Why Do Brakes Pulse?

We all know the feeling: You’re exiting a freeway or braking a little harder than usual and you feel the telltale pulsation in your right foot. It’s easy to jump to the conclusion that those rotors you bought have already failed. But maybe not. A pulsation in the brake pedal can have many causes, and sometimes it requires some investigation to identify and eliminate the vibration.

Brake rotor

What’s Really Happening?

Brake rotors are manufactured to exact specifications for material quality, thickness, and finish. And think about it, they live in a very hostile environment. Ambient heat well in excess of 100 degrees in summer, and sometimes below zero in winter. Water, sand, road salt, debris are all present. In hard use brakes can get very, very hot. And they’re subject to repeated shocks every time you hit a pothole or an expansion joint. Despite this, a good braking system can last well in excess of 100K miles.

Rusty brakes

What Warps Rotors?

Despite rotors’ careful design and manufacturing, they can suffer damage from an array of reasons. Let’s look at some of the possibilities.

  • Pad materials transfer: Brake pads can shed friction materials onto rotors, causing hot spots, or bumps in the rotor. This usually happens when rotors and pads get really hot, such as after a panic stop from highway speeds or driving in mountainous terrain. The heat spots are higher than the rest of the rotor, causing a pulsation.
  • Faulty calipers: Most low to mid-priced cars have what are called “floating” calipers. They have a piston on one side and slide pins that let the caliper shift when you step on the pedal, so equal pressure is applied to both sides of the rotor. If the slide pins are jammed or sticky, or not sliding evenly, the pads will apply uneven pressure to the rotor, and eventually it will warp. Calipers can also fail to release fully, causing one pad to drag. That can wear a brake pad unevenly, cause hot spots, or overheat the rotor, also causing warping.
  • Lug nuts too tight: Although some shops use torque wrenches, it’s common for repair shops to simply run their air wrench on your lug nuts until they’re good and tight. Unfortunately, this can stress rotors, causing them to warp.
  • Installation errors: Little things can make a big difference here. Debris under the rotor, rust on the caliper causing the pads to not fit properly, dirty or dry caliper slide pins, cross threaded caliper bolts causing crooked caliper mounting…even if the issue is minor, over time it can cause a pulsating pedal.

These problems can be prevented by proper installation and break-in of new rotors and pads.

Brake Pads + Rotor set

Rotors and pads: What to look for

The quality of the rotors and pads you purchase can make a big difference in both the performance and longevity of your braking system. Some things to look for:

  • High carbon rotors: Manufacturers use a variety of alloys in rotor manufacturing, but one component that can affect long-term rotor performance is the amount of carbon in rotors. High carbon rotors can be less susceptible to hot spotting and warping.
  • Pick the right pad for your use: In simple terms, brake pads can be softer (quieter and better pedal feel), and harder (can make noise when cold, may require harder pedal application).
  • Organic or semi-metallic pads may have more initial “bite” on application but can be prone to more brake fade in hard use. Ceramic pads may require harder initial pedal application and sometimes can squeal slightly when cold, but are less subject to fade in hard use. They also emit less dust than organic or semi-metallic pads.

Consider how you drive the car when choosing pads. Is most of your driving at lower speeds? Do you do a lot of highway driving and braking from highway speeds? Do you live in a mountainous area? Do you tow with your vehicle? Keep these items in mind when choosing brake pads.

Finally, rotors with painted hubs, or rotors that are coated, will rust less on the non-friction area over time. This may not affect rotor life, but they’ll look better. Almost all quality rotors are painted or coated.

Steering/Suspension

It might not be the brakes

In many cases the cause of vibration you feel in the brake pedal or steering wheel may not be caused by a warped rotor. Worn steering or suspension components can cause vibration or pulsation. Possible causes are:

  • Broken or worn suspension bushings that are allowing struts or control arms to move when brakes are applied.
  • Worn inner or outer tie rods (tie rod ends) that allow steering movement.
  • Failed or worn dampers (struts) or mounts that are loose and allow suspension vibration.

You can save time and money resolving a vibration issue by inspecting steering and suspension components for wear or failure before replacing brakes.

Newly installed brakes

Diagnosis steps

Here are some simple steps to take when determining what’s causing pulsation:

  1. After a drive check wheels for uneven heat, indicating a dragging caliper.
  2. Check lug bolts to ensure they’re tightened to spec, not more.
  3. Raise the suspect wheel and spin it and listen for sound of rubbing, either steady or intermittent.
  4. Inspect suspension bushings, suspension mounts, and tie rod end boots for cracking.
  5. Inspect struts for leakage and looseness.
  6. Remove a wheel and inspect rotor for materials transfer.
  7. Look at brake pads for signs of uneven wear.

Final thoughts

Encountering brake pulsation is frustrating and potentially dangerous, if not addressed. Following some simple guidelines when selecting new brakes and using good practices when installing them can ensure good brake performance and long life. And if you do encounter brake pulsation, diagnosing the real cause before replacing components can save you time and money.

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