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I am about to change the brake pads on my...

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Dear Tom and Ray:



I am about to change the brake pads on my car. I have heard that new brake pads need to be broken in, or "bedded." How does one do this? And if I don't do it, will I end up a smear on the road when I try to use the brakes next? -- Felix

RAY: Yes, it IS true that brake pads need to be broken in, Felix. But it's normally something your mechanic will do during his test drive, before he gives your car back.

TOM: The way the car stops is, the pads are squeezed together around the brake rotors. When new pads are installed, they are perfectly smooth. But if the rotors aren't replaced at the same time, the rotors are not perfectly smooth. They usually have grooves and ridges from the previous, say, 30,000 miles of stopping.

RAY: Because of those irregularities, the flat brake pads will not make 100 percent surface contact with the rotors at first. So they have to be used a few times, until the surfaces conform to each other. Then you get complete surface-to-surface contact, and you get all of the braking power you're supposed to have.

TOM: So if you change your own brake pads, you'll want to be very careful when breaking them in, because your stopping power will be diminished at first. We always do it for our customers, because we don't want to put them in any danger -- any more than they're already in from letting us work on their cars, that is.

RAY: Here's our procedure: We take the car out on a deserted road and drive at about 40 mph. Then every 100 yards or so, we apply the brakes and slow it down to 20 mph, then speed back up to 40 again. After doing that a handful of times, you can start to feel the pedal position and stopping power improving dramatically. And it never takes more than a few miles to completely break in new pads.

TOM: So, even if your mechanic doesn't do it, your pads will get broken in quickly from normal driving. You'll just have lousy brakes until they do.

RAY: So that's breaking-in brakes, Felix. By the way, you used the term "bedded" in your letter, which I've never heard. Sometimes we say that the pads have to "seat" to the rotors. Maybe bedding is what comes after seating? The pads sit down first, they get tired, they lie down, and then they get bedded. I don't know.
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