We have all experienced it, you put your foot on the brakes and the pedal shakes badly, sometimes clear up into the steering wheel. It is one of the most common complaints consumers have about brakes and not many people understand what is happening and what they can do to prevent it.
The way disc brakes work is when you push on the brake pedal the caliper pistons push two brake pads in and onto either side of the rotor, which is a disc, kind of like a Necco wafer. This slows your car down and stops it, at the same time generating a lot of heat. When you hit the brakes if there are any raised spots on that disc or variation in the thickness you will get that brake chatter or pulsation that you feel in the pedal.
One of the number one causes of brake pulsation is rotor runout. What this means is that for some reason the rotor, which should remain steady, firmly mounted to the hub, is able to move side to side some in the caliper, think of it as wobbling. Rotor runout will allow the brake pads, which are abrasive, to rub on the disc each time the high spot passes, even when you do not have your foot on the brakes, essentially sanding a spot on that rotor.
That causes a variation in the thickness of the disc and thus a pulsation. Rotor runout is caused by a bad wheel bearing, a piece of debris between the rotor and the hub or over torquing of wheels. If you have ever watched guys running wheels on with impact guns at a tire shop, that's what causes runout. Any half decent mechanic will run the lug nuts on with an impact gun just until they are snug and then will hand torque the wheels with a torque wrench so that each lug nut is torqued the exact same amount.
Your driving style can also lead to a brake pulse. If you ride your brakes just one time down a steep incline you can heat them up enough to cause hot spots on the rotor which will lead to a pulse. When coming down a mountain, for example, try pulling the car into second gear which will slow the car down without you having to ride the brakes.
Another cause of a pulse is rust, especially on a vehicle that sits for long periods of time. A little bit of rust is normal, but if too much builds up you will start to feel it.
When the brake pads get so worn out that there is little to no pad left they will start to cut into the rotors making a grinding noise and causing a vibration. At that point you are way past due for a brake job and in danger of doing damage to the braking system or causing an accident.
So what can you do when you get a pulsation in the rotor? If the problem is caused by rotor runout you first need to address what is causing it if there is a wheel bearing problem or debris between the hub and rotor. Once that is fixed, in theory you can remove that rotor and have it machined, taking a small amount of material off the rotor and leaving a flat surface for the pad to hit.
Twenty years ago this was the norm. Today's vehicles, however, are made lighter so that they get better gas mileage, meaning that the materials car manufacturers are using to make rotors is lighter and the way they are designed is such to allow the vehicle to be safe and at the same time light enough to be fuel efficient. In addition to being lighter, due to government regulations, nickel had to be taken out of the rotors. Because of this change rotors now rust faster and to a greater to degree. Sometimes you will even see large chunks of a rotor peel off.
The upside to all these changes is that the cost of rotors has come way down, unfortunately it means the days of machining rotors is about over. From brand new they are just handling the job of dissipating the heat, there just isn't enough there anymore to shave some off and still have a usable disc. Factor in the cost that you can often get new rotors for less than $20 a piece and the cost of machining that rotor is no longer economical.
Fixing a brake pulsation problem starts when you have your brakes replaced. You need to ask questions of your technician like are they measuring runout to make sure that there isn't a problem that needs to be addressed? Do they clean the surface so there is no debris between the rotor and hub or are they just slapping pads and rotors on it without doing the job right. Do they remove the calipers and sandblast them and lubricate the slides? If you are taking the vehicle somewhere that is running a "brake special", remember that you almost always get what you pay for.
The next thing you can do to avoid a brake pulse is to watch how you drive. Don't run up onto a stop sign and then jam on the brakes, this kind of abusive driving will overheat your brakes, wear them out faster and cause a pulse. Avoid riding with one foot on the brake and be realistic about how much weight you are hauling in your vehicle.
Yes your minivan can carry seven passengers, but is it designed to carry seven line backers? Probably not. The braking systems on new cars are designed to stop the gross weight of the vehicle, not the vehicle, four passengers plus suitcases. So if it seems like your brakes wear out and rotors warp faster today then they used to, there's a reason for that..
For more articles on automotive maintenance see these articles on flushing your brake fluid, flushing your coolant, flushing your transmission fluid, diagnosing a vehicle vibration, and carbon buildup in your engine.