Brake rotor warpage: Fact or myth? (Long)

Let me start by saying I have a ton of respect for the people of this (996) board, when someone writes wrong information, they are called out on it and usually the truth prevails. In a prior thread, I had got into a debate whether brake rotors could warp. I said they could and others called me out on my claims and said they could not. The others claimed they could not with info learned from racing and racing programs. I learned my info from hands on and word of mouth from others experiences outside of racing. After some rather convincing info from the opposition, I questioned if I had made a bad statement. I needed to get a solid answer. After a few days of analyzing the info presented (in my head) and my past experiences, I reached out to the Engineers of my company. I figured you can not get a better source of info. I had many Engineers at my disposal, I chose the ones who I had known to of worked on cars and done brake repairs themselves. I had 4 to be exact. These are there statements below. I do not claim that these are facts but rather opinions based off of Engineering concepts, if you will. This is a very, very short summary of what was discussed with each of them. The wording is not exact; I used parentheses to distinguish the Engineers response. Engineer #1 - Can a brake rotor warp? “Of course they can, if you overheat a rotor and drive through water”. What if I told you that warping a rotor is impossible? “That is bogus, it is metal”. “Anytime metals are involved in heating and cooling, warpage can occur”. What if I said it was brake pad material build up you feel? “I don’t think it is a direct cause”. Conversation was very brief, not sure how much thought he had put into it. Engineer #2 - Can a brake rotor warp? “I don’t think so”. I explained the pad build up and he said that is “highly likely”. “Anytime there is two surfaces rubbing together with friction, an uneven surface will form”. He went on to say that a rotor simply does not get hot enough and to warp a rotor, it would take a full submerging of water, then it is possible but he was not 100% it would warp. Engineer #3 - He claimed to of done 1000’s of brake jobs (of course exaggeration) and has focused heavily on brake systems. He had my ears perked. I asked, can you warp a brake rotor? He said, “You most certainly can”. I said, what if I told you experts claimed you could not? He said, “they are full of it and are not experts”. I said, what if I told you that the shudder you feel is brake pad material? He said, “I highly doubt it”. He claimed that the lip (furthest O.D) that does not have any pad touching on many rotors would be found to be out of spec using a dial indicator from warpage. Pad has never touched this area shooting down the pad transfer. He said there are inferior products out there that will indeed warp faster than others. That brought up a point I think that Porsche owners and high-end brake users can get mixed with other vehicles. These systems use very high-grade materials and are more resilient to warpage. He agreed about my point and added, when you have a higher content alloy, the properties of the metal become more resistant to heat and warpage. After a long explanation of heat treating and annealing of metals, I was somewhat convinced that my point had been validated. Engineer #4 - Jaguar aficionado (owns probably 6+) and a fan of many motor sports. Asked if warpage was possible, he said “yes”. When asked of pad build-up causing the shudder, he said “maybe”. This Engineer went on to talk about the “bell” shape of most rotors and drew a diagram of stresses from braking and heat. He went on to say that essentially a rotor could “potato chip” under extreme conditions when it is overheated followed by being rapidly cooled (the water puddle again). Of course, this was drawn on a piece of paper to describe his interpretation of “potato chipping”. After thinking awhile, he said, “I do not have the data but I feel it is very plausible”. So, after 4 Engineers I get 1 yes, 1 absolutely no, 1 absolutely yes, and 1 plausible. The point of this post was that I will never claim to know the answer to this question of rotor warpage if I can not get 4 Aerospace Engineers to agree on yes or no. I will never debate this topic ever again on this forum. I am not an expert on brake rotors.

Wow, sounds like I missed out on a real debate. Here's my $.02 (that's probably what it's worth) I have ruined a few rotors on my 986 with pad material transfer, but I have yet to warp one. I would not say that it is impossible to warp a Porsche rotor. However, it has not occured to me....................................yet (knock on wood).

yes, they can warp slightly but very rarely. most of the time it is pad transfer. repeated bedding will solve that problem. and even with bad material transfer and "warping" so what, you get a jutter when you brake, but you still have 99% of full braking power. i just ignore it until it goes away by itself.

