disk rotor choice

On 3/18/2011 2:36 PM, hls wrote:
>
> "dsi1" > wrote in message
> ...
>> On 3/18/2011 1:03 AM, George wrote:
>>> ('04 Cavalier) I need to replace the disk rotors. Everyone hereabouts
>>> seems to have two varieties: one at $22-ish, the other at $38. The $38
>>> ones have a longer warranty, but only for "defects".
>>>
>>> My guess is, the $38 ones are 'better' (more metal?); but, it could be
>>> just charging more for a pretty meaningless warranty. Does anyone know?
>>>
>>> Thanks,
>>> George
>>
>> Go the the store and ask to see both rotors. Buy the one with more
>> metal. In my case, I'd probably get the cheap rotors. My old Cavalier
>> was a great car and served me well. For me, only the cheapest parts
>> would go into that baby. :-)
>>
>> I read a startling article that said that the pulsating rotors are
>> actually caused by transference of pad material onto the surface of
>> the rotor at a molecular level which causes uneven braking. Beats the
>> heck out of me if this is true but if it is, then breaking-in the pads
>> could be critical. My understanding of the article is that applying
>> the brakes after heating them up will cause a visible imprint of the
>> pad onto the rotor. Weird stuff!
>
> I dont believe that. When I have chocked up the rotors in the brake
> lathe, there is definitely a significant warp. I have never seen much
> pad material transfer to the rotor surface, but then again I buy good
> pads.
>
> There are several factors at work here.

My guess is that most car guys won't believe it.

hls wrote:
>
> "Paul in Houston TX" > wrote in message >
>> You should be able to tell by looking at them.
>> Have them get both kinds for you then check them when
>> they come in.
>> Some really cheap ones are thinner, don't have as many
>> cooling fins, etc.
>> Everything is made in China, but some are better than others.
>
> I hear what you are saying, but when metallurgy is involved, I dont
> believe you can tell by looking. Certain good machine work looks
> better, usually, than crappy machine work. Metallurgy??? A different
> thing.

Good point. Hopefully good machining and more metal is
indicative of better metal. It would be a shame to
use poor metal with good machining.
There was an article on the subject in Pop. Mech. about a
year ago. I will see if I can locate it.

"dsi1" > wrote in message
...
>> There are several factors at work here.
>
> My guess is that most car guys won't believe it.

Im sure you are right.

On 03/18/2011 04:03 AM, George wrote:
> ('04 Cavalier) I need to replace the disk rotors. Everyone hereabouts
> seems to have two varieties: one at $22-ish, the other at $38. The $38
> ones have a longer warranty, but only for "defects".
>
> My guess is, the $38 ones are 'better' (more metal?); but, it could be
> just charging more for a pretty meaningless warranty. Does anyone know?
>
> Thanks,
> George


you may not have much choice with even "genuine" g.m. product since so
much of their stuff is made in china, but here's the skinny on cheapo
brake disks:

1. bad castings can crack. brakes go through significant thermal
cycling during use. that means a lot of internal stress and if the
casting molds are not made right, there can be stress risers between the
disk faces like this:

http://www.flickr.com/photos/38636024@N00/2700630344/

vs.

http://www.flickr.com/photos/38636024@N00/2700630346/

it's not common because, frankly, not many people drive that hard, but
with the unradiused casting, your worst case scenario could be disk
separation. more commonly, you just get cracks migrate through to the
braking face.

[cheapo disks are made unradiused because the molds are easier, and thus
cheaper, to construct.]


2. metallurgy can be very variable. paradoxically, you're looking for a
casting with less metal, not more as a visual on "good". less metal
means the base is a stronger alloy and thus you don't need to make up
with a bulk of cheap material to compensate for flaws and achieve
minimum strength.

[the alloy is gray cast iron. it's called gray because under the
microscope, you'll see it's full of graphite flakes. once machined and
to the naked eye, some of this graphite smears on the surface and
creates the gray appearance. if it's "too gray", carbon content is too
high and you can get increased propensity to crack and lower friction.
but it's also cheaper. if it's too white, carbon content is lower and
it doesn't conduct heat as well. a good disk requires a fine balance
between the two.]

