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Hello All,

I recently purchased a 2002 Saab 9-5 Arc. When I go to stop, whether at a high speed or a low one, there is significant vibration felt throughout the car. Additionally, the steering wheel seems to amplify the vibrations and this can be seen as the steering wheel makes small jittering movements to the left and right while braking.

Has anyone else experienced this? I would initially say that the brakes need replacing, but they seem to be at an acceptable level.

Thanks!
 

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Warped rotors.
According to the late and legendary Carol Shelby; what most people think of as 'warped' rotors is in reality 'rotor thickness variation', i.e. rotors don't 'warp', they develop a minute change in how thick the rotor is in different sections of the rotor. Google will easily find his thoughts on this matter, but I'll try and summarise:

This is caused by extremely thin depositions of pad material on the rotor faces, so thin that it can't be seen. This is a normal thing, i.e. rotors always have a microscopically thin layer of pad material on their surfaces that is continuously being deposited back and forth between the pad face and the rotor face.

The problem arises when this layer becomes uneven, and as the 'thicker' section of the rotor passes through the pads the braking force momentarily increases, then decreases as the thinner section of rotor passes through. This creates a 'hot spot' on the rotor, which then tends to pick up more pad material at that spot, and the problem gradually becomes worse and worse (in bad cases the 'hot spot' can be seen as an area of dark discolouration on the rotor faces).

The pedal may also start to noticeably pulsate as the problem becomes worse and worse. This is because as the thicker section of rotor passes through the pads it will force the piston(s) into the caliper body, and then allow it to move out again as the thinner section passes through, causing the volume of fluid in the caliper to cyclically change.

If you think about it, if a rotor were hypothetically to be only warped but still retained uniformity of thickness, all this would cause is for the caliper to compensate by allowing the pins to slide, or, with multiple opposed pistons, for one piston to move in while it's opposite equally moved out. This would create very little if any noticeable force variation at the pad / rotor interface, and no pedal pulsation because the internal volume of the fluid inside the caliper remains unchanged. I've found the proof of this in karting, i.e. the rotor can be misaligned quite noticeably but with no apparent affect on braking performance or feel.

At any rate, if you have this problem, you need to machine or replace the rotors. However, vibrations can be caused by worn suspension components, so don't assume it must be the brakes, it possibly might not be.

A tell tale for brakes will be if you also get a noticeable change in actual braking force as the rotors turn, most noticeable at low speeds. The car may be felt to cyclically 'lurch' a bit at low speeds with brakes applied, the affect sometimes being stronger and sometimes weaker depending on the orientation of the 'thick spots' relative to each other at the different rotors (if more than one brake is affected). If more than one brake is affected, then sometimes the effect is acting in unison at each brake, sometimes it's 180° out of phase and cancels out.

Probably too much information...

Regards,
John.
 

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In the Automotive world it's know as; Warped Rotors. Not the Carol Shelby; Ford definition.:roll:

According to the late and legendary Carol Shelby; what most people think of as 'warped' rotors is in reality 'rotor thickness variation', i.e. rotors don't 'warp', they develop a minute change in how thick the rotor is in different sections of the rotor. Google will easily find his thoughts on this matter, but I'll try and summarise:

This is caused by extremely thin depositions of pad material on the rotor faces, so thin that it can't be seen. This is a normal thing, i.e. rotors always have a microscopically thin layer of pad material on their surfaces that is continuously being deposited back and forth between the pad face and the rotor face.

The problem arises when this layer becomes uneven, and as the 'thicker' section of the rotor passes through the pads the braking force momentarily increases, then decreases as the thinner section of rotor passes through. This creates a 'hot spot' on the rotor, which then tends to pick up more pad material at that spot, and the problem gradually becomes worse and worse (in bad cases the 'hot spot' can be seen as an area of dark discolouration on the rotor faces).

The pedal may also start to noticeably pulsate as the problem becomes worse and worse. This is because as the thicker section of rotor passes through the pads it will force the piston(s) into the caliper body, and then allow it to move out again as the thinner section passes through, causing the volume of fluid in the caliper to cyclically change.

If you think about it, if a rotor were hypothetically to be only warped but still retained uniformity of thickness, all this would cause is for the caliper to compensate by allowing the pins to slide, or, with multiple opposed pistons, for one piston to move in while it's opposite equally moved out. This would create very little if any noticeable force variation at the pad / rotor interface, and no pedal pulsation because the internal volume of the fluid inside the caliper remains unchanged. I've found the proof of this in karting, i.e. the rotor can be misaligned quite noticeably but with no apparent affect on braking performance or feel.

