A friend's Audi sedan made some funky noises while braking.

Pulling the solid, one-piece, rear rotor revealed why. The outer surface
was fine, but the inner one showed the usual ~~smooth surface, and
several patches where the surface was anything but. It was as if the
surface was a plating or veneer....and flaked off. Underneath, the steel
was rusty and visibly porous. This was in several spots from dime to
quarter sized.

While none of us are 8-5/40Hr auto mechanics, between us we have multiple
engineering degrees, and many decades fixing our own cars; we've never
seen anything like such.

I'd assumed rotors are cast then turned, for both size and
thickness. True? Are they hardened at some point? What could explain
pores in such?



--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

Surface hardening due to excessive heat (read seizing caliper). The
rear calipers have park brake-actuated pistons. Seized park brake
cables will cause the rear calipers to stay applied. The inner pad
gets the brunt of the force.
JR
Dweller in the cellar

On Mon, 22 Jun 2009 01:12:11 +0000 (UTC), David Lesher
wrote:

A friend's Audi sedan made some funky noises while braking.

Pulling the solid, one-piece, rear rotor revealed why. The outer surface
was fine, but the inner one showed the usual ~~smooth surface, and
several patches where the surface was anything but. It was as if the
surface was a plating or veneer....and flaked off. Underneath, the steel
was rusty and visibly porous. This was in several spots from dime to
quarter sized.

While none of us are 8-5/40Hr auto mechanics, between us we have multiple
engineering degrees, and many decades fixing our own cars; we've never
seen anything like such.

I'd assumed rotors are cast then turned, for both size and
thickness. True? Are they hardened at some point? What could explain
pores in such?
--------------------------------------------------------------
Home Page: http://www.seanet.com/~jasonrnorth
If you're not the lead dog, the view never changes
Doubt yourself, and the real world will eat you alive
The world doesn't revolve around you, it revolves around me
No skeletons in the closet; just decomposing corpses
--------------------------------------------------------------
Dependence is Vulnerability:
--------------------------------------------------------------
"Open the Pod Bay Doors please, Hal"
"I'm sorry, Dave, I'm afraid I can't do that.."

JR North writes:

Surface hardening due to excessive heat (read seizing caliper). The
rear calipers have park brake-actuated pistons. Seized park brake
cables will cause the rear calipers to stay applied. The inner pad
gets the brunt of the force.
JR

Interesting theory. Thanks. I know the car's never had a seized caliper;
or parking brake, but it might have been dragged at some point. And, the
car does go to the track every so often.

That would say that the rotor could be turned until smooth and would be
OK. [They are way above the min. thickness spec.]



--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

We get some similar issues in the rust belt. Driving around all winter
in the salt spray leads to corrosion on the slide pins. This means the
piston side of the rotor takes all the wear, the opposite side gets
minimal wear. Then let the vehicle sit for a few days, you get heavy
rust pitting. In the winter, you never get to really hit the brakes hard
so you never really wear off the rust pitting. The fix is to lube or
replace the caliper mounting pins.

As for turning brake rotors, I've not had good results for high
performance vehicles. We usually see rotor warpage within a few thousand
miles. For track usage I'd go with new ones. I know, lotsa $$$ but it's
the only way to get the safety factor back where it should be. BTDT: ex
crew chief on a Porsche GT-3 Cup car.



David Lesher wrote:
A friend's Audi sedan made some funky noises while braking.

Pulling the solid, one-piece, rear rotor revealed why. The outer surface
was fine, but the inner one showed the usual ~~smooth surface, and
several patches where the surface was anything but. It was as if the
surface was a plating or veneer....and flaked off. Underneath, the steel
was rusty and visibly porous. This was in several spots from dime to
quarter sized.

While none of us are 8-5/40Hr auto mechanics, between us we have multiple
engineering degrees, and many decades fixing our own cars; we've never
seen anything like such.

I'd assumed rotors are cast then turned, for both size and
thickness. True? Are they hardened at some point? What could explain
pores in such?

RoyJ writes:

We get some similar issues in the rust belt. Driving around all winter
in the salt spray leads to corrosion on the slide pins. This means the
piston side of the rotor takes all the wear, the opposite side gets
minimal wear. Then let the vehicle sit for a few days, you get heavy
rust pitting. In the winter, you never get to really hit the brakes hard
so you never really wear off the rust pitting. The fix is to lube or
replace the caliper mounting pins.

I grew up in the rust belt and am WAY too familier with that issue.
Idle rotors do get surface rust, but it's gone after a day's use.

This is quite different. No seized pins, stuck calipers, etc.

I'll see if we can get pictures.



As for turning brake rotors, I've not had good results for high
performance vehicles. We usually see rotor warpage within a few thousand
miles. For track usage I'd go with new ones. I know, lotsa $$$ but it's
the only way to get the safety factor back where it should be. BTDT: ex
crew chief on a Porsche GT-3 Cup car.

He's replaced the rotors but we remain curious.
--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

On Jun 21, 7:12*pm, David Lesher wrote:
A friend's Audi sedan made some funky noises while braking.

Pulling the solid, one-piece, rear rotor revealed why. The outer surface
was fine, but the inner one showed the usual ~~smooth surface, and
several patches where the surface was anything but. It was as if the
surface was a plating or veneer....and flaked off. Underneath, the steel
was rusty and visibly porous. This was in several spots from dime to
quarter sized.

