Bad Brake Vibration

17 years ago

I have a 1999 Mercury Mystique with discs up front, drums in the back. For about the last 500 miles or so, when you apply the brakes (not a panic stop but just to slow down), you get a very bad vibration, especially on the highway at high speeds (pretty scary). The whole car shakes and the pedal pulses. However, at low speeds, sometimes you get it, sometimes you don't. I know what the ABS feels like when it kicks in and this isn't it.

The car has 52K on it. I replaced the rotors at 41K, so they have less than 12K on them. I also pulled them to see if they were warped, but except for a little minor pitting here and there, they appear to be straight with a lot of material left on them. I also pulled the backs and could see no problems. Pads were replaced front and back with a premium brand when I replaced the rotors. They look fine with plenty of wear left on them. Also, I replaced springs, hardware, etc. in the rears at the same time as the pads.

The brake fluid is clean, brakes have been bled; no sponginess in the pedal. The car has a 2.5 V6 auto with air, ABS. No warning light from the ABS system has appeared. New tires last year (balanced and alignement), same size and specs as what was previously on it. I can't believe the rotors are warped after so few miles and that they spin fine with no high spots, but it sure feels like it. Any thoughts?

Comments (4)

jemdandy
17 years ago
Did you check the rotors for run-out and variation in thickness? The rotors could be warped. It takes only a few thousandths of either thickness variation or run-out to get brake pulsing. But, if your rotors are in good shape, suspect the fit of the pins with the calipers. If the pins and calipers have a sloppy fit, the caliper can cant out of true and create pulsation in the braking force. If the calipers are stuck on the pins, the calipers can not slide to adjust for wear differences between the inner and outer sets of pads and the rotors will be pushed out of shape causing problems. Typically, pads on the piston will wear faster than the ones on the caliper when the caliper is stuck on its pins. The piston pads tend to wear a bit more than the caliper pads becasue of friction between the caliper and pins.

Of course, the discussion above assumes that the source of your vibration is caused by the front disc brakes.
john_g
17 years ago
Rotor thickness variation is a condition that can be imagined as the two sides of the rotor not being parrallel to each other. This can be caused by a "crushing" of the rotor by the brakes. It can be caused by uneven wear. Or it simply could have developed over time from a combination of forces. This problem is normally felt in the brake pedal by it pushing back at you with a rythm consistent with the current wheel speed.

Rotor run-out is often caused by debris (rust) being caught between the new rotor and the old wheel hub, or by improperlly torquing the wheels, which can bend the hub, rotor and wheel. There is always a chance that excessive heat build up from heavy braking without sufficient time for the system components to cool is playing a role as well. This by the way is the most common cause for the report that we often see where a new car keeps having to get the rotors machined. It's not the cars fault, it is how the driver is using the brakes in heavy traffic!

We also have rotor delamination, or rusting if you will. This causes a combination of runout and thickness variation issues.

Typically rotor thickness variation of .002" is plenty to feel when driving. Otherwise you would have to use a dial indicator, and mark the high and low points on both sides of the rotor to see it.

Rotor runout's of .005 or less will normally give no real signs they are present as the calipers of today have sufficient movement built into the system to "follow" the rotor. Between .005, and about .009 you will often feel the pulsation, but quite often you will hear the brake pads "saw" on the caliper supports as they get pushed back and forth. This "saw" is the grunting sound you get on a light brake pedal application as the car moves slowly such as 15mph or less.

As I alluded to in the beginning of this response, a good technician does not even need to put a dial indicator on a car where the problem is rotor thickness variation. He/she will feel that right from the drivers seat. However, the technician DOES need to use a dial indicator, and measure wheel runout, then remove the wheel and (for most cars) put the lugnuts back on and measure rotor runout, then measure the front hub runout so that this problem is repaired the first time. Sadly this is one of the times where the good technician that does the job correctly needs to be paid to do so, yet the allure of the "free" brake inspection often prevents them from getting good at it because many are rewarded for being fast and getting the work in and out and as "cheap as possible" which in some occasions turns around and bites them for the effort.

For the record, don't do "free" brake inspections, but I fix them.
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mmichaelk
Original Author
17 years ago
Thanks for the good info, John and Jemdandy. For the record, the car isn't in a lot of stop and go type situations and when I drive, I use the brakes sparingly, preferring to let the car slow itself down ahead of the light and then coast up to it. Personally, I just think these brakes run hot -- always have, compared to my previous cars. Maybe they're just poorly designed.

Either of what you both suggested sounds like a probable cause, but is definitely beyond my realm of expertise. Just was originally having a hard time believing it could happen after such short mileage, but it sounds like it can. And did.

I'll take it up to my local mechanic whom I had good success with before and let him take it from here. If he doesn't come to the same conclusion as you two, I'll show him the forum with your responses. Don't want to offend him by waltzing in there and telling him what to do right off the bat. Thanks again for all the input.
noelmarchestra
13 years ago
Hey there! Good day and apologize for the unecessary disturbance. Got the same problem and badly need some help. Hope you don't mind it. I have an 01 f150 7700 and it had a brake problem. As I step on the brakes I get a pretty good vibration. rotors have been replaced. somebody told me it could be a sensor in the antilock system sound right? it feels like its mostly in the right front but it shakes pretty good and sounds and feels like a rotor but replacing them did not fix the problem? any ideas? thanks in advance.

Related topic: Auto repair