Brake rotors ─ Why it seems like you need new ones every brake service

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Question: Traditionally, when I needed brake service, I’d go to Hong Kong Auto Service and you would machine the brake rotors. Now, you usually say I need to replace the rotors. We have three cars in the family, so we do our share of brake jobs. Why do we always need new rotors? Why can’t we machine the old ones?

Answer: Those are great questions! It’s true that, until recently, machining or resurfacing the rotors was the norm. This was done to ensure a flat, even stopping surface for the new brake pads.

But now, replacing the rotors is the norm. This is because they are typically too thin to be machined by the time the brake pads wear out. Why is that?

1. Brake rotors start out being much thinner than ever before. In the push to increase fuel economy, every pound counts. New rotors can weigh four to five times less than they used to.

Sounds great, right? After all, that means we’re conserving raw materials. The challenge is that the thickness of a brake rotor enables it to dissipate the heat that’s created during braking. When a rotor is too thin, it overheats and warps, leading to an annoying vibration or pedal pulsation.

When rotors were thicker, they could take the wear from the brake pads, be machined and still have enough metal left to dissipate the heat.  Today, when they’re new, they are often intended to have one life cycle─to survive one set of brake pads─when they are put on the car.

2. Brake pads today are typically made from ceramic or semi-metallic materials. Semi-metallic pads provide strength and conduct heat away from rotors but also generate noise and are abrasive enough to increase rotor wear. Ceramic pads accommodate higher temperatures with less heat fade, shorter stopping distance, and generate less dust and wear on both the pads and rotors.  They also provide much quieter operation because the ceramic compound helps dampen noise.

3. Many rotors are made from recycled materials, which are lighter and better for the environment than the raw steel used previously. However, they also wear faster and are more prone to corrosion because the metals have already been degraded or deteriorated once before.

So the next time your rotors need to be replaced, we hope you’ll know that no one’s being wasteful, trying to speed the job along or looking to make more on the brake service. It’s simply because the rotors are too thin or too corroded to be of use to you.