Car shakes when braking — The most common causes

Why Your Car Shakes When Braking (And How I Fix It Permanently)

Quick Summary

If your car shakes when braking, the problem is almost always due to rotor thickness variation, also known as disc thickness variation (DTV). Lateral runout, NOT warped rotors, is what causes DTV. In other words, the brake rotor isn’t sitting perfectly parallel with the wheel hub or steering knuckle, causing it to rotate in a wobble. All it takes is .003″ or more of lateral runout to cause this problem.  In almost every case I diagnose, the vibration comes from rusty wheel hubs, cheap rotors, improper lug nut torque, or sloppy installation practices.

Article

When a customer comes in and tells me their car shakes when braking, I already know I’m dealing with brake work that wasn’t done right. When your car vibrates when braking, it’s easy to assume the rotors are “warped.” But let me set the record straight once and for all: street-driven cars do NOT warp rotors. They cannot reach the temperatures required to soften cast iron. Your brake pads would vaporize long before the rotor ever warped.

So why does your brake pedal pulsate when braking and make the steering wheel shake? The answer is simple: disc thickness variation caused by lateral runout.

It’s Disc Thickness Variation — Not Warped Rotors

Disc thickness variation occurs when the rotor does not sit perfectly parallel to the wheel hub. Even a 0.003″ or more deviation can make your car vibrate when braking. As the rotor wobbles, one side of the rotor contacts one pad more than the other, either depositing pad material (with soft pads) or wearing metal unevenly (with semi-metallic pads). That thickness difference between sides is what makes the brake pedal pulsate when braking.

The Most Common Causes of Lateral Runout

• Sloppy brake installation is the most common — Not cleaning rust off the wheel hub is the #1 cause of lateral runout.

Corrosion on the wheel hub causes lateral runout

All you need is .003″ of corrosion buildup on the hub to prevent the rotor from sitting perfectly parallel with the hub.

• Not using a torque wrench to tighten lug nuts is the #2 cause of lateral runout— Uneven lug nut torque causes the rotor to cock toward the tightest lug nut, preventing it from sitting parallel with the hub.

• The wheel hub has lateral runout— It’s pretty rare, but you can distort the wheel hub by hitting a curb or a huge pothole. If you’ve removed all the rust, used a torque wrench, and installed new rotors, and your car still shakes when braking, it’s either lateral runout on the wheel hub or a worn wheel bearing. There aren’t any other possibilities.

Other causes of brake pulsation

1) Installing cheap brake rotors — Cheap rotors are thinner and lighter, and they lack the intricate factory proper cooling vane designs. For example, OE rotors often use curved vanes for directional cooling, whereas cheaper rotors use straight vanes. Poor cooling leads to heat-related pad deposits—another reason a car vibrates when braking.

Brake rotor comparison

I purchased a store-brand rotor and a rotor made by a name-brand manufacturer. Look at the images below. First, compare the thickness difference between a cheap rotor and a high-quality OE-grade rotor. Then compare the weight difference. Lastly, compare the cooling vanes; the economy rotor vanes and doesn’t match the factory design.

See the difference between economy brake rotors and OE quality brake rotors.

2) Not cleaning new rotors properly — Many rotors come from the factory with an anti-rust coating. Aerosol brake cleaner removes the coating, but it doesn’t remove the machining residue left over from manufacturing. If you don’t wash the rotors with hot, soapy water before installation, those metallic particles embed in your new pads, causing noise and uneven wear. Yes, it’s a hassle, but it’s the new best practice for rotor prep.

Here’s the result of three rounds of scrubbing with hot water and soap. The cloth on the left shows how much machining residue was removed by the first wipe. The middle and right-hand photos show the successive wipes.

3) Not Cleaning and Lubricating Caliper Hardware — Sticking caliper pins or rusty pad abutments cause dragging pads. Drag builds heat, heat causes pad deposits, and pad deposits cause the brake pedal to pulsate when braking.

Use synthetic brake grease—never anti-seize.

4) Not performing the proper brake pad bedding procedure — A brake pad bedding procedure ensures that the pads lay down even transfer film of friction material onto the face of the rotor. The bedding procedure varies by manufacturer, but commonly it involves these steps:

• 30 stops from 30 mph
• 30 seconds cooling between stops
• Avoid hard braking for one week

© 2012 Rick Muscoplat