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#1
Old 01-10-2006, 04:32 PM
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Car shakes when braking from high speeds

When I am braking from more than say 50 mph and coming to a stop, my car vibrates really bad. The pads are practically new and the rotors are fine. What might be causing this?
#2
Old 01-10-2006, 04:48 PM
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The most common cause is warped brake rotors. "The rotors are fine" is vague. Has the runout (deviation from planar flatness) been measured with a dial indicator? The maximum runout specs are usually in the .002-.004" range, not something that the naked eye can detect.

Usually it's the front rotors that are warped, and often if they're warped there will be steering wheel shimmy. However, there can be front rotor warpage without said shimmy, and there can be rear rotor warpage, or with rear drum brakes, rear drums out of round.

It's less likely, but possible, that something is loose or badly worn in the suspension or steering linkage.
#3
Old 01-10-2006, 04:54 PM
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Quote:
Originally Posted by Gary T
The most common cause is warped brake rotors. "The rotors are fine" is vague. Has the runout (deviation from planar flatness) been measured with a dial indicator? The maximum runout specs are usually in the .002-.004" range, not something that the naked eye can detect.

Usually it's the front rotors that are warped, and often if they're warped there will be steering wheel shimmy. However, there can be front rotor warpage without said shimmy, and there can be rear rotor warpage, or with rear drum brakes, rear drums out of round.

It's less likely, but possible, that something is loose or badly worn in the suspension or steering linkage.
I did not use a dial indicator. I just examined them to see if they have any deep grooves...the front brakes are disc and the back brakes are drums.

If they are warped is the best solution to just replace them altogether?
#4
Old 01-10-2006, 04:57 PM
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If you are sure that the brake rotors are ok and you don't have bent wheels, then the next suspect item is the tie rods, which run between the steering rack and the steering knuckles. These do loosen up over time and should be replaced about every 100k miles depending on the vehicle. Not knowing what vehicle you have, I can't say what other things it might be. If you have a McPherson strut type front suspension then I would say the lower ball joints are probably bad too. If you have double A arm (or wishbone) suspension then either the lower or upper ball joints are bad. It would be a good idea to have a mechanic look at this. You could experience major failure of a steering component which would ruin your day and maybe your life.
#5
Old 01-10-2006, 05:03 PM
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Quote:
Originally Posted by StewieGriffin420
I did not use a dial indicator. I just examined them to see if they have any deep grooves...the front brakes are disc and the back brakes are drums.

If they are warped is the best solution to just replace them altogether?
Scoring is irrelevant to the symptom. Sometimes you can get a quick indication by spinning the wheel and listening for a regular mild scraping sound once per revolution.

Sometimes the rotors or drums can be resurfaced to solve the problem. Whether that's possible depends upon how distorted they are and how much metal is there to work with. However, if this is possible, it often leaves the rotors/drums at or near their minimum thickness, which makes them more susceptible to future warpage.

Occasionally rotor/drum warpage, or even warpage of the hub, can be caused by overtightening the lug nuts. When you mention nearly new pads, I wonder if this might have occurred when they were replaced.
#6
Old 01-10-2006, 05:08 PM
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I meant to add -- a quick test for out of round drums is to apply the parking brake at the speed indicated. If you get the vibration, that clearly indicates it's from the rear brakes. If you don't get the vibration, it's unlikely, though not impossible, it's from the rear.
#7
Old 01-10-2006, 05:28 PM
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Posts: 16,451
To add to Gary T's excellent answer, if the steering wheel shakes the problem is in the front. If the brake pedal pulsates it's in the rear. This pulsation is particulary noticable with drums that are out of round.
Another cause of brake vibration is thick/thin varration on a rotor. Say you measured a rotor at four points around it. The first measurement is 21.00mm. You would expect the other three to also be at that value. But sometimes you can get measurements like
21.00mm
21.05mm
21.10mm
21.05mm
21.00mm
#8
Old 01-10-2006, 06:03 PM
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Join Date: Dec 2005
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as others have posted, it sounds like the rotors.

If you are handy with tools, you can replace these yourself pretty easily. Brake jobs in general are pretty straight forward, and if you are like me and don't mind the occasional screw up, you can save money and learn something too!

I will say that if you don't know what you are doing, it's best to not mess with something like brake fluid. This can get messy if you do something stupid (like depress the brake pedal before the line is hooked up correctly. My friend hit the pedal. Honest!) However, if you take your time, you can change brake pads and rotors pretty damn easily without unhooking the brake line from the caliper. Drum brakes aren't as bad as people think, but you need the right tools. Taking those damn springs off with pliers instead of a spring removing tool can be frustrating. You can also bend the springs. Drum brakes are an engineering marvel the first time you look at them. After the initial thrill wears off, however, you'll never understand why anyone still puts them on a car (the answer, of course, is MONEY. They're cheaper).

I disagree with turning down a rotor. I've had it done before, and it's just a band-aid. If you have an American car, you can get a couple of new rotors fairly cheap. Remember, when you smooth the surface of a rotor or drum, what you are doing is removing metal. Therefore, it's closer to being unsafe and under its minimum safety thickness than before. So your steering wheel might not shake (which is good), but you are going to need new rotors anyway pretty soon.
#9
Old 01-10-2006, 06:16 PM
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Do you have anti-lock breaks? It might be a bad anti-lock brake sensor.
#10
Old 01-10-2006, 07:22 PM
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Quote:
Originally Posted by Khadaji
Do you have anti-lock breaks? It might be a bad anti-lock brake sensor.
That's what I was going to say. Sometimes anti-lock brakes can act like that.
#11
Old 01-11-2006, 01:42 PM
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My car does the same thing when I brake except it only shakes when I hit from 30mph to about 20mph. Any other speeds when stopping are fine. It's odd. Good to hear that I'm not crazy.
#12
Old 01-11-2006, 02:50 PM
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Join Date: Apr 2003
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My car has been doing something very similar for a while, shaking and making a "thump thump thump" noise when I brake from higher speeds. I'm going to have this checked out as soon as possible.
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