3. casting quality can be revealed in machining, so look out for areas
where there are surface holes [porosity], particularly tiny ones, and
also uneven color.

4. examine the outside circumference of the disk for where it may have
been machined for balance. balanced disks are always higher quality.


regarding brake shuddering during use, while brake disks /can/ warp
subsequent to heat treatment and machining, it's not actually very
common because the temperatures needed to cause it are very high and few
disks are operated at that level. ["apparent" warping visible on disk
lathes is more commonly a setting problem than actual material
distortion.]

more likely, it's an issue with the wheel/hub interface that can be
cured by scraping off surface rust from the hub wheel interface [and the
bearing carrier/disk interface], applying a little antiseize, and
torquing the lug nuts to spec in the correct sequence, and in a two or
more stage process. this is particularly relevant on modern lightweight
hubs where the hub can be elastically distorted by uneven torque because
it's so thin. the procedure above allows the hub to set evenly
minimizing elastic distortion, and thus run-out and will cure even
"badly warped" disks on problem vehicles like hondas without machining
or replacement.

this is also more of an issue on "modern" single piston calipers since
the mass of the moving parts is different on each side of the disk. you
can't even out the mass without using opposing piston calipers, but you
can mitigate the issue by maintaining the caliper's sliders so they can
float properly.


--
nomina rutrum rutrum

jim beam wrote:
>
> On 03/18/2011 04:03 AM, George wrote:
> > ('04 Cavalier) I need to replace the disk rotors. Everyone hereabouts
> > seems to have two varieties: one at $22-ish, the other at $38. The $38
> > ones have a longer warranty, but only for "defects".
> >
> > My guess is, the $38 ones are 'better' (more metal?); but, it could be
> > just charging more for a pretty meaningless warranty. Does anyone know?
> >
> > Thanks,
> > George
>
> you may not have much choice with even "genuine" g.m. product since so
> much of their stuff is made in china, but here's the skinny on cheapo
> brake disks:
>
> 1. bad castings can crack. brakes go through significant thermal
> cycling during use. that means a lot of internal stress and if the
> casting molds are not made right, there can be stress risers between the
> disk faces like this:
>
> http://www.flickr.com/photos/38636024@N00/2700630344/
>
> vs.
>
> http://www.flickr.com/photos/38636024@N00/2700630346/

The foundry is not the party that designs the rotor and decides whether
to put a radius or not. The foundry would prefer a radius but if the
customer says no radius then that is the way its done. Usually rotor
internal ribs do have a radius, but I don't see any evidence in your
picture that the no radius fins on that rotor are causing a problem.
In the rust belt where salt is used in the winter a big problem with
rotors is that rust builds up in the space between the internal ribs and
that rust blocks the airflow and cooling capability of the rotors.



>
> it's not common because, frankly, not many people drive that hard, but
> with the unradiused casting, your worst case scenario could be disk
> separation. more commonly, you just get cracks migrate through to the
> braking face.
>
> [cheapo disks are made unradiused because the molds are easier, and thus
> cheaper, to construct.]

Actually no. It is more difficult and expensive to eliminate the radius.
You just make stuff up to support your fantasies.



>
> 2. metallurgy can be very variable. paradoxically, you're looking for a
> casting with less metal, not more as a visual on "good". less metal
> means the base is a stronger alloy and thus you don't need to make up
> with a bulk of cheap material to compensate for flaws and achieve
> minimum strength.

>
> [the alloy is gray cast iron. it's called gray because under the
> microscope, you'll see it's full of graphite flakes. once machined and
> to the naked eye, some of this graphite smears on the surface and
> creates the gray appearance. if it's "too gray", carbon content is too
> high and you can get increased propensity to crack and lower friction.
> but it's also cheaper. if it's too white, carbon content is lower and
> it doesn't conduct heat as well. a good disk requires a fine balance
> between the two.]

