Michael Pardee wrote:
> "jim beam" <spamvortex@bad.example.net> wrote in message
> news:r5Wdnc1D2fOgUNjbnZ2dnUVZ_s2vnZ2d@speakeasy.ne t...
>> Michael Pardee wrote:
>>> I don't want to trigger another never-ending thread, but in research for
>>> another forum I stumbled across this essay on brake pulsations at
>>> http://www.powerbrake.co.za/download..._01_judder.pdf
>>>
>>> It seems to pull together the cementite issue with the issue of disc
>>> thickness variation and supports it with pictures. I've seen the
>>> individual pieces in various places but thought this did a good job of
>>> making sense of it all. There is also a good treatment of brake pad
>>> selection that mainly reinforces my preference for staying with OEM.
>>>
>>> Mike
>> dude, the moment they start talking about cementite, and getting it wrong,
>> they lose all credibility.
>>
>> bottom line, honda disk hubs are ultra-lightweight. they elastically
>> distort when the lugs are tightened. if the torque is incorrect, the disk
>> starts to sit out of plane, and the brake judders. it's real simple!
>>
>> metallurgical problems can exist, and they show disk cracking as one of
>> them, [but get that explanation wrong too], but cementite transformation
>> at normal braking temperatures is pure b.s.
>>
>> i've permanently fixed this issue several times now on different civics
>> and most recently on my crx. simply apply a thin layer of antiseize to
>> the hub surfaces, torque in a 2 or more stage process and bob's your
>> mother's brother.
>>
>> i've been thinking about this for a while, but i guess this article
>> ratchets things up my agenda - i must go to a junk yard and take some pics
>> of things to look out for in the disk brake department. there's one thing
>> particularly i want to show for cheap chinese disk castings.
>>
>
> In spite of your doubt about the role of cementite, did you read it all? And
> do you have disagreements about the rules for avoiding pulsations on page 7?
>
> I am a skeptic about cementite formation as a factor in brake disc
> deterioration, but looking back on the odd visual patterns I've seen on some
> brake discs after being in service a while I am not so skeptical. I'm sure
> you've seen those, too - scaly looking areas with irregular shapes. Given
> that cementite will form below 700 degrees C (according to the link) and
> that braking under adverse conditions such as short downhill freeway
> off-ramps dumps a whole lot of energy into the brakes in a hurry, it makes
> sense. As the link points out, cementite isn't necessarily the root cause of
> brake pulsation. It places heavy blame on poorly cleaned surfaces and
> anything else that causes uneven friction. The disc with darkened areas over
> the dividers in the cooling vanes (figure 4) really turned on the light bulb
> for me. I've seen that.
>
> I too have experienced much less brake trouble since I began torquing wheels
> with a torque wrench. But the pulsations often still appear down the road -
> just farther down the road. Something is happening to what were carefully
> installed brakes, and not just on Hondas. My Volvo is even touchier.
>
> Does this prove cementite is to blame for all cases of brake pulsation or
> worsening pulsations? No, but it is a good fit in that the conditions to
> create cementite are known to exist in heavily used brakes. It isn't proof,
> but it sure is more credible than global warming theories. It has my
> attention.
>
> Mike


the #1 issue with "cementite theory", even if the metallurgy weren't
bogus, is that brake pulsing disappears if you take the wheel off,
clean, antiseize and correctly torque. even if you're unsure on the
metallurgical argument, that alone identifies it as a mechanical issue,
nothing else.

regarding casting quality, there are indeed a multitude of issues that
can be present including non-homogeneity of the casting, voids,
inclusions, etc. there can also be heat treatment issues too, lots and
lots of things. but to suppose there's any significant metallurgical
transformation going on at the temperatures cited, for the few seconds
it can be sustained, is massively underinformed.

if we're looking at patchiness on the disk, my money's 80% on surface
contamination. a thumb print for instance leaves oils which carbonize
on heating. then you have a glazed patch with different friction and
wear properties to the rest of the disk. add to that a brake pad with
insufficient silica [abrasive] content, and this patch will remain while
the the remainder of the surface wears. etc.

my other 20% is on insufficient post-casting heat treatment. disk irons
are usually "gray" - that means carbon flakes are precipitated
throughout the material. if there's a region where it's been
insufficiently heated for not long enough, there my be insufficient
precipitation and too much retained cementite, but that's /retained/,
not formed as the result of service.

so, inasmuch as the article tries to address the problem, it's making a
good effort regarding cleanliness, torque, q.c., etc. but it's well
wide of the mark on the metallurgy and only part way there on contamination.

oh, and why do disks "warp" over time? believe it or not, the wheel
moves about on the hub, even when bolted tight. not a lot, but a
little. if something "settles" as the result of this movement, it'll
"warp" the disk. likewise corrosion - that can creep in under bolted
surfaces and have the same effect. keep things clean and antiseized,
there will be no problems.

and as a final reality check, always bear in mind that brakes are
supposed to be able to cope with severe service. [despite the
"standards" considered acceptable in detroit not so many years ago] if a
brake can't stop a fully loaded car from it's maximum speed, fully
loaded, on a steep grade, it's a potential killer. cast irons can
retain sufficient strength and hardness for this function well into the
red heat zone. that's a good deal hotter than our "avoid 610 C" friends
seem to be able to envisage.