Wed08202014

On The Road

Matt solves the mysterious case of the professor's brake shake

A car’s ability to brake effectively is obviously one of the most important components of safe driving.

Braking problems come in all shapes and forms – from intermittent squeaking to complete failure.

Last month one of my customers called me about a problem I’ve heard quite often. The customer explained that his brakes would shake (or pulsate) on the freeway but not on city streets.

The customer, a professor at a local university, drives on Interstate 280 twice a week and only notices the problem when driving at higher speeds. He mentioned that when he drives on surface roads around town, he never experiences the pulsating brakes, but after driving 5-10 minutes on the freeway, the car starts shaking when he brakes.

He asked me what to do, and I explained that unless he could avoid highway driving indefinitely, he better bring the car in to be checked out.

Types of brakes

To diagnose the problem, I needed first to determine the type of brakes installed. Brakes come in two standard models – disc and drum.

The most widely used is the disc brake system. The disc brake configuration uses a brake disc the size and shape of a dinner plate and brake pads the size and shape of a smartphone. The brake pads are depressed onto the brake disc by a brake caliper, approximately the size of a cantaloupe. The caliper’s job is to squeeze the brake pads.

The brake shoe and drum configuration is different. The brake shoes are roughly the size and shape of a banana, and the drum the size and shape of a round cake pan. The shoes sit inside the drum and are pushed out to the inner walls of the drum to slow down the car.

My customer’s car, a 2001 Subaru Outback, has front and rear power disc brakes. I took the Subaru on a short test-drive and did not notice any brake problems. I then drove the car onto Highway 85 to see if I could feel the pulsating at higher speeds. After passing three off ramps, the car started to shake. I felt the shake in the brake pedal and the steering wheel. I drove the car back to the shop and lifted it.

The first thing I noticed was that the right and left front brake rotors had a blue tint. The left front pads seemed to be equal to each other, but the right side was not. I then pulled off the wheels and calipers to check the pads up close, where I discovered that the right side was wearing very unevenly. The inner pad width was down to 1 mm and the outer pad was at approximately 2.5 mm, which suggests that the brake pads are getting jammed or stuck.

I also noticed that the caliper carriers were rusted. The caliper carriers are where the brake pads sit. Upon further investigation, I found that each pad was wearing unevenly (high on one side and low on the other). The pads were not floating and instead were getting stuck in the carrier and preventing the brake pads from properly addressing the rotor surface. This started to cause abnormal heat buildup in the brake pads and rotors. When the brake rotors overheat, they lose their shape and thus cause the pulsation.

The reason my customer was only noticing the shaking on the freeway is that city driving does not get the rotors hot enough to cause the distortion. The blue coloring I mentioned earlier was caused by the constant overheating of the brake rotors.

After diagnosing the car, all I had to do was change the front brake pads and rotors and the pulsating went away. My customer can now drive happily on the freeway without worrying his brakes could go out. Problem solved.

Matt Pataky is the longtime owner of Sunnyvale Foreign Car Service, which recently moved to 15 Pioneer Way in Mountain View. For more information, call 960-6988, email This email address is being protected from spambots. You need JavaScript enabled to view it. or visit sfcarservices.com.

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