On 10/5/2022 1:39 am, 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.

Dial indicator with the disc mounted on a brake lathe is my choice. Much
more accurate and allows you to ascertain exactly where the runout is
and enables the mechanic to differentiate between such things at
thickness variation, taper, hard spots and warp. A long flat edge and
feeler gauges isn't going to get you there. What you also need, in
particular for assessing thickness variation and taper, is a disc
micrometer;

https://www.starrett.com/metrology/product-detail/458BXR

With this micrometer the mechanic can check thickness variation and
taper in the disc running surface as well as groove depth, an asset no
brake mechanic should be without. You can also get disc rotor vernier
calipers which, though not as accurate, will allow the mechanic to get
the job done with a minimum of fuss.

https://m.media-amazon.com/images/I/61ObCDKm14S._AC_SX679_.jpg

A long flat edge and feelers just doesn't cut it if you are looking for
accuracy. You're just not getting meaningful measurements.

> 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"

Well, as I said, I have seen it, felt the symptoms and measured the 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.

Deposition creates a *different* symptom to that caused by warp when
driving. Any good mechanic should be able to differentiate.
>
> 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

Of all those materials, the only one in *common use* is cast iron. The
downside in cars is the increase in unsprung weight. Steel is for racing
cars and they are very prone to warping. Laminated steel is less prone
to warping but this material/type isn't common in production passenger
vehicles. More an aftermarket thing for the racing fraternity. Yje high
carbon iron and ceramics are only to be found on the upper end cars.
>
>> 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."

What is *normal driving*? I used to hammer the brakes on my cars but I
no longer do that - I'm aging gracefully. There is no longer an
imperative to save 5 or 10 seconds on a short journey - I've all the
time in the world these days and I no longer live in a capital city.
I've done the tree/sea change and now live in a small rural/seaside city
of 75,000 people. Drive 1 kilometre from my home and I'm out in the boonies.
>
> 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?

When assessing serviceability of brake components I *never* rely on or
use intuition. I always measure.
>
>>> 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

Warp will *create* runout. It has to.

> 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. :)

The manufacturers specify a *minimum thickness* for rotors. It's but a
simple calculation from *measurements* to work out whether a rotor will
clean up and still be within spec. The is no need to measure, machine,
*remeasure* then discard.

> "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."

With sliding/floating calipers, it is quite common for rotors to get
hotter on one side. It's the nature of the beast. That is why,
particularly on sliders, that one pad wears out before 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).

Not everyone drives their daily driver like a grandpa. I certainly
didn't back in the day.
>
> "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."

It depends......
>
>>> 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).

Well now you can say you've met one - *me*. What's more, I taught brake
assessment at an automotive technical college for some 20 years and
there the focus was on measure, measure, measure.
>
> 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.

They aren't trying.
>
<snip>

--
Xeno

Nothing astonishes Noddy so much as common sense and plain dealing.
(with apologies to Ralph Waldo Emerson)