
There are 3 basic kinds of brake pads
Aside from racing pads, there are 3 kinds of brake pads for street use: non-asbestos organic (NAO), Semi-Metallic and Ceramic. Each is made from 10-20 different raw materials and in different percentages to match the requirements of each year, make and model vehicle. Even though brake pad manufacturers label their brake pads according to the type, no brake pad is completely organic, semi-metallic or ceramic. NAO and Ceramic pads for example may contain some metal (10%-30%) and semi-metallic pads may contain come ceramic material. In reality, the types should be referred to as mostly organic, semi-metallic or ceramic. In addition, there’s no single type of brake pad that’s best for all driving conditions, so you must match the brake pad to your vehicle and your particular driving condition. Every pad type has pros and cons.
There is SO much more to this topic. This post is just a primer on brake pads. For more information see these other in-depth posts.
Brake pad anatomy. See how brake pads are assembled
Brake pad quality levels. What’s the difference between economy and premium?
Brake pad backing plates—they’re even more important than the friction material
Brake pad backing plate types. Buying the right type can extend brake pad life.
Brake rotor quality—what’s the difference between economy and premium brake rotors?
Why brake pads fail. Yeah, there’s a reason you don’t get the expected life from brake pads.
First, Understand how braking friction works
The friction material on brake pads works by converting kinetic energy (motion) into heat using either an abrasive or adherent friction method. Although, even here, no brake pad formula is completely abrasive or completely adherent. Here’s how the two friction methods work.
Abrasive brake pad friction
Abrasive friction transforms motion into thermal energy by breaking the chemical bonds of the brake pad and the rotor. Abrasive friction material contains abrasive crystals, metallic fibers and metal powders, lubricants and a binding agent to form a brake pad that’s harder than the rotor. As the name implies, abrasive friction works by “abrading” the rotor. It’s really that simple.
Need a real world analogy? Chuck a round piece of hardwood into a lathe and hold a sanding block against it while it’s spinning. The abrasive particles in the sandpaper convert the spinning motion into heat. If you press hard enough, you can stop the lathe from turning. There, that’s abrasive friction. When you’re done, you’ll wind up with saw dust (the rotor) and abrasive particles that have broken off from the sandpaper (the brake pad). Which component wore faster? The wood. The same applies to abrasive friction in vehicle brakes; the rotor wears faster than the pad because it’s softer.
Abrasive brake pad friction Pros:
Maximum stopping power. Handles high heat loads with less fade. Used in heavy cars and trucks and full size SUVs that weigh more and are designed to carry heavy payloads. Also used in racing to provide maximum braking at high speeds.
Abrasive brake pad friction Cons:
Faster rotor wear. More noise. Metallic brake dust that rusts and deposits on your alloy wheels. More vibration and harshness. Brake pads cost more than adherent NAO style pads (described below).
Adherent brake pad friction
Adherent friction also transforms motion into thermal energy but does it by rubbing a sticky brake pad against a sticky “transfer layer” of the same material that’s been deposited on the rotor. The initial transfer layer is deposited onto the rotor during a “burnishing/bedding” process. The burnishing/bedding procedure varies depending on the brake pad formulation, but usually consists of a set number of complete stops from a recommended speed with a cooling interval between each stop cycle.
The burnishing/bedding not only deposits a layer of brake pad material onto the rotor face, but also smooths any roughness or irregularities on the pad and rotor contact areas while physically and thermally converting the pad and rotor surfaces. From that point on, every time you brake, you’re literally rubbing two sticky surfaces together.
Need a real world example? Get a kid’s sucker. Wet it and wipe the sugary liquid onto one hand. Let your hand and the sucker dry. Your hand is the now the rotor and the dried sugary coating is the transfer layer. Now rub the sucker back and forth against your hand until it warms up. As things heat up, it will get harder and harder to rub the sucker against your hand. That’s adherent friction. In vehicle braking, the heat from friction continually disintegrates the transfer layer from the rotor and the hot brake pads continually wipe on a new layer. The dust you see from adherent brake pads is mostly burned up friction material. In an adherent system, the brake pad wears faster than the rotor.
Adherent brake pad friction Pros
Less rotor wear and less noise. Adherent braking systems are used on most small-to-medium sized passenger cars, crossovers and some light trucks and small SUVs.
