Brakes again

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/downloads/tech_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.

 

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.

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

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.

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.

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

 

if we saw honda disks with patches on them, then sure, but since it's
rare in comparison to the "other cause", and the "other cause"
disappears with simple remedies, i say it's clutching at straws.

perhaps, but metallurgically unsound. my casting metallurgy is a little
vague these days, but iirc, unless there's some crazy prolonged heating
involved, combined with alloying problems, i don't see how gray iron is
going to re-transform itself into cementite. casting and subsequent
heat treatment errors leading to /retained/ cementite are /much/ more
likely in comparison, and they'd show up in machining.

slightly tangential to this, the europeans are big into abrasive pads on
their disk brakes - the disk wear rate on bmw, mercedes, volvo, the euro
fords, etc., is very high compared to japanese [honda] disk wear rates.
while disk replacement on each pad change is pretty much inevitable,
with the higher expense involved, there is some method to this madness.
because they're effectively always being skimmed, these disks are
much less susceptible to surface contamination problems, including the
effects of rust.