There were some insightful observations made by your experts, and although I'm far from an expert since I've changed brakes and rotors only around a half dozen times over the last 40 years, I doubt that rotors will warp. It just so happens that I was reading about this topic a few days ago, and I invite anyone who is interested to access this site: http://www.performanceoiltechnology....ingsystems.htm

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Oh dear God, here we go again. Do the guys that told you that rotors warp deal with metals or are they just brake repair guys? Is it possible? Of course. Can we create enough heat (and cooling) to warp metal from our braking systems. Please. This is one of the most ignorantly debated topics in the car world. Did these engineers actually have expertise in working with metals or does working on cars or changing their brakes qualify them?

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To be an Engineer at my company, you either have to redesign a part, or create a part for a jet engine. Do you think a simple brake rotor is as complicated as a 29,500 lbs of thrust or 16,000hp jet engine part that spins 15,000RPM in burner? I think they have the bases covered, haha If you read my last paragraph, I stated I will not take a side. I am neutral even though in the past I claimed to be on the warped side. I will not debate this. I will back our Engineers though.

Not asking you to take a side and I am not going to debate this further. I do not disagree that a brake rotor does not have the complications that your jet propulsion guys parts do. However, do they have EXPERTISE working with brake rotors and their metal properties? That's the question! Funny how 4 engineers that you have at your company don't have a DEFINITIVE answer for you but are adamant in their statements. By the way, nobody ever said it was impossible to warp, it was stated that it is DAMN NEAR IMPOSSIBLE. Of course metal can deform but it needs to be subjected to those properties that will allow for it. Do you think the metals used in brake rotors are such that can warp from heat generated by the average street driver?

I will chime in that from my experiences it is very difficult to warp a rotor. I have done many brake jobs over 35 years and have never seen it. On my race car I over heated the front brakes so much that the pad backing plate warped from the force of the single piston in the caliper but the rotor stayed straight! Not saying it can't happen but in my extreme racing conditions have never had it happen. And these vintage cars we race don't even have vented rotors so they get real hot!

What do they know about clear bras?

Years ago when I read the article that sort of drove this debate (link below) it made perfect sense to me. While I don't doubt that a rotor can theoretically be warped and that it was common on early or cheap rotors I really can't see how it can happen on our cars. I am relatively confident that the stuttering we have all experienced is the result of material transfer. Especially since, like Moot pointed out, it will go away in a few laps. I think the more important question to ponder is why is it spelled warping and not warpping? I think there�s something fishing going on there. http://www.stoptech.com/tech_info/wp...rakedisk.shtml

Thanks Jim.......After I completely wake up from reading that exciting article I will work on the spelling issue

Yes you can warp a brake rotor--but not from regular driving. In fact, you normally won't warp a brake rotor even from heavy braking. (Excluding discussions about manufacturing defects) -- rotors seem to warp when they heat or cool irregularly. A large amount of water on a hot rotor--especially on just a part of it--might do it (somewhat unlikely). But the most common cause seems to be Heavy braking to a stop--then holding the car in a single stopped position with your brakes (like a panic stop to a stoplight--then just sitting there). The pads hold in high heat at one spot from the stop, while the rest of the rotor cools. So....if you find yourself coming to a dramatically sudden stop or at the bottom of a Really Long hill that would heat up the brakes unusually, then try to stop a little early and every few seconds move forward a few inches...thus moving the pads to new sections of the rotors. That will tend to even the cooling and reduce the chance of any warpage. But, warping the rotors while continuing to drive (like on a track) or under anything approaching normal use is highly unlikely.

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That's funny. I didn't even notice that you had mis-spelled it. I was refering to my own mistake which the spell checker found for me. Trust me, I'll be the last to correct others. Something about the glass house I live in. Jim

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Dell, don't stop debating, we want to know the details about what you know. I'm not an engineer.. oh, wait actually I am, but not in metallurgy... oh, wait, I guess Materials Science qualifies... But I don't work on brake rotors!!! So tell us the gory details. I have become over the years very sensitive to vibration under braking. Every Honda, Acura, Volvo, Infiniti, and Toyota I had drove me nuts because of this. New rotors every time. My C4S is as smooth as Bens chin, I don't think it will ever vibrate. As you probably know, all the mechanics said warpage. Whatever it was, it was so very common on common cars.

I can't debate anymore. I have lost that fire

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Why in Blue Hell would you say that.....

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Do we have any other metallurgists on the 996 forum staff?