Adherent brake pad friction Cons
Faster brake pad wear and more brake dust. Most adherent brake pads can’t handle the braking requirements of heavy vehicles or vehicles that carry large payloads because the high heat causes brake fade. Also, adherent friction efficiency depends on a uniform thickness of the transfer layer over the entire face of the rotor. To achieve that uniform transfer layer, you need near-perfect parallelism between the rotor and the wheel hub. If the rotor has more than .0006” lateral run out, the rotor will develop disc thickness variation where the transfer layer is thicker on opposite sides of the rotor. Disc thickness variation causes brake pedal pulsation and “judder” that’s often misinterpreted as “rotor warp.” Even something as simple as uneven lug nut torque can cause lateral run out and disc thickness variation.
3 types of brake pads
All brake pads contain abrasives to create friction, fillers to stabilize the brake pad and reduce cost, reinforcing fibers to remove heat, provide structural support and prevent pad breakup (like rebar in concrete), lubricants to provide some degree of slide to prevent brake lockup and binders that hold the mixture together
Non-asbestos Organic (NAO) brake pads
NAO brake pads stop your vehicle using adherent friction. They’re the least expensive of all brake pad types. NAO brake pads usually contain some metallic strands (typically less than 20%) as well as organic fibers like fiberglass and aramid to maintain structural stability. The pad also contains fillers like potassium, fiber wool, silica, zirconia, alumina and silicon carbide, friction powder or friction dust, lubricant (usually graphite) and a binding agent (glue).
NAO pads are quiet and provide smooth braking with little vibration. They’re the softest and most compressible of the three types, so they wear faster than the rotor. They require added braking pressure as they heat up due to their compressibility and their tendency to “out gas” and fade at high temperatures. Depending on the formulation, they have excellent braking performance and wear at temperatures below 400°F, but poorer braking and more wear at higher temperatures and under heavy braking. Some NAO brake pads tend squeal during the first application on cold mornings. Fewer car makers are installing NAO pads at the factory. Most have switched to ceramic friction materials.
Pros: Least expensive of all three types. Quiet, except on initial braking in the morning. Easy on rotors.
Cons: Wears quickly. Create lots of brake dust. Requires more brake pedal effort as they heat up. Most fade of all three types with longest recovery (cooling) time between brake applications.
Semi-Metallic brake pads
Semi-metallic brake pads stop your vehicle using abrasive friction. They contain 30%-65% chopped steel, iron and brass metal fibers and flakes to create friction and dissipate heat. They also include fillers, which can include everything from rubber dust, cashew dust, mica, and vermiculite to calcium carbonate and potassium titanate, as well as lubricants and binding agents. Semi-metallic brake pads provide aggressive braking even at high heat and have excellent fade resistance. They provide poorer braking at temperatures below 212°F, but offer the best braking and excellent wear at temps above 400°F. Semi-metallic brake pads provide excellent stopping properties for heavy vehicles and heavy loads, but the pads make more noise, create more vibration, harshness and dust than the two other types. They also wear rotors faster than either of the other two types.
Pros: Most aggressive of all three types. Excellent high heat braking with less fade.
Cons: More costly than NAO. Most noise, vibration and harshness. Poor cold braking. Brake dust contains metallic particles that rust and discolor wheels. Most rotor wear.
Ceramic brake pads
Ceramic brake pads stop your vehicle using adherent friction. They’re made with porcelain bonded to metal flakes or filaments to provide excellent structural stability and heat dissipation when exposed to tremendous heat. The pads include ceramic fillers such as ceramic powder, potassium titanate and abrasives like zirconia or alumina, as well as lubricants and binding agents.
Pros: Provide better braking and last longer than NAO brake pads. Ceramic pads produce less noise and dust than NAO or semi-metallic.
Cons: Highest cost. Easy on rotors. However, since ceramic materials don’t dissipate heat as effectively as semi-metallic brake pads, they can overheat brake rotors on heavy brake applications or during extended use. Ceramic brake pads are most susceptible to brake pedal pulsation caused by disc thickness variation brought on by lateral run-out. Extreme care is required when installing ceramic brakes pads to insure the wheel bearings and wheel hub are within specification and clean, and lug nuts are torqued evenly with a torque wrench to avoid lateral run-out.
©, 2019 Rick MuscoplatPosted on by Rick Muscoplat
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