need opinion of hyundaitech

Matt - you get too hung up on little things and overlook the obvious. Claim
what you will, but if you've never mounted and turned a rotor I'm not going
to spend any time arguing about what "could" be happening. I'm happy to let
you believe what you read on a web site as the hidden answer to rotor
problems, and I'll simply bumble along fixing cars in my ignorance. I'm
really not going to spend a lot of time trying to satisfy your needs for
empiracle evidence to refute something you read.

And I do stand corrected on this point, in that I should have said dial
indicator. Don't know how the word caliper slipped in there unless it was
in the front of my mind - this being a brake discussion. Regardless, if
you'd have worked on pulsing pedal problems you'd certainly have verified
warped rotors with an indicator.

Let me challenge that article this way Matt - how much depositing and
imprinting is necessary to cause a noticeable pulse in the pedal? How much
to create wild pulsing at highway speed? How does this buildup occur in
such varying depths around a 10" disk that is turning at highway speeds with
(near) constant brake pressure, while 5 hard stops is all it takes to clean
it up?

 

The buildup thing kind of puzzled me also. I'd think that if there was
buildup turning the rotors would remove it and leave the rotor in spec.
Certainly some shop would have noticed they are cutting brake lining, not
metal.

Do you think it is possible to generate enough heat and the spinning would
actually unwarp the rotors? Since repeated stops are necessary, that is the
only possibility I can think of.

 

I've never mounted a rotor on a lathe as I never had a lathe. I've
always used a dial indicator with the rotor on the car as that is THE
most accurate way to check for warp. And thickness variation is checked
just as easily on a car as well. And thickness variation would cause
much more severe pulsing anyway. Warp just moves the calipers side to
side in their mount. Since they are designed to move this way anyway,
this doesn't cause much problem. The severe pulsing is caused from
variations in friction as the rotor turns and this is either from
variations in coefficient of friction around the circumference of the
rotor or from variations in clamping force around the circumference.
The former is caused by uneven pad material deposition or nonuniform
changes in the metal properties as described in the article. The latter
is caused by uneven thickness of the rotor (also extremely rare). Warp
just about never happens and even if it did, it would cause very slight
vibration and would NOT cause the car to shudder as will the other two
problems.

No problem, brain farts happen. However, I have to admit when I hear
someone use the incorrect nomenclature it causes me to question the
veracity of their message.

Very little is required. All it takes is enough to change the
coefficient of friction from one circumferential location to another.
One pad imprint can do it. It is a little bit like oil. It takes a
very thin film to radically change the coefficient of friction of a surface.

Matt

 

I don't think the issue is build up in the sense of changing the
thickness measurably. I think it is more an issue of changing the
coefficient of friction in a nonuniform way around the circumference.
That only takes a layer a few atoms or molecules thick... I'd have to
read the article again as I don't remember all of the details, but I
think they also talk about other phenomenon occurring as well.

I think the odds of changing a mechanically warped rotor back to a true
one are simply astronomical. That is the reason that I started to
believe this article had merit. That and measuring a handful of rotors
that pulsed severely and finding that they were true well within the
manufacturers specs. After "fixing" a few "warped" rotors with very
hard stops, I knew that I wasn't "unwarping" the rotor and something
else had to be going on here. I also break in new pads per the article
and I now almost never have a problem with pulsing brakes.

However, I'm not saying you should believe me or even the article. Try
it yourself. Next time your brakes pulse, put a dial indicator on them
and see if they are really warped (moving side to side as they spin).
I'll bet the odds are very high that you have nothing outside of the
manufacturer's specs. Then go out and make several very hard stops per
the article and see if the pulsing changes. Then measure axial run-out
of the rotors again and see if anything has changed. I'm betting
nothing has changed, but I'll also bet that your brakes pulse less. It
will never go completely away as you can't undo completely the damage
caused by improper brake-in or other use such as making a hard stop and
then holding the brakes on at a light or such and severely imprinting
the brakes. Only shaving metal off the rotors or replacing them will
restore you completely to normal.

However, as I said earlier, if you are at the point of replacement
anywhere, what have you got to lose by trying the hard stop treatment?

Matt

 

That's the part I don't buy Matt. How fast are those rotors turning at
highway speed? How does this deposit occur from one circumfrential location
to another? If imprinting was as real a factor as this guy suggests, every
single car that has ever put a mile on a road would have suffered this after
the first stop at the end of a highway exit ramp. It's a great sounding
theory for a guy's web site, but it's bull - at least as he presents it.

 

Well, for a 30" diameter tire, the RPM at 60 MPH is 672. Alternatively,
this is 11 revolutions per second. I don't follow your point though
about the RPM. Can you elaborate?

Most cars I've ridden in have suffered from this. I travel on business
fairly often and rent a lot of cars and almost every rental I drive has
pulsing brakes. Some fairly minor and some downright obnoxious.

Matt

 

What I meant is that the rotor is tuning fast eniugh that if layering were
truly the culprit, it would be a much more consistent thing given the
rotational speed of the wheel. It would be difficult at best to create
layering of variable density.

Well, I don't drive rental cars anymore, like I used to. I've been more
local for the past two years (almost). I have not found brake pulsing to be
common in any rentals I ever did drive - and I used to be in 2-3 different
rental cars every week. I know that none of our cars display it unless
things are wearing out.

Let's go back for a sanity check though... We *are* talking about a wobble
and not a very faint low level pulse - right?