Some easy reading from http://www.performanceoiltechnology....ingsystems.htm (these guys advertise Amsoil, so who knows), but I like the Disc Thickness Variation (DTV) idea, which to many would seem like warpage. I am leaning to Dell's side of the fence. "What can occur over a period of time is that whatever runout is in the system coupled with improperly torqued wheel nuts and/or misadjusted or loose wheel bearings and rust and corrosion forming between the rotor and hub surface leads to increases in runout. As you drive your vehicle without using the brakes, such as on the highway, every rotation of the rotor high spot or multiple high spots contacts your brake linings in the caliper, even when you are not using the brakes and wears the high spot or multiple high spots off the rotor which causes a thin spot or multiple thin spots. Over a period of time this repeated process causes what is called Disc Thickness Variation (DTV). DTV is when the rotor thickness is not the same all the way around the rotor. DTV is typically caused by lateral runout. DTV can only be measured with very specialized laboratory testing equipment or with special on vehicle capacitance probes. When you apply your brakes, and a brake rotor has DTV, the thick and thin spots on the rotor cause the brake pads to move in and out. This in-and-out pad motion causes increases and decreases in brake system pressure, which the driver can feel in the brake pedal. This in-and-out pad motion causes a varying brake force, which is passed to the steering wheel. As the rotor gets hot, it is much more likely to increase thickness variation, thus increasing pedal pulsations as well as steering wheel and other vehicle vibrations. This phenomenon is what many technicians refer to as "warping", however they actually think the rotor warped and needs replacement. Typical acceptable values for DTV are around 0.0004 in. That's 4 ten thousandths of an inch! As DTV increases beyond 0.0004 in., brake pedal, steering wheel and vehicle vibrations and pulsations will almost always occur. "

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parts. Even for those iron castings that have been left to age/stablize -- some foundries put the castings outside to experience a year's worth of heating/cooling cycles -- that over time will eliminate most if not all stresses arising from the casting process, upon initial machining which removes the skin of the casting, these stresses -- uneven by definition -- can see the part warp. At this point, depending upon various things, the rough machined casting can be subjected to another round of stress relieving before being final/finish machined. But subsequent cuts to bring the part to its final shape and finished size should not see any warpage although improper fixturing of the part during machining can introduce features that resemmble warpage. For instance, grip a cast iron cylinder too tightly in a 4-jaw chuck, even though the jaws are adjusted to eliminate measureable runout, and after boring the inside diameter and releasing the part from the jaws, one can measure out of roundness, unequal cylinder wall thickness -- usually just before he's fired... For brake rotors, it is possible -- although I have to believe rare -- that some residual casting stresses can remain -- rotor blanks are rushed through manufacturing for some reason -- and upon being subjected to heating/cooling cycles especially if these are severe before the remaining stresses can work themselves out with less severe cycles -- this residual stress can result in the rotor warping. Rotor thickness should remain unchanged around the rotor, but the rotor can develop a lateral run out. If continued to be used, this can cause uneven rotor wear and eventually the rotor thickness will develop unevenness. For most brake rotors, though, no significant stresses remain, or brought forth by severe usage, and the rotor does not warp. And let us not forget wheels that are installed with incorrect wheel lug nut/bolt tightening sequencing or with the wrong torque settings (too much torque or to little) or against rotor surfaces that are not flat, that have a burr or something else that prevents the rotor from turning without some runout. But by and large, I believe uneven braking feel -- pulsing -- can develop which can of course be mistaken as warped rotors and it develops from braking usage that results in uneven material deposition from pad to rotor surface under some conditions. I experienced this when after washing my VW Golf and failing to take it for a drive to dry the brake rotors. Then upon driving -- and in fact the rusting was so severe the parking brakes stuck and let go with a loud crack when I took of from a the parking place -- and I had to make an emergency stop and upon stopping left the brakes applied. The rust combined with high heat developed during the braking resulted in this uneven material transfer from pads to rotors and afterwards resulted in a slight pulsing feel upon light braking thereafter. It was not severe enough to warrant resurfacing the rotors and never developed into a severe case before I sold the car. BTW, this occurred after I had put quite a few miles on the car and had used the brakes in some cases heavily many times before this instance. Brake rotors can warp. But most rotors labeled as warped rotors are in fact not warped, at least not initially, but have been used in such a way that there has been an uneven transfer of material from the pads to the rotor surfaces which results in varying degrees of friction as the rotor turns past the pads which will cause some pulsing feel in the brake pedal. Then a few warped rotors arise from incorrect assembly (or manufacturing -- other than improper/insufficient aging/stress releiving of the castings). Sincerely, Macster.