At any rate, if you have this problem, you need to machine or replace the rotors. However, vibrations can be caused by worn suspension components, so don't assume it must be the brakes, it possibly might not be.

A tell tale for brakes will be if you also get a noticeable change in actual braking force as the rotors turn, most noticeable at low speeds. The car may be felt to cyclically 'lurch' a bit at low speeds with brakes applied, the affect sometimes being stronger and sometimes weaker depending on the orientation of the 'thick spots' relative to each other at the different rotors (if more than one brake is affected). If more than one brake is affected, then sometimes the effect is acting in unison at each brake, sometimes it's 180° out of phase and cancels out.

Probably too much information...

Regards,
John.
 

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In the Automotive world it's know as; Warped Rotors. Not the Carol Shelby; Ford definition.:roll:
My mistake, it's Carroll Smith, not Shelby (I knew who I meant, brain fart).

You may roll your eyes, trouble is 'warped' means warped, not what the problem actually is. The Smith definition is the correct one, what it's very commonly known as is wrong, leading to millions of people misunderstanding the true nature of it. The incorrect use of terms and misunderstanding of definitions causes a great deal of confusion and trouble in engineering.

At any rate, whatever anyone calls it, replace or machine will fix it.

For anyone who doesn't know, Smith was an engineer who wrote numerous books including; 'Tune To Win' and 'Engineer To Win', among other respected racing related texts. He was also deeply involved in a number of notable racing projects, including development and oversight of the Ford GT40 program (quote; "the most intensive brake development program in history" to that time), where he worked with Shelby (too many Carrolls...).

Regards,
John.
 

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Definition of warp (v)
warp[ wawrp ]
get twisted:to become twisted or out of shape, or make something become twisted or out of shape
deviate from course:to make something deviate from its usual or correct course, or deviate from a usual or correct course

Synonyms: twist, bend, distortion, deviation, change, alteration
 

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Definition of warp (v)
warp[ wawrp ]
get twisted:to become twisted or out of shape, or make something become twisted or out of shape
deviate from course:to make something deviate from its usual or correct course, or deviate from a usual or correct course

Synonyms: twist, bend, distortion, deviation, change, alteration
Exactly, and none of these things are what has actually happened to what is commonly and incorrectly called a 'warped' rotor. What has actually occurred is that an uneven deposition of pad material has built up on the rotor surface, which can also lead to uneven thickness wear of the rotor itself.

With a 'warped' (sic) rotor, if you were to place a dial indicator onto the outer surface of the rotor and turn it, you'd see a 'run-out' that looks like the rotor is actually warped, but if you were to place another dial indicator on the inner face you'd find a more or less equal 'run-out' in the same section of the rotor, i.e. it's not warped, it thickness varies.

This creates an area of increased friction on the rotor, which in turn creates a 'hot spot' where the thicker section of the rotor passes through the pads (thickness as a result of pad material deposits), and these two things exacerbate the deposition problem.

The localised heat at the 'hot spot' can cause the metal in this section of the rotor to become a lot harder and thus more resistant to wear, so over time you can end up with a condition whereby the softer iron wears away more quickly than at the hardened spot. This results in thickness variation created not only by the difference pad deposition, but the metal itself also can also develop thickness variation.

Since the pad material is constantly being transferred from the pad to the rotor and from the rotor back to the pad, there must be times when the metal itself is abraded (at differing rates dependant on hardness). The deposited pad material doesn't completely protect the rotor from wear, though it must lessen it.

Regards,
John.
 

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Whatever.
Someone overtightened your lug nuts and screwed up your rotors.
Mike,
At the risk of seeming pedantically argumentative, I strongly suspect this is another very common myth, though possibly a useful one for car companies / dealers trying to avoid warranty claims on brakes...

I can't see any possible mechanism by which unequal torqueing of the wheel nuts (or bolts) could actually warp a rotor. The stud / bolt is clamping several layers of robust metal together where there are only machined flat surfaces with no lumps, bumps or depressions that might introduce unwanted forces being fed into the rotor faces (also keeping in mind that if any such unequal force actually existed it would still need to pass through the very robust 'tubular' section of the rotor 'top hat' in order to manifest at the friction surfaces, which I don't buy that it would).

Only if the clamping forces at the studs were so high that the metal directly between the stud / nut were crushed would this be possible even to a minor degree, and the clamping forces just aren't that extreme. I've had occasion to attach three wheels using only three of the four nuts on each wheel (as an emergency measure, lost four nuts), and not experienced any braking issues as a result. Surely this is approaching the ultimate in unequal torqueing?

