"jim beam" <spamvortex@bad.example.net> wrote in message
news:n_WdnSmVCvKeqtvbnZ2dnUVZ_uKknZ2d@speakeasy.ne t...
>
>
> 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.
>
I've never had that experience (possibly because I lived in Phoenix so
long - no corrosion.) My Volvo has disks that are essentially removed when I
remove the wheel and once they pulsate they only get worse. When I learned
the importance of torque I tried removing and retorquing the wheels without
any improvement. New rotors were as smooth as butter the first few years,
then they too gradually started pulsating.

> 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.
>
Dunno - that's outside my areas of expertise. But the models and
descriptions fit my experience very well.

> 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.
>
Brake cleanliness is something I've always been a fanatic about. I clean the
discs and my hands well before installation and clean the disc with brake
cleaner and isopropyl alcohol (and lots of paper towels) before final
assembly. I still get the scaly spots occasionally - I just figured they
were casting anomalies. They may still be.

> 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.
>

The major problem with the cementite theory is that it doesn't take us very
far into the practical realm: what to do to prevent trouble. The link covers
your points and more (including intelligent pad selection for the use and
breaking in brakes) but the cementite theory only offers an explanation as
to why the problem is progressive, not anything new to do about it.

> 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.
>

Although - the brakes will do that, cementite formation or not. Brakes are
made to be serviced based on inspection and performance, so if the
performance has degraded the brakes are fixed, regardless why it got that
way.

I agree the argument for the role of cementite is not conclusive but it
isn't something to be rejected out of hand either. Whether it is *useful* or
not, I'm not prepared to debate. But it is interesting.

Mike