You don't see many gray iron rotors nowadays. Back a few decades ago
there were some gray iron rotors in use.

Anyway, your analysis of gray iron is a bunch of made up hooey. but
since it irrelevant to modern rotors we won't go there.

>
> 3. casting quality can be revealed in machining, so look out for areas
> where there are surface holes [porosity], particularly tiny ones, and
> also uneven color.

Sure but with those sort of visible defects they will likely have been
already scrapped before they get to parts store.

>
> 4. examine the outside circumference of the disk for where it may have
> been machined for balance. balanced disks are always higher quality.

balancing would certainly cost more. Achieving balance thru good QA
processes also costs money.

>
> regarding brake shuddering during use, while brake disks /can/ warp
> subsequent to heat treatment and machining, it's not actually very
> common because the temperatures needed to cause it are very high and few
> disks are operated at that level. ["apparent" warping visible on disk
> lathes is more commonly a setting problem than actual material
> distortion.]

Rotor disks warp and wear unevenly and when they do that causes
additional warping and wear due to uneven loads and heat. So once it
starts it usually gets worse.


>
> more likely, it's an issue with the wheel/hub interface that can be
> cured by scraping off surface rust from the hub wheel interface [and the
> bearing carrier/disk interface], applying a little antiseize, and
> torquing the lug nuts to spec in the correct sequence, and in a two or
> more stage process. this is particularly relevant on modern lightweight
> hubs where the hub can be elastically distorted by uneven torque because
> it's so thin. the procedure above allows the hub to set evenly
> minimizing elastic distortion, and thus run-out and will cure even
> "badly warped" disks on problem vehicles like hondas without machining
> or replacement.

That is a fairly good description of one mechanism that will cause
warped disks.

>
> this is also more of an issue on "modern" single piston calipers since
> the mass of the moving parts is different on each side of the disk. you
> can't even out the mass without using opposing piston calipers, but you
> can mitigate the issue by maintaining the caliper's sliders so they can
> float properly.

I don't know why you think single piston calipers are more modern, but
you are correct that sticking calipers can quickly cause a tiny amount
of warp to turn into a greater amount of uneven wear and warp.

-jim



>
> --
> nomina rutrum rutrum

On 03/19/2011 09:54 AM, jim wrote:
>
>
> jim beam wrote:
>>
>> On 03/18/2011 04:03 AM, George wrote:
>>> ('04 Cavalier) I need to replace the disk rotors. Everyone hereabouts
>>> seems to have two varieties: one at $22-ish, the other at $38. The $38
>>> ones have a longer warranty, but only for "defects".
>>>
>>> My guess is, the $38 ones are 'better' (more metal?); but, it could be
>>> just charging more for a pretty meaningless warranty. Does anyone know?
>>>
>>> Thanks,
>>> George
>>
>> you may not have much choice with even "genuine" g.m. product since so
>> much of their stuff is made in china, but here's the skinny on cheapo
>> brake disks:
>>
>> 1. bad castings can crack. brakes go through significant thermal
>> cycling during use. that means a lot of internal stress and if the
>> casting molds are not made right, there can be stress risers between the
>> disk faces like this:
>>
>> http://www.flickr.com/photos/38636024@N00/2700630344/
>>
>> vs.
>>
>> http://www.flickr.com/photos/38636024@N00/2700630346/
>
> The foundry is not the party that designs the rotor and decides whether
> to put a radius or not. The foundry would prefer a radius but if the
> customer says no radius then that is the way its done. Usually rotor
> internal ribs do have a radius, but I don't see any evidence in your
> picture that the no radius fins on that rotor are causing a problem.

that's because i haven't posted a picture of it!



> In the rust belt where salt is used in the winter a big problem with
> rotors is that rust builds up in the space between the internal ribs and
> that rust blocks the airflow and cooling capability of the rotors.

which has nothing to do with casting process.