At any rate, if we were to assume that I'm wrong and that it did happen (for the sake of argument), we would still have a rotor that has warpage but no thickness variation. It's still thickness variation that will cause uneven braking as the rotor turns, not warping.

This is because (as I said earlier), all a pure warp will do is cause the caliper to compensate by sliding the pins back and forth, or with opposed multiple pistons, by causing the opposed pistons to move oppositely in / out in unison, with no significant variation in how hard the pads are gripping the rotor as it rotates (as per my kart example with the visually obviously run-out rotor that still braked perfectly with no issues at all).

In neither case (different caliper types) would there be a significant change in clamping force as seen at the pads. There will be some very minor force transferred to the pads, but only what is required to overcome the friction in the (greased) pins, or to flex the rubber in the pistons seals. Very small forces in the scheme of things.

Regards,
John.
 

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While this is an interesting topic (I agree that lug torquing cannot cause rotor warping by the way), the solution to the OP's problem is a new pair of front brake rotors.

I'm not certain I buy that the caliper would be able to ride out an actual warped rotor though by sliding on the pins, I'm of the opinion that the mass of the caliper and pads is too high for it to slide quickly enough side to side in order to compensate for any rotor warping...at least not at higher speeds where the theoretically warped surface is coming into contact with the pads about 20 times per second.

Agreed for the most part that rotors do not warp per se and that the issue is varying thickness though.
 

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Perhaps the 80 lbs recommendation is prevent the wheel from cracking under stress. I am willing to stand corrected as I am now so.
 

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Warped rotors.
No.

John Lear is absolutely right. Please read this. I've posted it multiple times here...

Pulsing under braking is a sign of brake pads that weren't correctly bedded in...

Or it could be a loose suspension component, since it's isolated to the front. I'd check the front control arms and the steering attachments to the front uprights..
 

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I see you're detemined to change automotive termanology? Call it what you want, describe it as you please. It's like having a 300 horsepower engine with no (actual horses) under the hood; Bottom line is the OP has whats called warped rotors that need to be changed. I never said the rotors are acctually warped/bent, nor did I need a 5 page explaination on brake rotors. I've read all this before; this is good information if you want to learn about brake vibration and possible causes. I've done many brake jobs in my day; and the term warped rotors still refers to the OP's problem. So, if you ever have this problem; when you go to Autozone tell them your car has lateral runout and see what reaction you get.

Definition of warp (v)
warp[ wawrp ]
get twisted:to become twisted or out of shape, or make something become twisted or out of shape
deviate from course:to make something deviate from its usual or correct course, or deviate from a usual or correct course

Since you want to quote me; explain the difference?

Exactly, and none of these things are what has actually happened to what is commonly and incorrectly called a 'warped' rotor. What has actually occurred is that an uneven deposition of pad material has built up on the rotor surface, which can also lead to uneven thickness wear of the rotor itself.

With a 'warped' (sic) rotor, if you were to place a dial indicator onto the outer surface of the rotor and turn it, you'd see a 'run-out' that looks like the rotor is actually warped, but if you were to place another dial indicator on the inner face you'd find a more or less equal 'run-out' in the same section of the rotor, i.e. it's not warped, it thickness varies.

This creates an area of increased friction on the rotor, which in turn creates a 'hot spot' where the thicker section of the rotor passes through the pads (thickness as a result of pad material deposits), and these two things exacerbate the deposition problem.

The localised heat at the 'hot spot' can cause the metal in this section of the rotor to become a lot harder and thus more resistant to wear, so over time you can end up with a condition whereby the softer iron wears away more quickly than at the hardened spot. This results in thickness variation created not only by the difference pad deposition, but the metal itself also can also develop thickness variation.

Since the pad material is constantly being transferred from the pad to the rotor and from the rotor back to the pad, there must be times when the metal itself is abraded (at differing rates dependant on hardness). The deposited pad material doesn't completely protect the rotor from wear, though it must lessen it.

Regards,
John.
Synonyms: twist, bend, distortion, deviation, change, alteration
Weather he's right or wrong; the termanology remains the same I'm sure the OP could'nt care less what termanology is used; he just needs to change his warped rotors to make his problem go away.


No.

John Lear is absolutely right. Please read this. I've posted it multiple times here...

Pulsing under braking is a sign of brake pads that weren't correctly bedded in...

