Why Does My Car Shake When It Brakes?

Jack R. Nerad | May 20, 2020

You're pulling up to a stop sign and your car shakes when braking. As you apply the brakes your car begins to jiggle and shake like it's going over a long patch of corrugated iron. But it's not going over corrugated iron, cobblestones, or even a bunch of "Botts’ dots." No, the road surface is perfectly smooth but your car is giving you an unwanted full-body massage. Naturally, you ask yourself, "Why does my car shake when it brakes?"

why car shakes when braking

There are several explanations for car shaking when braking:

  • One of your tires could be badly "out of round”
  • A wheel could be unbalanced having lost its counterweight
  • Your car's front-end alignment could be way out of whack
  • Your car's steering might suffer from a loose control arm or damaged knuckle 
  • You might have a damaged axle shaft that's acting up
  • You might even have a case of loose lug nuts, giving the wheel a chance to wobble that becomes especially noticeable as you slow down

Most Likely Cause of Car Shakes When Braking

Clearly, there are several potential causes for the shakes-when-it-brakes syndrome, but according to Eric Charles, a Midwest-based certified mechanic we spoke to about the issue, far and away the most likely cause of car shake when you brake is a problem with the braking system itself.

"The other problems, like an unbalanced wheel or bad alignment, will typically cause vibration all the time, not just when braking," he told us. "So, if your car shakes only when it brakes, the first place to look is at the brakes themselves. Most often that's where you'll find the problem."

How Brakes Work

Even the least technically astute drivers understand that brakes slow a vehicle by impeding the ability of the wheels to turn. If you slow the rotation of the wheels, you slow the car. 

As you compare cars you'll find two types of brakes are fitted on virtually all current vehicles. The predominant type is disc brakes, while a few cars still use drum brakes, typically on the rear wheels.

In a disc brake, a metal disc called the rotor is coupled to the wheel. Each rotor is fitted with a clamping device called a caliper that enables the rotor-and-wheel combination to rotate freely until the driver pushes the brake pedal. That pressure actuates hydraulic fluid in the braking system to bring the brake pad within the caliper into contact with the disc, slowing the wheel. More pressure on the brake pedal creates more pressure in the hydraulic system, clamping the brake pad onto the rotor more tightly.

With drum brakes, a hollow metal drum is attached to the wheel and when hydraulic pressure is applied via the brake pedal and braking system it forces "shoes" within the drum to apply friction to its inside circumference, slowing the wheel. Again, more pressure applied to the brake pedal results in more pressure being applied by the brake shoes to the drum, slowing the wheel more quickly. 

Why a Car Shakes When Braking

In a vehicle with disc brakes, the most likely cause of shaking is a warped or otherwise damaged rotor. Warping can be a consequence of normal wear. The repeated application of the brake pad onto the rotor will wear away the rotor material in that contact area. Over time the heat caused by the friction of the pad on the rotor can cause the rotor to warp. Brake rotors can be resurfaced ("turned") to restore their trueness and lessen the possibility of warpage, but if too much rotor material is removed in the process the rotor will be prone to warp again.

Mechanic Charles told us another frequent cause of brake rotor issues is lack of use. In vehicles that sit undriven for periods of time, the area of the rotor under the brake pad has a tendency to corrode or even collect brake pad material. When the vehicle gets back on the road, the effect will be much like that of a vehicle with a warped brake rotor. A rotor with surface corrosion can be turned to restore its flatness, but, as we noted earlier, if too much material is machined away it will be very likely to warp again and thus be unusable. In this case, the brake rotor should be replaced.

In vehicles with drum brakes, out-of-round drums can cause pedal pulsation and vibration when the brakes are applied. A mechanic can measure the drums to see if they are out of specification and also check for scoring and other signs of damage. Like disc brake rotors, brake drums can be machined to restore roundness as long as the thickness of the drum remains within manufacturer guidelines. The alternative is replacing the drum.

A seasoned technician should be able to diagnose the source of your car's shaking during a test drive. Again, if shaking occurs only during braking, it is likely due to a damaged rotor or out-of-round drum. 

While the shaking is annoying, it doesn't imply that you need to consult a car finder and car loan calculator in a search for a new car. Brake issues like the ones we talk about can typically be repaired quickly and relatively inexpensively. We don't think you'll have to apply for a second mortgage.

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