Why does my brakes pulsate

Brake pulsations in cars with ABS

Table of Contents Show

The Science of Stopping
Main Cause of Pulsing Brake Pedals
High Spots on the Rotor
How to Prevent Brake Pedal Pulsation
Step1: Make 5 Moderate to Aggressive Stops (from 40 mph to 10 mph)
Step 2: Make 5 Moderate Stops (from 35 mph to 5 mph)
Step 3: Drive for As Long as Possible Without Coming to a Stop
Have a Question? PowerStop Can Help

Two common faults can cause your brake pedal to pulsate when braking: dirty sensors and warped disc brake rotors.

Here is a simple way to detect a warped rotor. Go to a deserted open smooth area to perform this test. Accelerate the car up to 10-15 mph, and then let it coast in a straight line. Apply the brakes GENTLY until you feel them pulsate under your foot. If the pulsations are absolutely regular, then they are synchronous with wheel rotation…. which is 99% indicative of a warped rotor. This test does NOT tell you whether it is the fronts or rears at fault.

Disc brake rotor warp occurs when the disc fails to trace a absolutley flat planar path when it rotates. This deviation is often very small, on the order of thousandth’s of an inch, and is normally detected by removing the rotor from the axle, mounting it so that it can rotate freely in a shop, the rotating it by hand and measuring the run-out with a standard dial gauge. Warp occurs from rotor overheating from hard-braking. Aggressive drivers can cause this type brake rotor distortion in a few hundred miles…. others may not see it for many tens of thousands of miles, or ever.

Having a factory published repair manual is invaluable in tracking down and servicing ABS brakes. ABS sensors are cylindrical magnets surrounded by a coil, and mounted inside the disc brake support. They are subject to collect metallic debris from the pads, which will degrade the generated signals to the ABS computer, causing it to actuate ABS inappropiately, evident by the brake pedal pulsating. Most are secured with a single hex bolt. Removal and cleaning the sensors is a straight-forward operation, easily done at home.

Replacing brake rotors are another matter; although possible in a home garage, it may be best performed by a competent brake mechanic. Brakes are a crucial safety measure in a car, and there is no reason to put repair cost before driver safety. It is advisable to replace rotors in pairs: both fronts or both rears. Since the pads need removal anyway, good time to replace them as well, as well as checking the disc brake fluid seals of the pistons.

Infrequent causes of ABS generated brake pusations are factured sensor coils, fractured wiring between sensor and ABS computer, dirty/corroded connections to the ABS computer, intermittent power connections to the ABS computer (ie a fracture in its fuse), or internal component failure inside the computer. Tracking down these faults is time-consuming and can be quite expensive. Each manufacturer has a step-by-step process clearly outlined in their manual.

When you apply the brakes while driving and notice a wobbly pulsation coming from the brake pedal, you should determine the cause of the pulsating feeling.

Pulsating brakes can be felt in a shaking steering wheel, a pulsating brake pedal, or in the seat as it vibrates when braking. If you feel the brakes pulsate when coming to a stop, it’s most likely an issue with the brake rotors.

The Science of Stopping

It is hard to imagine the energy required to stop a vehicle traveling at 110 mph. Kinetic Energy is ½ mass times the velocity squared: KE = ½ mv2. Stopping a 2500 lb car requires 352,800 ft-lb of energy, the equivalent of dropping the car from an 11-story building.

When you stop, almost all of this energy is converted into heat through the rotor in a very short time. Brake rotors are designed to get hot, often in excess of 1400 degrees F.

Contrary to popular belief, the temperature does not cause the rotors to warp, and this is not the reason why you get pulsating brakes.

So what causes the brake pedal to pulse or feel like it’s vibrating?

Main Cause of Pulsing Brake Pedals

The answer to what is causing your pulsing brake pedal lies in understanding how friction works, and how high spots form on the rotor to cause the pulsating brakes. There are two primary mechanisms of friction at work during the stop: adherent friction and abrasive friction.

Adherent Friction: For adherent friction, a thin layer of material is continuously transferred between the brake pad and the rotor. The breaking of the chemical bonds creates a resistant force to stop the vehicle.
Abrasive Friction: Abrasive friction is the mechanical wearing of the rotor and friction surface, like sandpaper on wood.

High Spots on the Rotor

Brake pads use both adherent and abrasive friction, but at higher temperatures, adherent friction is what stops the car. The brake pad deposits material on the rotor, and as the pad gets hot, more material gets deposited. These deposits are caused by the resin (glue) in the pad.

All brake pads use a binding resin that holds the friction component materials together. This resin turns to liquid as the pads get hot, and the resin material along with some other material in the pad gets deposited on the rotor.

On overheated rotors, you can sometimes see the glazed resin material. It can look like someone brushed syrup on the rotor. The resin glazing forms a high spot on the rotor surface.

Another way to form a high spot on the rotor is if you clamp down on the brake after a hot stop.

When the brake pad gets hot, it wants to transfer friction material to the rotor and it does not care if the rotor is stationary or rotating. If you are at a standstill, the pad will imprint material on the rotor in the shape of the pad. The next time you brake, as the pad passes over the high spot, the temperature increases relative to the rest of the rotor. This causes more material to get deposited on this spot until it gets high enough to cause pulsating brakes.