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Outside as in, out of doors? C'mon Dell, you gonna let this one go?

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will let mother nature do what would cost considerable extra money to do with other forms of energy. The tradeoff is the foundry can have a considerable amount of castings laying about but this represents less cost than feeding the castings through some accelerated stress relieving process. One can put the castings in an oven and raise the temperature to whatever is necessary to allow stresses to relax, then shut off the oven and let it cool enough the castings can be removed without the risk of uneven cooling introducing stress. (And not all stress relieving need be done with heat. It was found some years ago castings that traveled by rail and subject to considerable vibration/banging against one another were releived of stress as well as if they'd been subjected to the heating slow cooling process.) Or the castings can be left outside and while the heating/cooling temperature swings are less, there are more of then and the stresses come out over time, as much as can come out with the outer casting "skin" still present. In some cases, the castintgs are rough machined then placed outside for further aging. There's as many different techniques, almost, as there are different iron castings. It depends too upon what the casting will be used for. Air or space flight hardware and manufacturing paper work/blueprint will spell out post casting heat treatment , post rough machining heat treatment, post secondary machining heat treatment, etc., in very detailed fashion. A cast iron part used for something less critical may go from the shaker table/cooling rack to the machine shop while still warm. However, even so, the machining process will be such that initial and intermediate machining steps will see (ideally and usually ) all stresses are relieved so the part upon final inspection will meet the requirements specified by the blueprint, most of the time. The part may on the granite inspection table meet the print but can while on the rack see residual stress work out that can leave the part slightly out of print. Sincerely, Macster.

Nothing beats-on rotors like race cars -- race cars (and their drivers ) are relentless on rotors. In the 11 years I have worked in professional motorsports, I have never seen a warped rotor. Pad deposition, rather uneven pad deposition, certainly.

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Dell, WHAT IN THE BLUE HELL IS GOING ON HERE?!?!?!?!?!? "I have lost that fire" you say? Well, at least be truthful about it. Just tell the man the truth and admit that....... you have turned into a *****!

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OH NO DI-INT!!!!!!! Time for me to get back into form..............

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Hey Ray, I had a feeling that might knock some sense back into him!

Whilst I have had the problem at some stage in at least half the cars I have owned (from Pajero to M5) - a Sierra Cosworth had six sets of front rotors in 20,000 miles all at Ford's expense - I have never once experienced it in a Porsche (and I am on my tenth). Something for which my wallet is eternally grateful! Anyway why would you ask an aerospace engineer about disc brakes on a sports car? Bit like asking an orthopaedic surgeon to interpret an ECG - over qualified in the wrong area. Or a physician to comment on metallurgy .... so I'll shut up!

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Because they are all car buffs and have mechanical experience.

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That's what I said....buut just because I am not a PROFESSIONAL I guess it didn't count......welllll excuuuseeeeee meeeeee

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Steven, we had to discount your comments because you drive RED cars! Sorry, but that's just the way it is!

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Maybe there is confusion, sounds like he asked lawyers, not engineers. As lawyers, we always have different answers because we don't have to base our opinions on science, it is just always a hypothesis with us and we always reserve our right to modify our answer based on new information. Being a lawyer is much better than being an engineer. *** Flame Suit ON and ZIPPED ***

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No, they are Engineers, it is Dell who said they were adament, there was only two of the four. He misintepreted. I wont say anything, I dont want to be sued for slander.

adamant=lawyer??? Try as you might, but I have no interest in this thread any further. Unsubscribing......

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Oh, Dell, come on, we haven't tried this hard to get someone to do something in a long time. DEBATE, DEBATE, DEBATE. :

Maybe it's the impending birth of my 1st child that has done it to me. Besides, with the hormones wreaking havoc on my wife, I get debated to death about every night when she gets home from work

Dell: Okay, then with that information I'll grant the FREE PASS. Damn, I feel for you man.

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****, you think it's bad now, wait until the "post partum" sets in! You're in for a treat!