Why Does Your Car Shake When Braking

Some Things That Can Cause a Rough StopModern automobiles benefit from more than a century of engineering refinement, and some components like engine control computers are complex products of the current age. The brake system you depend on to bring your car to a safe stop dozens of times a day, however, shares many features with the brakes on cars built decades ago.

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Like the classics, your car might sometime not stop as smoothly as it did when it was new. If your car shakes when braking, here are some things you might look for to identify and repair the problem (or have it fixed by a reliable mechanic).

My Brakes Were Fine Last Week – Why Do They Shake When I Step on the Pedal Now?

Generally, brakes that usually work but begin vibrating or rattling when you hit the pedal are trying to tell you something is wrong – or more specifically, something is worn, warped, loose or broken.

To get an idea how problems like this can translate into that annoying vibration every time you try to slow down or stop, let’s consider what happens when you step on the brake pedal.

The job of the brakes is to turn the kinetic energy, that momentum of your car’s motion, into heat energy and dissipate it into the air surrounding your wheels. Until the brakes have converted all that rolling energy into heat by rubbing stationary brake pads or shoes against moving parts attached to your car’s wheels, your car will keep right on rolling.

This concept of motion turned into heat is key not only to how automobile brakes work (now, and, in one form or another, since they were invented), but also why they might start shaking this afternoon when they were fine just a few days ago.

When you step on the brake pedal, your car’s wearable friction surfaces, sometimes called “brake linings,” are pressed against either brake drums or rotors, sometimes also referred as the “discs” that give “disc brakes” their name.

The force with which you hit that pedal is multiplied using hydraulics, and usually boosted with vacuum pressure from your engine (the so-called “power brakes). The harder you press, the more firmly the brake shoes or pads press against the spinning rotors or drums, and the more rapidly the heat builds up in those rotating parts until the energy conversion process is complete.

Here is an example of what brake drums, rotors, shoes and pads look like on a typical passenger car (heavier vehicles have larger components to more effectively dissipate the heat resulting from braking).

How Worn Parts Translate into Warping, and Why the Car Shakes When Braking?

When your brake rotors and drums are new, they are constructed heavily enough to absorb and dissipate the heat generated by slowing and stopping your car. Like any mechanical components rubbing together with great force, these rotating parts wear out, slowly losing metal mass until they can no longer withstand the high temperatures involved without beginning to fail.

Just as a piece of wire heated in a camp fire may eventually soften and deform, the rotating components of your brake system can suffer damage from overheating – and the deformation often takes the form of a warped brake drum or rotor.

After the warping occurs, applying the brakes will often cause a rhythmically uneven friction level between the rotating parts and the linings – and so, you could slow and stop smoothly a few days ago, but now the car shakes when braking.

What Might Be Loose – And Causing Brake Vibration or Shaking?

Your car is made up of thousands of parts, and after many years and miles, other brake components might wear enough to become loose. One item in particular that can suffer from increased tolerance due to wear is a disc brake component known as a caliper.

The caliper is a sort of clamp that grabs the brake disc or rotor from both sides when you step on the brake. The caliper applies pressure to brake pads, trapping the spinning disc between them and generating the heat necessary to slow and stop the attached wheel, and, in turn, the car.

This is what a typical brake caliper looks like. See the two mounting bolts with smooth shafts? They are designed to allow the caliper to slide in and out of the stationary structure that holds the brake to the car. This ensures that the brake pads are applied with equal force on both sides of the rotor, preventing one pad from wearing out faster than the other, and ensuring maximum, even pressure is applied to the rotor when you hit the brakes.

Over the course of many years of braking, these sliding pins can wear until they may fit loosely, and rattle in the two holes that hold them; this is sometimes one more condition that can cause a car to shake when braking.

Another simple – but critical - item to check for a loose fit if your car exhibits shaking when you brake is your lug nuts – if you have a loose wheel, it is likely to vibrate or even wobble when the brakes are applied.

Is It Important That I Check Out Any Shaking When I Brake?

Absolutely! If your car suddenly begins to shake noticeably when braking, gradual wear may not be the problem; a more immediate failure may have occurred. There are a number of other components that may have broken or become misaligned, including steering and suspension systems.

Anything that might impact the effectiveness of your vehicles brakes is an obvious threat to safety, and needs to be assessed and repaired as soon as possible.

What Will Be Required to Stop the Shaking When I Brake?

Most brake shaking can be remedied by simply replacing the worn parts related to the problem. Installation of new front brake pads and rotors may completely solve the shaking on many vehicles, and can usually be easily accomplished by your mechanic – or by you, if you have the tools and mechanical ability

Obviously, if you are unfamiliar with brake repair, a knowledgeable and qualified person should either make the repairs or guide you through the process.

Even if you plan to have a qualified technician make your brake repairs, you may be curious about what the job involves. There are many videos on line that show the steps in brake component replacement; here is one example.

The Bottom Line

Your brakes need to work correctly for you to be safe. If you detect any abnormal sounds, feeling or effectiveness in your brakes, it is not worth your life to put off checking into the problem – you need to check it, or have it checked.

For more information about solving brake shaking, check out Brake Works and Popular Mechanics.

About the Author Chris Lawrence

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