If you are into working on your vehicle and getting involved in its routine maintenance then you’ve probably already been involved in bedding in your brake pads and rotors and understand the need to do it. If you don’t do the work yourself, then you may or may not have come across this procedure. Bedding in your brake pads and rotors is extremely important and you should make sure that the procedure is always done correctly.
The simple reason to ALWAYS bed-in your rotors and pads is that it is the only way to ensure you get the full performance benefits of your system. If you don’t, driving under normal conditions will result in an extremely uncomfortable braking experience and a severely limited braking performance.
You can expect that a system that had not been bed-in will have uneven, random deposits of brake pad material on the rotor. When the brake pedal is pushed, this uneven layer will cause (to varying levels) vibrations in the steering wheel, shaking of the vehicle, and pulsing in the pedals. People usually believe these symptoms are the result of defective or “warped” rotors but in almost all of the cases, it is caused by the uneven pad material deposits on the rotor… which is caused by not bedding in the pads and rotors correctly.
The image below shows how incorrectly bedding-in your pads and rotors can leave “pad imprints.”
The Background to the Why.
If you’re looking for a more in-depth explanation, let’s talk about the brake mechanism. Brakes work by converting kinetic energy to thermal energy through friction. (Read the first couple of paragraphs of THIS article). There are TWO types of friction when considering friction in brakes:
- Abrasive friction: This is when two surfaces are rubbed against each other and the friction breaks down both surfaces. Pads which rely mostly on Abrasion would tend to generate more brake dust particles, more heat, and more wear. Brake fade tends to be clearly noticeable as the friction mechanism creates a lot of heat.
- Adherent friction: involves laying down a very thin layer of pad material on the rotor surface. This thin layer of material bonds with the pad when the brakes are applied. As the pad moves over this surface, the bonds break and reform and it is this breaking down and reforming that creates Adherent friction. Pads which rely mostly on Adherent friction tend to last longer as the pads do not solely grind down the surface of the rotor. In addition, they’ll have less brake dust. The downside is that at high temperatures the adherent mechanism of friction breaks down.
These two types of friction work together to provide you with the friction needed to stop your vehicle. There is no pad out there that uses purely Abrasive friction or purely Adherent friction. They both work together (although some may rely on one mechanism more than the other) where Abrasive friction cleans the rotor surface and Adherent friction forms, breaks, and reforms pad material layers.
When pads and rotors have been correctly bed-in, there will be a thin, even layer of pad material on the rotor surface. This thin material lays the foundation of how your brake system will efficiently stop your vehicle. As the brakes are applied in a correctly bed-in system, Abrasive friction cleans off debris on the rotor and allows for a smooth surface on which Adherent friction can work. Adherent friction then breaks and reforms the bonds of the pad material and rotor surface, leaving a smooth layer of pad material to work with at the next rotation.
If the pads and rotors have not been bed-in correctly, the mechanism of Abrasive and Adherent friction will not work well and use of the brake system, especially at high temperatures, will result in random and uneven deposits of brake pad material on the rotor surface. As mentioned earlier, this creates vibration in the steering wheel, vibration of the vehicle and pulsing of the brake pedal.
The picture below shows black spots on the rotor… these black spots are the uneven pad deposits. Although they may look small, they do have a large impact on the performance of the rotor. The owner of vehicle these rotors were used on complained of extreme shaking of the vehicle when the brakes were used.
With heavier brake use the deposited material becomes hotter than the rotor. This creates heat spots at those deposits and when temperatures increase significantly, the heat can change the crystal structure of the metal under the deposits. The metal in that area then becomes very hard and brittle and as the brakes continue to be used, the depth of the hard and brittle metallic structure increases making braking of the vehicle more and more uncomfortable… and evidently dangerous as it will be more prone to cracking and failure.
So how do you prevent this? There is only one way: BED-IN YOUR BRAKES.