On 2022-05-09 8:39 a.m., Andy Burnelli wrote:
> Xeno wrote:
>
>>> You can't test warp on the vehicle (not reliably anyway).
>>> It has to be tested on a known flat bench (or using known flat tools).
>>
>> Or mounted on a brake lathe
>
> Agreed. All you need is a long flat edge and feeler gauges as a minimum.
>
> I'm all about facts. I'll change my mind in a split second if that's
> where the facts lead.
>
> Let's look at the facts in this discussion between two adults on Usenet.
>
> My point is that nobody who says warp (as in potato chip) measures that
> warp (as in potato chip), where I already know that warp (as in potato
> chip) isn't what happens when a brake rotor is subject to intense heating &
> cooling cycles.
> "High quality brake rotors that pass SAE J2928 Brake Rotor Thermal
>  Cracking Procedure for Vehicles below 4,540 kg GVWR proves
>  they don't warp"
>
> As I said, intuition is a terrible thing indeed.
> "Calling it brake rotor warp demonstrates a complete misunderstanding
>  of the metallurgy and the braking process"
>
>> I wasn't saying anything about rebedding at all.
>
> OK. But the fact that rebedding works sometimes means that what it fixed
> wasn't warp (as in potato chip). What it fixed wasn't runout either.
>
> What it fixed is the only thing it _could_ fix, which is deposit buildup.
>
> Intuition is a terrible thing indeed.
> "The brake pads and rotors in a street vehicle can't possibly  generate
> enough heat to warp a brake rotor."
>
>>> Almost nobody could machine a warped rotor (as in potato chip) back to
>>> perfection simply because the amount of metal needed isn't going to be
>>
>> Depends on the amount of warp and whether the rotor still meets
>> minimum specs afterwards. It only takes a very small amount of warp
>> for the driver to be able to sense it.
>
> Agreed.
> However, any rotor that was subject to enough heat to truly warp it (as in
> potato chip) was subject to _tremendous_ heat - almost impossible to attain
> in a passenger vehicle (again, I'm not talking about the space shuttle
> here).
> "The heating and cooling that people refer to when discussing  'warped
> brake rotors' would cause cracking, not warping."
>
> There are half a dozen materials rotors are made of though, so we'd have to
> look up the melting point of each of them to be more precise about melting.
> 1. cast iron
> 2. steel
> 3. stainless steel
> 4. laminated steel
> 5. high carbon iron (e.g., Sparta CX3.5)
> 6. ceramic
>
>> The car showed *symptoms* of warp, the rotors, when measured showed
>> *evidence* of warp. I drove the car, I felt the symptoms, I measured
>> the rotor warp.
>
> I'll leave it at that since I respect your experience.
>
> Read this please, though (it's just one random hit of many of course):
> *What causes warped brake rotors?*
> <https://ricksfreeautorepairadvice.com/warped-brake-rotors/>
> "brake rotors DO NOT WARP from  normal driving or even race track use.
>  Yes, you read that correctly. A typical street vehicle can't possibly
>  generate enough heat to warp a brake rotor."
>
> All the quotes in this one post are from that reference, but I can easily
> find more because I know what I know and most people are intuitive.
>
> Did I mention yet that intuition is a terrible thing?
>
>>> I doubt it. Rotors are made of a variety of steels (e.g., motorcycle
>>> rotors
>>
>> I don't, I saw the evidence.
>
> Again, I respect your experience.
> However, you have to respect my knowledge that _reliable_ sources say
> otherwise and that runout isn't warp (as in potato chip) and that if
> rebedding worked, it wasn't warp (as in potato chip) after all.
> "Even if you're traveling straight down the side of a mountain with your
> brakes applied the entire way. Your brake pads will fade and start to
> disintegrate long before you come close to heating your rotors enough to
> soften them to the point where they could possibly warp"
>
> I even doubt machining could fix any appreciable warp (as in potato
> chip), and even if it did, I wouldn't want any rotors from _that_ shop. :)
> "In addition, if your brake system is working properly, it's applying
> equal pressure to both sides of the rotor which means BOTH SIDES are
> heating at the same rate. To warp a brake rotor, you must have more heat
> on one side of the rotor than the other."
