Brake pad bed-in, definitions and procedures
Regardless of the finish of brake discs braking surface, either fine-turned or grinded, brake pads must be perfectly seated to increase the area of contact between pads-discs and to ensure maximum efficiency while braking.
Therefore, after maintaining a vehicle where the discs and pads have been replaced, the bed-in process is essential for maximum performance of the brake system.
What is bed-in?
Simply stated, bed-in is the process of depositing an even layer of brake pad friction material, or transfer layer, on the braking surface of the disc. Technically speaking this is called the transfer layer or the third body layer.
Although bed-in is quite basic in definition, achieving this condition in practice can be quite a challenge, and the ramifications of improper or incomplete bed-in can be quite annoying and/or dangerous to the driver.
Abrasive friction and adherent friction
There are two basic types of brake pad friction mechanisms: abrasive friction and adherent friction. In general, all pads display a bit of each, with abrasive mechanisms dominating the lower temperature ranges while adherent mechanisms come more into play as pad temperature increases. Both mechanisms allow the conversion of motion (kinetic energy) into heat (thermal energy) which is the basic function of any braking system.
The abrasive mechanism generates friction or energy conversion by the mechanical rubbing of the brake pad material directly on the disc. This results in mechanical wear of both, the pad and the disc.
The adherent mechanism is altogether different. In an adherent system, a thin layer of brake pad friction material actually transfers and sticks (adheres) onto the disc braking surface. The layer of pad material, once evenly established on the disc, is what actually rubs on the brake pad. The bonds that are broken, for the conversion of Kinetic to Thermal energy, are formed instantaneously before being broken again. It is this brake pad-on-transferred brake pad friction material interaction on a molecular level that yields the conversion process (montion into heat), and brakes the vehicle.
With the adherent mechanism there is much reduced disc wear as compared to abrasive mechanism, but it's not a free lunch – pads now become the primary wear element in the braking system.
The all-important transfer layer
As stated above, the objective of the bed-in process is to deposit an even layer of brake pad friction material, or transfer layer, on the braking surface of the disc.
Note the emphasis on the word even, as uneven pad deposits on the disc braking surface are the number one, and almost exclusive cause of brake judder or vibration. It only takes a small amount of thickness variation, or DTV - Disc Thickness Variation, in the transfer layer (we're only talking a few ten thousandths of an inch here) to initiate brake vibration.
While the impact of an uneven transfer layer is almost imperceptible at first, as the pads start riding the high and low spots, more and more DTV will be naturally generated until the vibration is much more evident, and causes discomfort and insecurity while driving. With prolonged exposure, the high spots can become hot spots and can actually change the metallurgy of the rotor in those areas, creating “hard” spots in the rotor face that are virtually impossible to remove.
In general, bed-in consists of heating a brake system to its adherent temperature to allow the formation of a transfer layer. The brake system is then allowed to cool without coming to rest, resulting in an even transfer layer deposition around the rotor circumference. This procedure is typically repeated cyclically in order to ensure that the entire rotor face is evenly covered with brake pad material.
Because the adherent temperature range for brake pads varies widely (typically 100°F-600°F for street pads and 600°F-1400°F for race pads), each bed-in needs to be application-specific, and it’s recommended be careful in the first 300 km after the maintenance of the brake system.
Failure to respect the bed-in period or to generate a one-size-fits-all procedure will have consequences. Too little heat during bed-in may be insufficient for the adhering mechanism to start, and therefore transfer a uniform layer to the brake disc braking surface, while overheating the system system can generate uneven pad deposits causing hard points on the disc, and/or at least vibrations which compromise the driveability of the vehicle.
In summary, the key to a successful bed-in is to bring the pads up to their adherent operating temperature in a controlled manner and keep them there long enough to start the pad material transfer process. Different brake system designs, pad types, and driving conditions require different procedures to successfully accomplish the bed-in. Be cautious and use common sense when hitting the brakes in the first 300km after maintenance.