>
>
>
>>
>> it's not common because, frankly, not many people drive that hard, but
>> with the unradiused casting, your worst case scenario could be disk
>> separation. more commonly, you just get cracks migrate through to the
>> braking face.
>>
>> [cheapo disks are made unradiused because the molds are easier, and thus
>> cheaper, to construct.]
>
> Actually no. It is more difficult and expensive to eliminate the radius.

nope. to get the internal vents, the sand molds are made in two parts.
to get radii on both disk halves, you have to make both molds with
complex structure. if you don't radius, you only need one complex
structure, and you eliminate any need to align. it's a significant cost
saving.


> You just make stuff up to support your fantasies.

no, i just have some experience with iron casting.


>
>
>
>>
>> 2. metallurgy can be very variable. paradoxically, you're looking for a
>> casting with less metal, not more as a visual on "good". less metal
>> means the base is a stronger alloy and thus you don't need to make up
>> with a bulk of cheap material to compensate for flaws and achieve
>> minimum strength.
>
>>
>> [the alloy is gray cast iron. it's called gray because under the
>> microscope, you'll see it's full of graphite flakes. once machined and
>> to the naked eye, some of this graphite smears on the surface and
>> creates the gray appearance. if it's "too gray", carbon content is too
>> high and you can get increased propensity to crack and lower friction.
>> but it's also cheaper. if it's too white, carbon content is lower and
>> it doesn't conduct heat as well. a good disk requires a fine balance
>> between the two.]
>
> You don't see many gray iron rotors nowadays.

almost /all/ automotive rotors are gray iron. section, polish and put
under a microscope. you can see it for yourself.

http://www.scribd.com/doc/25391619/M...aphy-Cast-Iron


> Back a few decades ago
> there were some gray iron rotors in use.
>
> Anyway, your analysis of gray iron is a bunch of made up hooey. but
> since it irrelevant to modern rotors we won't go there.
>
>>
>> 3. casting quality can be revealed in machining, so look out for areas
>> where there are surface holes [porosity], particularly tiny ones, and
>> also uneven color.
>
> Sure but with those sort of visible defects they will likely have been
> already scrapped before they get to parts store.

yes you'd think, but you'd be surprised.


>
>>
>> 4. examine the outside circumference of the disk for where it may have
>> been machined for balance. balanced disks are always higher quality.
>
> balancing would certainly cost more. Achieving balance thru good QA
> processes also costs money.
>
>>
>> regarding brake shuddering during use, while brake disks /can/ warp
>> subsequent to heat treatment and machining, it's not actually very
>> common because the temperatures needed to cause it are very high and few
>> disks are operated at that level. ["apparent" warping visible on disk
>> lathes is more commonly a setting problem than actual material
>> distortion.]
>
> Rotor disks warp and wear unevenly and when they do that causes
> additional warping and wear due to uneven loads and heat. So once it
> starts it usually gets worse.

most disk brakes do not get hot enough to physically permanently warp.
to warp them, you need either out of plane loading while at a high
temperature, or a residual stress left over from casting which is then
raised to a high temperature. like cherry red. since almost all brake
disks are adequately heat treated after casting in order for them to be
machinable, the most common cause of brake problems is as described.


>
>
>>
>> more likely, it's an issue with the wheel/hub interface that can be
>> cured by scraping off surface rust from the hub wheel interface [and the
>> bearing carrier/disk interface], applying a little antiseize, and
>> torquing the lug nuts to spec in the correct sequence, and in a two or
>> more stage process. this is particularly relevant on modern lightweight
>> hubs where the hub can be elastically distorted by uneven torque because
>> it's so thin. the procedure above allows the hub to set evenly
>> minimizing elastic distortion, and thus run-out and will cure even
>> "badly warped" disks on problem vehicles like hondas without machining
>> or replacement.
>
> That is a fairly good description of one mechanism that will cause
> warped disks.
>
>>
>> this is also more of an issue on "modern" single piston calipers since
>> the mass of the moving parts is different on each side of the disk. you
>> can't even out the mass without using opposing piston calipers, but you
>> can mitigate the issue by maintaining the caliper's sliders so they can
>> float properly.
>
> I don't know why you think single piston calipers are more modern, but
> you are correct that sticking calipers can quickly cause a tiny amount
> of warp to turn into a greater amount of uneven wear and warp.
>
> -jim
>
>
>
>>
>> --
>> nomina rutrum rutrum