Or it could be a loose suspension component, since it's isolated to the front. I'd check the front control arms and the steering attachments to the front uprights..
Now if you want to call it lateral runout instead of warped rotors; that's your choice. But, so that everyone understands the term WARPED ROTORS is used to describe lateral runout.

 

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... I never said the rotors are acctually warped/bent, nor did I need a 5 page explaination on brake rotors....
First reply above:

Warped rotors.
Actually, if you read the white paper on brake rotors you'd understand why calling the OP's symptoms "warped" rotors just means that there's a strong possibility of it happening again.

If you mistakenly start chalking up brake judder to "warped" rotors, then you miss the most likely reason it happens: lack of proper brake pad bedding. If you call it the right thing, and not some -- wrong -- term learned years ago, you can reduce the chances of it happening.
 

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DWMCC,

Apologies for the off-topic lessons on brake components, but the brakes might not be the culprit here. There are a couple of things that could give that same brake judder, so it might be worth taking the front wheels off to inspect the steering and suspension connections.
 

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Guessing only leads to buying unnecessary parts and the wrong advice. If you have the rotors turned on a lathe; it will happen again, if you replace the rotors; I would hope you would change the pads as well to eliminate the problem; because the next thing is the caliper. Something as simple as a twisted brake line can cause the OP's problem. So take your choice, replace parts, have the whole front end check or look at the obvious. The OP asked a question, I posted my answer; had he asked for an explanation/cause of the problem; I probably would’ve post the same information. If the question is ever asked again; my answer will be the same Warped Rotors”.

First reply above:

Actually, if you read the white paper on brake rotors you'd understand why calling the OP's symptoms "warped" rotors just means that there's a strong possibility of it happening again.

If you mistakenly start chalking up brake judder to "warped" rotors, then you miss the most likely reason it happens: lack of proper brake pad bedding. If you call it the right thing, and not some -- wrong -- term learned years ago, you can reduce the chances of it happening.
 

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Guessing only leads to buying unnecessary parts and the wrong advice. If you have the rotors turned on a lathe; it will happen again, if you replace the rotors; I would hope you would change the pads as well to eliminate the problem; because the next thing is the caliper....
Bingo. It will happen again because it's not actually the rotors that are the problem, it's the improperly bedded in pads. If you take "warped" rotors and get them turned, the deposits will reappear because of the bad technique, not from too much heat or a faulty casting. Yet, so many people hear "warped" rotors and go replace them...and have the same thing happen in 3-6 months.

Now do you get why it's not the "warped" rotors?

Something as simple as a twisted brake line can cause the OP's problem. So take your choice, replace parts, have the whole front end check or look at the obvious. The OP asked a question, I posted my answer; had he asked for an explanation/cause of the problem; I probably would’ve post the same information. If the question is ever asked again; my answer will be the same Warped Rotors”.
Sigh. OK.
 

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Discussion Starter #18
Thank you all for the incredibly long and interesting read. I'm not kidding, it was quite informational and at the same time comical to see several arguments about 'warped rotors', and 'improper bed in' along with the terminology for each. Seriously though, I really appreciate all of your opinions and I have learned much from each one.

What is a good plan of action in order to diagnose the problem that is causing the jittery braking on this car? What should I do first in order to prove/disprove the hypothesis of improper bed-in and deposits on the rotors?

Thanks again to everyone who contributed to this thread!
 

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You didn't prove otherwise; my answer was based on fact not theory. If you don't hear faulty front end parts when not braking or turning the steering wheel. why look there? What I do get is; it's not the front end, it's the rotors; the "term", "warped" rotors, is the "term" used to identify the problem if you repair/know about cars so until the "term" changes. The OP should start by changing the brakes pads and rotors to rectify the problem known as "Warped" rotors.

Now do you get why it's not the "warped" rotors?



Sigh. OK.
 

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What is a good plan of action in order to diagnose the problem that is causing the jittery braking on this car? What should I do first in order to prove/disprove the hypothesis of improper bed-in and deposits on the rotors?

Thanks again to everyone who contributed to this thread!
If you get the rotors turned on a lathe, or new ones entirely (nearly as cheap in some places) and ensure that the new pads are properly bedded in, then you shouldn't feel the vibration...if the rotors and pads were actually the problem. I usually do pads and rotors myself and follow the bedding procedure in the link I posted. I haven't had an issue with rotor-related jittery braking in maybe 10 years. (Brake shake from a worn bearing is a another story ;) )

If you don't follow the bedding in, the problem might return...and your new rotors will have become magically "warped."

This will eliminate rotors and pads as a source of the issue if it doesn't go away.
 
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