While none of us are 8-5/40Hr auto mechanics, between us we have multiple
engineering degrees, and many decades fixing our own cars; we've never
seen anything like such.

I'd assumed rotors are cast then turned, for both size and
thickness. True? Are they hardened at some point? What could explain
pores in such?

All my experience has been with cast IRON rotors, porosity is a
manufacturing defect, all too common on off-shored parts. I've never
seen steel rotors, the friction coefficient will be different and they
probably won't last as long. The graphite pockets in the cast iron
help with longevity. Only hardening that cast iron gets is chill from
the molds, if incorrectly cast, you can get hard spots. This leads to
problems if and when you try to get them turned. Stresses left from
casting will also lead to warping after getting heated up with hard
braking. Unfortunately, most of the aftermarket brake parts you can
get are from the Far East, mostly China and are made to the lowest
possible price. Leads to the situation where it's cheaper to go get
new rotors than to try to get the old ones turned true. Sounds like
you're not going to have that problem, new ones are in the cards no
matter what.

Stan

writes:


All my experience has been with cast IRON rotors, porosity is a
manufacturing defect, all too common on off-shored parts. I've never
seen steel rotors, the friction coefficient will be different and they
probably won't last as long. The graphite pockets in the cast iron
help with longevity.

This is the factory rotor in question.


--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

wrote:
On Jun 21, 7:12 pm, David Lesher wrote:
A friend's Audi sedan made some funky noises while braking.

Pulling the solid, one-piece, rear rotor revealed why. The outer surface
was fine, but the inner one showed the usual ~~smooth surface, and
several patches where the surface was anything but. It was as if the
surface was a plating or veneer....and flaked off. Underneath, the steel
was rusty and visibly porous. This was in several spots from dime to
quarter sized.

While none of us are 8-5/40Hr auto mechanics, between us we have multiple
engineering degrees, and many decades fixing our own cars; we've never
seen anything like such.

I'd assumed rotors are cast then turned, for both size and
thickness. True? Are they hardened at some point? What could explain
pores in such?

All my experience has been with cast IRON rotors, porosity is a
manufacturing defect, all too common on off-shored parts. I've never
seen steel rotors, the friction coefficient will be different and they
probably won't last as long. The graphite pockets in the cast iron
help with longevity. Only hardening that cast iron gets is chill from
the molds, if incorrectly cast, you can get hard spots. This leads to
problems if and when you try to get them turned. Stresses left from
casting will also lead to warping after getting heated up with hard
braking. Unfortunately, most of the aftermarket brake parts you can
get are from the Far East, mostly China and are made to the lowest
possible price.

Yes and no. Stick with Raybestos, Wagner, Bendix and your parts will
probably come from overseas, but will be to OE or better standards.
We have been there with the false economy of generic rotors, and we went
all Raybestos at this distributor. The difference is apparent.

David Lesher wrote:
writes:


All my experience has been with cast IRON rotors, porosity is a
manufacturing defect, all too common on off-shored parts. I've never
seen steel rotors, the friction coefficient will be different and they
probably won't last as long. The graphite pockets in the cast iron
help with longevity.

This is the factory rotor in question.



Doesn't mean much. GM, Ford and Chrysler ALL have had problems with the
cast iron rotors used on their vehicles. GMs had slag inclusions and
delamination issues as a result.
Ford had a similar problem with rotor rust causing problems
Chrysler as well had problems.

--
Steve W.

No, these are deep pits, major areas of the tin worm. Particularly
noticeable after driving on a snowy day then leaving town for a week or
two for someplace sunny.

David Lesher wrote:
RoyJ writes:

We get some similar issues in the rust belt. Driving around all winter
in the salt spray leads to corrosion on the slide pins. This means the
piston side of the rotor takes all the wear, the opposite side gets
minimal wear. Then let the vehicle sit for a few days, you get heavy
rust pitting. In the winter, you never get to really hit the brakes hard
so you never really wear off the rust pitting. The fix is to lube or
replace the caliper mounting pins.

I grew up in the rust belt and am WAY too familier with that issue.
Idle rotors do get surface rust, but it's gone after a day's use.

This is quite different. No seized pins, stuck calipers, etc.

I'll see if we can get pictures.



As for turning brake rotors, I've not had good results for high
performance vehicles. We usually see rotor warpage within a few thousand
miles. For track usage I'd go with new ones. I know, lotsa $$$ but it's
the only way to get the safety factor back where it should be. BTDT: ex
crew chief on a Porsche GT-3 Cup car.

He's replaced the rotors but we remain curious.

I'll see if we can get pictures.


Better late than never.

See http://www.panix.com/~wb8foz/rotor/

That's the inside surface of a rear rotor. Those are potholes you see;
not just rust on top. you can feel the edges. As I said; it was as if the
polished surface was peeling off.

--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

On Jul 29, 1:02*pm, David Lesher wrote:
I'll see if we can get pictures.

Better late than never.

See http://www.panix.com/~wb8foz/rotor/

That's the inside surface of a rear rotor. Those are potholes you see;
not just rust on top. you can feel the edges. As I said; it was as if the
polished surface was peeling off.

--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

Ouch!

Dave