--
nomina rutrum rutrum

jim beam wrote:

> >> [cheapo disks are made unradiused because the molds are easier, and thus
> >> cheaper, to construct.]
> >
> > Actually no. It is more difficult and expensive to eliminate the radius.
>
> nope. to get the internal vents, the sand molds are made in two parts.
> to get radii on both disk halves, you have to make both molds with
> complex structure. if you don't radius, you only need one complex
> structure, and you eliminate any need to align. it's a significant cost
> saving.

You are about 100 years behind the times. The process you are describing
is how they used to manually mold sand cores. Even in China the
foundries are automated. Rotors made in China are not machined manually
either.

-jim

On 03/20/2011 06:39 AM, jim wrote:
> jim beam wrote:
>
>>>> [cheapo disks are made unradiused because the molds are easier, and thus
>>>> cheaper, to construct.]
>>>
>>> Actually no. It is more difficult and expensive to eliminate the radius.
>>
>> nope. to get the internal vents, the sand molds are made in two parts.
>> to get radii on both disk halves, you have to make both molds with
>> complex structure. if you don't radius, you only need one complex
>> structure, and you eliminate any need to align. it's a significant cost
>> saving.
>
> You are about 100 years behind the times. The process you are describing
> is how they used to manually mold sand cores. Even in China the
> foundries are automated.

no, i'm describing current process for the design and deployment of the
actual mold form. and the casting strategy.

whether a form is made by machine or by hand is a red herring. look up
"lost wax casting".


> Rotors made in China are not machined manually
> either.

red herring - machining is not casting.


>
> -jim


--
nomina rutrum rutrum

"jim" <"sjedgingN0Sp"@m@mwt,net> wrote in message >
> I don't know why you think single piston calipers are more modern, but
> you are correct that sticking calipers can quickly cause a tiny amount
> of warp to turn into a greater amount of uneven wear and warp.
>
> -jim

Remembering a few decades ago, a lot of calipers had dual pistons and
some even had four (Corvette and perhaps a few others). I would
guess that this added complexity, weight, and expense, and flew in the
face of the rule of thumb that the more working parts you have, the
more failures you will have.

I agree that one should clean the hub and the mounting face of the
disc to be sure that there is no runout due to dirt or rust. If you
really wanted to go down that road, you can try the rotor disc at
all five possible mounting positions and test the runout with a dial
indicator. I've never done this, but may take the time, if I ever have
to work on my own brakes again.

I've seen the ventilation channels get pretty dirty or rusty, but I never
took the time to clean them. Have you? Agreed that if you are going
to use cheap rotors, it might be easier just to buy new.

It is beginning to look like most brake mechanics these days are
parts changers, and many have no idea nor really care what makes
the rotors wobble. Not that parts changing is bad, mind you. It can
sometimes be the cheapest, quickest, and surest cure.

On Mar 18, 7:03*am, George > wrote:
> ('04 Cavalier) I need to replace the disk rotors. *Everyone hereabouts
> seems to have two varieties: one at $22-ish, the other at $38. *The $38
> ones have a longer warranty, but only for "defects". *
>
> My guess is, the $38 ones are 'better' (more metal?); but, it could be
> just charging more for a pretty meaningless warranty. *Does anyone know?
>
> Thanks,
> George

Another thing you need to consider when purchasing cheap rotors
(vented type) is the layout of the cooling fins. Usually cheap rotors
have fewer fins and are layed out differently, and they also usually
have thinner metal making them non turnable, but they are above the
minimum thickness. Same with the solid rotors they are usually just
thinner.

A lot of shops in my area usually purchase the cheap rotors, but put
the good pads on. They usually don't have any trouble with them with
this combination.