It does not take a lot of material to cause this brake judder, only .0007 inches is enough to start to feel the pulsation.

PowerStop friction is engineered with lower resin content to maintain a uniform boundary layer of friction on the rotor.

Another way to prevent glazing or non-uniform friction deposits is to install drilled & slotted rotors. Powerstop’s drilled & slotted rotors help keep the pad cooler to prevent pad glazing.

How to Prevent Brake Pedal Pulsation

To prevent your brake pedal from pulsing, consider taking the following steps:

Step1: Make 5 Moderate to Aggressive Stops (from 40 mph to 10 mph)

Make five moderate to aggressive stops from 40 mph down to 10 mph in rapid succession. Do not come to a complete stop and do not let the brakes cool.

If you’re forced to stop, either shift into neutral or give room in front so you can allow the vehicle to roll slightly while waiting for the light. The rotors will be very hot and holding down the brake pedal will allow the pad to create an imprint on the rotor. This is where the judder can originate from.

Step 2: Make 5 Moderate Stops (from 35 mph to 5 mph)

Make five moderate stops from 35 mph to 5 mph in rapid succession without letting the brakes cool. You should expect to smell some resin as the brakes get hot.

Step 3: Drive for As Long as Possible Without Coming to a Stop

Once you have made a succession of moderate stops, drive around for as long as possible without excessively heating the brakes and without coming to a complete stop (Try for about 5 minutes at moderate speed). This is the cooling stage. It allows the heated resin in the brake pads to cool and cure.

After the brakes have cooled to standard operating temperature, you may use the brakes normally.

Have a Question? PowerStop Can Help

At PowerStop, we are committed to providing top-quality performance brakes for every vehicle on the road. If you have a question about any of our products, contact us today online or toll-free at (888) 863-4415. Our customer service team is available to answer your questions Monday through Friday from 8 a.m. to 5 p.m. (CST). Se habla español.

Few things are as dreadful as hearing a squeaking or grinding sound, feeling the steering wheel shake, or the brake pedal pulse while applying the brakes in your car. Sure, everyone needs to change their brakes every once and awhile, but it’s not always the end of your brake’s expected lifetime when these symptoms start to show their faces. Knowing the various causes of brake pulsation can help you avoid the potentially costly problem and save you money by getting the most out of your brake system.

First things first, if you have an anti-lock brake system (ABS) and you’re feeling pulsation when you have to brake quickly, that’s normal. ABS’s automatically pulsate the brakes for you while braking hard to prevent them from locking up and causing the vehicle to slide into whatever you’re trying to avoid.

Unfortunately, one of the most common causes of vibrating, pulsating or grinding brakes is also the reason they sometimes seem to persist even after being “fixed”. Improperly-installed brakes, rotors, calipers, and even wheels can cause the problem to persist or even get worse. All that hardware is designed to have extremely narrow margins for error when installed and having your brake pads or rotors offset by just a fraction of an inch can cause serious damage and potential brake failure in your vehicle. For that reason, it’s extremely important that you use proper techniques when reinstalling these parts and your wheels after you have service done. Be sure you properly measure the clearance on all the parts you replace, clean away any debris from new parts before they’re installed, and consider using sandpaper to add a nondirectional rough finish to rotors you install for better braking friction. When you’re finishing up, use an accurate torque wrench, and remember that the nuts of your wheel should be torqued in a star pattern and in two-step increments. Avoid using lubricants while you measure the torque as it can lead to inaccurate readings and potentially over-tightened nuts. During the installation steps, it’s important that you thoroughly clean and remove rust from any parts you’re reusing to avoid excess friction and grinding, and make sure you use all new hardware every time you do service on your vehicle.

Another common cause for vibrating or pulsating brakes is heat spots on your rotors. The parts in your brake system are designed to operate within certain temperature conditions and exposing your car to excess heat due to extreme temperatures or friction from hard braking can actually damage your brake pads and rotors. When your rotors overheat, material from your brake pads can melt and glaze onto your rotors, and the friction forms uneven spots on the braking surface, known as heat spots. In minor cases this can cause shaking, noise, and vibration, while more serious cases can lead to increased braking speeds or even brake failure.

Whether it’s caused by heat, driving habits, normal wear-and-tear, or manufacturing error, imperfections in your parts can also cause brake pulsation. Checking that your rotor surface is flat and isn’t warped when you install it can save a lot of time and hassle later. As stated before, making sure to clean parts and use new hardware in installation ensures secure fits and helps to avoid unnecessary damage and grinding parts later on. Finally, ensuring your parts all have the proper alignment and clearance can go a long way in maintaining the health of your vehicle.

There are plenty of variables that can cause shaking, wobbling, noisy, or pulsating brakes in your car. By ensuring you use high-quality clean parts, new hardware, and properly align and install your brakes, rotors, calipers, and wheels you can significantly reduce the chances of encountering brake issues later on. Not only does this keep you safe behind the wheel, but it saves you valuable time and money down the road. If you’re looking for discounted new parts or tested pre-owned OEM brake pads, rotors, calipers, hub bearings, brake cleaners or even someone to do the service for you, Holbrook Auto Parts has everything you need to get back On the Move!

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