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Hello All,
I have a '99 XC with just over a 100,000 miles. Several months ago, after driving in stop and go traffic, (allowing the rotors to heat up), when I applied the brakes, just before the car came to a complete stop, I would hear a "crunching" noise coming from the front brakes. My independent shop diagnosed the problem as warped rotors. It replaced the front rotors with (Brembos at my request to save $$) and installed Volvo pads. About a month or so later, the "crunching" noise returned. The rotors were replaced again (Brembos under warranty). Again, about a month or so later, the "crunching" noise again returned when applying the brakes. Again, the rotors were replaced, this time using Volvo OEM rotors (again under warranty). The noise went away for about 3 weeks, and again returned. The pads are fine, plenty of pad left on them. Does any one have any idea what is causing the rotors to prematurely warp? My independent shop hasn't seen this problem before. Any advice will be appreciated. Also, the noise only develops after you drive the car in stop and go traffic for several miles. Thanks.
Hugh
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Make sure the sheet metal dust covers for the brakes are not rubbing on the wheel. I had a similar symptom and bending the dust covers slightly cured it.
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My vote is with Smitty's. My only input to this conversation will be that if the rotors are warped, they will have high and low pressure inputs by the calipers as they hit the high spots in the rotors. This provides the classic pulsating feeling. I can't explain the noise you are hearing, but it is NOT a warped rotor. Look else where for rubbing. Do you see shiny spots where there shouldn't be? Then as Smitty stated, this might be normal. I’m a mechanical engineer by trade, and he makes the most sense so far.
Jerry
01 V70 T5a
104K miles
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Jerry,
I'm thankful for every one's reply, including yours. I agree, Smitty's reply seems to make the most sense, because I'm not experiencing the typical "pulsation" in the brake pedal as I apply the brakes. I'm going to share Smitty's response with my mechanic, not to be a "wise guy" but to offer an explanation so if another customer has these experience, a pausable answer will be available.
Hugh
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Like checksix, I'm a mechanical engineer by trade... My advice, however, is to avoid getting too deeply technical with your mechanic. A discussion involving coefficients of friction varying with temperature, or "stick-slip" phenomena, may earn you a "that's a lot of hooey" response, because everyone knows we engineer types never leave our desks and deal with real-world hardware and problems. (Sarcasm now switched OFF.)
Not demeaning mechanics by any stretch, but in my experience it is a rare one indeed who will listen to a technical explanation from someone who doesn't repair cars for a living.
--
In God We Trust. Everyone else, keep your hands where I can see them.
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Smitty,
You're right about how people who make a living doing something may react, when someone, who doesn't offers an explanation to a problem they can't solve.
I usually cough my offer of information I've obtained by saying self-deprecating remarks like "I'm no mechanic or expert, but I came across this information I want to share with you in case it is helpful." I find that approach usually protects egos. Sometimes you can get help from the most unexpected sources. I also couch my "suggestion" that perhaps this information might help another customer too. But again, your point is well taken.
Hugh
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Hi, are you sure the rotors are twisted? is braking from high speed pulsating, is low speed braking uneven as wheels are rotating? Low quality soft rotors do warp easily when braking from high speed and stopping without propper cooling but your noicee can have other sources as well; broken spring? or wheel fixing point.
br tapsa
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Hugh,
The number 1 reason for warped rotors is improper torque on the bolts.
They should be torqued to 89 foot lbs. on dry threads.
You may want to check the torque with a quality torque wrench to insure that all bolts are equal.
Bryan
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Bryan,
My independent garage has had my car for a week and could not duplicate the "crunching" noise that I hear just before the car comes to a stop. The garage checked the rotors, pads and calipers (not stuck) and found nothing wrong. The rotors have been replaced 3 times. Each time the "crunching" noise went away for about a month and then returned. The rotors were replaced under the belief they had become warped.
I have to add though, I never experienced any "pulsation" in the brake pedal when I heard the "crunching" noise. This noise is heard after driving the car for several miles in stop and go traffic. When you first drive the car, especially when its been parked overnight, you don't hear any noise from the brakes as I described. You hear this noise just before the car comes to a complete stop.
I've talk my independent garage that as soon as I hear the "crunching noise" I'll bring it to them immediately so they can hear and experience it too. It's just frustrating to have to deal with this problem for so many months.
Hugh
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"You hear this noise just before the car comes to a complete stop."
You also stated this only happens when you;ve been in stop-and-go traffic, which tells me it happens when the brakes are warmed up.
I think what you describe is annoying but perfectly harmless. When your brakes are heated up from use, at low rotor speeds and only light to moderate brake pedal pressure, the friction linings are doing a "stick/slip" action on the rotors. The pads, which are not a tight fit on their locating guides, move back and forth in their clearances (not left to right, but relative to the direction of rotor rotation). This results in vibration which you perceive as the noise you describe.
I had the exact same problem on my 850 - I'd get a "graunch" just before the car came to a complete stop. I tore into the brakes to find the problem but found nothing. Careful experimentation showed I could make the noise appear pretty much on demand except when the brakes were cold (first or second stop of the day) or hot from some hard use.
Absent any pulsation or vibration from rotor runout or deposits ("warping" may be a myth, according to some brake experts), the noise is *just* annoying and is not indicative of problems with the brakes.
If your rotors are fairly new (like under a couple thousand miles), you might try a set of premium/heavy duty pads up front. I think I used Axxis Metal Masters which minimized the problem, but more reliable was to do about 3 or 4 hard stops a week from 60 or 70 mph, to get the brakes hot. You may not want to put up with that kind of regimen. Basically, the source of the problem comes down to the coefficient of friction between the pads and the rotors. This will vary with the type of pad used, and of course with the temperature at the friction interface.
--
In God We Trust. Everyone else, keep your hands where I can see them.
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Smitty,
Thank you for your detailed response. The "noise" is still present, but I have Volvo pads (with plenty of pad still on them) , and new Volvo rotors that were installed a few hundred miles ago.
At this juncture, I was coming to conclusion that this was an annoying noise that I would have to learn to live with. With your explanation, it's more palatable. Thanks!
Hugh
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