The perfect brake pad application to a rotor would happen between two parallel surfaces. The face of the friction material would apply the same amount of force across the entire surface, and the pad would not flex under the force applied by the piston(s). Also, the friction material would generate the same coefficient of friction across the entire surface.
In the real world, the rotor might not be parallel to the pad due to cornering forces and the flexing of the hub. The shape of the brake pad may not be optimal because of engineering, packaging and economic compromises. Also, the brake caliper, bracket and hardware might not be in the best condition after being on the vehicle for an extended period.
When the brake pad’s friction surface is not in harmony with the caliper and rotor, the result is usually noise. But, brake pad manufacturers have some tricks to prevent this problem or at least shift it outside of the range of human hearing.
Brake Pad Shape
Another factor that can influence brake noise is the shape of the friction material on the replacement brake pad. OEMs tune the shape of their brake pads to the entire system, which includes elements like overall length, width and shape of the friction on the backing plate.
Following the OEMs lead, some aftermarket manufacturers have tuned the shape of their pads to suit older vehicles and the characteristics of their friction material blend.
Some replacement brake pads will further optimize the shape of the friction material for noise reduction and to better utilize an improved friction material.
Brake Pad Chamfers
A chamfer is an angled cut on the friction puck’s leading and trailing edges. Chamfers can control how the edge of the pad interacts with the rotors and can help prevent noise. Chamfers do this by making sure the largest possible edge of the pad makes contact with the rotor.
They can also help to compensate for flex in the pad on some applications that use a long pad. Some manufacturers also claim it can improve the structure of the pad while helping it bed into the rotor more effectively.
There is a lot of engineering behind a chamfer. A chamfer can be application specific and even proprietary to the manufacturer. But, some pads do not need chamfer because the type of friction material used or the design of the overall brake system.
Some shops and technicians still grind chamfers into new brake pads. This was a practice started in the 1970s when the quality of brake pads was not what it is today. Some of the old pads had flashing attached to the edge that needed to be knocked off. Most modern brake pads without chamfers should not be ground to a rounded edge. It is a waste of time in most cases.
Brake Pad Slots
Slots or channels have two functions in a brake pad. First, they can shift the natural frequency of the brake pad by breaking up friction material into smaller components. Second, they allow for gasses from the pad to escape.
Brake Pad Backing Plates
The backing plate also plays a role in brake pad noise. The more consistent the brake torque, the less excitation and noise. If a backing plate is flexing under the mechanical forces of the caliper, the friction footprint on the rotor changes as the driver modulates hydraulic pressure. Brake pad manufacturers can either add thickness to the backing plate or change the attachment methods so that the holes for rivets or integral molding voids are minimized.
The next time you take a new brake pad out of the box, take some time to examine the pad. Chances are many engineers spent a lot of time making sure that pad set is quiet and effective when it is installed on the vehicle.