>
>>> are often stainless steel) but even for the worst quality cast iron
>>> rotors,
>>> look up the temperature it takes to melt them.
>>
>> Automotive rotors were pretty much all cast iron back in the era
>> concerned. And the rotor did *melt* at the periphery, as I stated. It
>> had been damn hot all over but the periphery was clearly melted.
>
> I'm not going to argue with you on that as cast iron has a relatively low
> melting point where our question is can a rotor typically get to that point
> on a passenger vehicle whose owner suspects his rotor warped (as in potato
> chip).
>
> "To get cast iron hot enough to soften the metal you'd need to generate
>  almost 2,300�F range. There isn't a factory stock automotive brake
>  system in the world that's capable of generating that kind of heat.
>  In fact, you would experience brake pad fade, pad disintegration,
>  brake fluid boiling and rotor discoloration long before you  reached
> 1,000�F."
>
>>> I'm not saying it's impossible. But I doubt it happens for normal
>>> passenger
>>> vehicles (I'm not talking space shuttle stuff or fighter plane stuff).
>>
>> Nor am I. I wasn't and never have been an aviation mechanic.
>
> The only thing I harp on is I've never met a person who claimed it was
> "warp" (as in potato chip) who actually measured it, and, worse, the remedy
> often is something (like rebedding) which couldn't possibly have fixed warp
> (as in potato chip).
>
> Worse than even that, is all the reliable sources say passenger vehicle
> rotors can't warp (as in potato chip) simply because brakes can't generate
> enough heat.
>
>>> Look it up.
>>
>> No need, I saw the evidence directly.
>
> I knew what I'd find in brake warp when I just looked it up as I did my
> research long ago (just as I did with cellphone caused accident rates).
>
> Most people work solely on intuition.
> Humans have the intuition of evolved monkeys.
>
> Intuition serves humans well... sometimes.
> But not always.
>
> Read this _one_ reference for a starting point on fixing that intuition.
> <https://ricksfreeautorepairadvice.com/warped-brake-rotors/>
>
> If you don't like it, find another (there are plenty).
>
>>> Yes. I know. But did you measure warp (as in potato chip)?
>>
>> Yes, I did, with a dial indicator. Definitely warp.
>
> "To pass the J2928 rotor test, rotors must withstand at least  150 heat
> cycles on a dynamometer without cracking or showing  any structural or
> dimensional failure. "
>
>>> You can't measure warp without a known straight surface to compare
>>> against.
>>> A mic won't measure warp.
>>> A dial gauge won't measure warp (unless it's a special setup).
>>
>> It was a special setup - a brake lathe. Runout and warp shows up very
>> clearly when the hub and rotor assembly is correctly mounted.
>
> Note that the link I am giving you has information from the brake rotor
> manufacturers who themselves claim that their rotors can't warp (as in
> potato chip) in passenger vehicles under _any_ circumstances.
>
> Raybestos video titled "Rotors Can't Warp":
> <https://youtu.be/LVRVe1cEBDI>
>
>>> HINT: I've never seen anyone who did who said their rotors warped
>>> when what
>>> really happened was something else (e.g., runout or pad deposition).
>>
>> Depending on the site, runout can be a symptom of warp. You verify it
>> as warp by checking both sides.
>
> I'm not going to harp on the issue other than to repeat my salient points.
> a. Rotors don't warp (as in potato chip) in passenger vehicles. Period.
> b. People who _say_ they did, didn't measure warp (as in potato chip).
> c. The temperatures needed are unattainable in passenger vehicles.
>
>>>> If they wouldn't clean up and still remain within specs, they would
>>>> be replaced, and that could be determined from the measurements.
>>>
>>> I seriously doubt an actual warp (as in potato chip) could be
>>> machined, but
>>> if it's only slight, then maybe, but what fool would want rotors that
>>> were
>>> actually warped (as in potato chip) even after they were machined?
>>
>> Well, these days the rule is discard and renew, don't machine.
>> Procedures have changed.
>
> OK. But I can _easily_ find industry references that say rotors can't warp
> (as in potato chip) simply because they are designed _not_ to warp (as in
> potato chip), and, because the temperatures to cause warp (as in potato
> chip) are unattainable.

<https://youtu.be/glIik3KHcOs>

What's that if it's not warp, Arlen?