Many people know that their owner’s manual outlines engine break-in procedures that should be followed when their car is new, but not as many know that it also contains a brake pad break-in procedure. Just like when break-in procedure is repeated after a re-manufactured engine is installed, brake break-in procedures should be repeated whenever the brake pads are replaced. In this article I’ll explain why bedding in the pads is necessary, and how to do it.
Let’s start with the “why”. There are two reasons for bed-in after a brake job. One is the percentage of the brake pad that’s contacting the rotor. The other, at least for the type of cars that we repair here at Art’s, is that a “transfer layer” of pad material must be deposited on the brake rotor before the brakes become fully effective.
Before explaining pad contact and the transfer layer, I’m going to talk a little bit about how the brakes work for those who aren’t familiar. The brake rotor is a disc that rotates with the wheel. When the driver steps on the brake pedal, the caliper, which is fixed to a stationary part of the knuckle, squeezes two brake pads against the spinning rotor, causing the rotor and wheel to slow.
When we replace brake pads, we machine or replace the brake rotors. When a rotor is machined, it is mounted to lathe and material is cut off until the surfaces the brake pads press against are smooth and flat. When the rotor is mounted onto the lathe’s spindle, its powerful motor turns the rotor against sharp carbide cutting bits, one on each side of the rotor. The bits are slowly moved from the inside of the rotor to the outside, sort of like the reverse of a needle on a record player (for those of you old enough or hip enough to get the reference). When finished, the surface looks and feels smooth, but if you were to look at it under magnification, you’d see what might look like a plowed field, with rows of troughs and mounds where the bit cut through the metal. When the rotors are installed on the car and the pads are pressed against the rotors to slow the car, the pad’s friction material rides on the top of the peaks and doesn’t touch the troughs. This means that until the peaks and troughs are worn flat, only a portion of the pad is touching the rotor. The more contact between the rotor and pad, the more effective the brakes, so having partial contact is a bad thing. The longer the car is driven, the more the peaks will be worn and the better the contact will become. This is part of the break-in process, and doesn’t really require any special procedure; it will happen with normal driving.
Obviously, the smaller the troughs and peaks are after machining, the less time it will take to wear them down. This is one of the many reasons that one shop’s “brake job” is not the same as another shop’s “brake job”. We use either a Hunter Engineering bench lathe or on-car lathe (both very nice machines) and when we make the final pass we use the slowest possible feed rate. This means the bits move out less per rotation, leaving a smoother surface. In addition, we also use fine sand paper to remove any hanging swarf and knock off the tips of the peaks, giving the rotor a head start on break-in.
Now on to transfer layer. This is the part of bed-in that won’t necessarily happen satisfactorily on its own. What needs to happen is for a layer of pad material to transfer to the brake rotors. This happens when the brake pads get hot and “melts” onto the rotor. The brakes won’t be 100% effective until the rotors have been coated. Here’s the catch: the transfer layer needs to be uniform. If lumps of pad material develop, you may notice a pulsation when braking.
When we do a brake job, we start the bed in process for you. Procedures vary a little from mechanic to mechanic, but they all have the same goal: get the pads hot, then brake with uniform pedal pressure for as long as possible. Some mechanics like to slow from 50 MPH to 30 MPH. Others 40 MPH to 10 MPH. These controlled slow-downs will be repeated several times to build the layer. Regardless, the important aspects are even pedal pressure and avoiding coming to a complete stop when the pads are at temperature because that might leave a glop of pad material, making the rotor thicker in one spot and causing a pulsation.
After we deliver the car to you, you won’t need to worry about creating artificial braking events to bed in the pads, but smooth even braking for a period of 100 miles or so will help maintain and build the transfer layer. Just avoid jabbing at the brake pedal for short periods of very heavy braking. This is pretty much what you’ll find in your owner’s manual. The following is from a Honda owner’s manual: “Avoid hard braking. New brakes need to be broken-in by moderate use for the first 200 miles (300 km).” That pretty much sums up everything you know in two sentences, which I’ve placed at the end of 1,000 word article 🙂
If you’re like me and like to know more than you need to know about things, there are some great articles on StopTech’s website written from an automotive engineer’s perspective, that go deeper into how brake systems work. Here are a few: