Arjan B.

A often discussed item is 'can rotor warp or not'??

I had hughe vibratons in my steering wheel last couple of months, it p d me off that I installed new ones last week. I brought the old ones to the "Machine man" to check them. 0,6 mm out of line......... so the are warped.

The car drives comfortable as new, calipers work fine [needed to do some work on the pads before they fitted well] pistons go out and in easy. I tested the brakes for over a week very intensive and still no vibrations........

I thouht I would be interesting for everyone..........

burgass

What kind of pads do you use?
Cheers

Arjan B.

Porsche original pads.

joey bagadonuts

It was Carroll Smith who wrote about the misperceptions surrounding rotor problems.He explained that Apparently, measuring rotor thickness for variability does not accurately determine if a rotor is warped. Arjan, was this how your mechanic determined if the rotor was warped? What does "0.6mm out of line" mean?

Carroll's explanation

Oddjob

That is interesting. I would think that being 0.6mm from flat, you would be getting a vibration while driving, not just under braking. Was that the case? Were both fronts warped or just one?

Everyone correct me if Im wrong, but I think thats unusual for what Im guessing to be a street driven car.

By chance, were they worn well past the wear/thickness limit? Were the pads worn relatively even inside to outside, top to bottom, and axle to axle?

Our rotors are relatively thick and it will take a lot of abuse to damage them. And they usually show stress/heat cracking first.

Ive cracked through several sets of rotors and had various brands of pads score and leave deposits on rotors that caused a pulsing vibration while braking. But this was on track cars with competition pads. And the only time I ever warped a rotor was when I had a bad front wheel bearing (this was on a 944), and I think that one just wore unevenly; it didn’t actually warp from heat.

Ruairidh

Arjan - The explanation Joey quotes mirrors what I've been told on a number of occasions in relation to race cars - it is not warping rather uneven wear.

Arjan B.

Joey B.:

Out of line [ I have no other word for that] was ment: Install a rotor on a machine, take measurement gauge and install the finger of one gauge on the inside brake the rotor. Install the second gauge on the outside of the rotor and let the finger touch the outside bake area.

Same gauge's give a 0,6 mm difference if you rotate the the disk. A shimmy is the right word then I suppose.

I don't know what people believe what certain people say about warped rotors, I see at least once a week warped rotors, some little some really bad. Why are people machining disks when the never warp.........??
We have most of the time warped rotors with cars who have automatic transmission, people hold their brake at a traffic light after a hard brake action. The pad area doesn't cool as much as fast as the 'free' area of the rotor.

I totally agree that pads can cause certain dirt inside the surface of the rotor, oh yes that is a big problem sometime, but rotors can warp for sure, If you believe me or not, the 'un important Dutchman'..................

Some people will think that I am stubburn, but I really know where I talk about, almost 20 years of Automotive experience learned me a lot.

springer3

I agree other things can cause pulsing brakes, but anything made of steel can warp. I have seen warped rotors getting machined. The tool starts cutting in one spot, and will remove as much or more than Arjan's 0.6 mm before it starts to cut evenly around the disc. Cutting is usually a waste of time, as I try to explain below.

How does a disc warp?

Arjan's theory is one way. Brakes get red hot during hard used. Even very good steel is soft when red hot. If you keep the brake pads over the same place while the rest of the rotor cools off, the hot spot under the pads is put under compression by thermal shrinkage of the rest of the disc. In an extreme case, the hot spot is still red hot, and the compressive stress gets high enough to permanently yield the softer hot metal. When the hot spot finally cools, the metal thermally contracts enough to go into tension. The tensile stress exerted by the former hot spot is high enough to elastically distort the entire disc. A crack is the extreme case, where the local tensile stress in the former hot spot is so high that it exceeds the strength of the steel.

Another way to warp a disc is to get it red hot, and then splash water on one side. The cold side will undergo thermal shrinkage. The hot side does not have the strength to resist the force applied by the cooler side, and local yielding will occur. When the hot side finally cools, it will be in tension, and pull the disc out of flat. Warpage is almost always asymmetrical, so it causes pulsing brakes.

Machining rotors rarely succeeds in getting back to 100% flat. Machining removes steel that is under stress, and that changes the distortion with each cut. The only way to get back like new condition is to stress-relief anneal the part before machining. That would require removal of the disc from the hub, and would probably cost more than a new disc.

What can we do? First, don't get the brakes extremely hot when there are puddles to run through (good advice for a lot of reasons). Second, allow the car to roll a few feet after each high speed stop. That will prevent a hot spot under the pads.

pete000

I have had after market rotors warp very quickly. OEM ones seem to last much longer, but even they can warp. One trick is after hard braking never leave your foot on the brake if possible when you stop. This can cause warping to hapen. Cars with automatics tend to have more warping issues due to the fact you cant stay stopped with the brake off unless you drop it into "N"

Arjan B.

Springer, thanks for the reasonable support! It's thru what you say. Everytime a lot of people believe what some people write about certain experiences. As for the 'warped roter' story, liftime race experience or not, highly educated or not, I nor what I see, I know what I experience every week. People can call me stubborn or whatever they want. People in the racing world experience different things then in the ordinairy road cars. Ordinairy mom and dad brake more things on a car then you ever want to know. Extremely forced test by factory can not brake what citizen can do to a car, thus also for brakes.

Let me tell you one thing we had in a few months ago......

We had a 993 tip tronic in for fibrating brakes..... the guy held always the padal in front of a traffic light, and not selecting 'P' or 'N' to save his brakes.
He bought new front and rear rotors and pads at the Porsche dealer in March, in may we replaced them again in April/May , warped, so bad, the car was shaking all around while braking. Now he uses the 'P' or 'N' while waiting and he has no fibrations in the car when he brakes.

Let everybody believe what they want, I know what I see, and that's a lot of misery in 'Auto Land'


Have a nice Sunday!!!

springer3

Arjan:

You are not stubborn as most. You have facts and you know you are correct. Porsche has brillient engineering, and I know their brakes are the best available. However, they used cast steel for the 946 brake rotors, and any steel will warp if one spot is heated above the metallurgical transition that occurs about the same time the metal glows red. Iron workers take advangage of this phenoninon to straighted bent steel beams. Body shops use heat to shrink body panel areas stretched in an accident. Anyone can warp steel brake rotors if they trap a red hot spot under the brake pads while the rest of the rotor cools below the transition temperature..

Instead of shifting into P or N, why not stop 2 - 3 meters before absolutely necessary, and then let the car to roll slowly for a meter or so to prevent a hot spot under the pads? That is the technique I started to use after warping the rotors on my first 911. My wife and teenage drivers (sigh) also know to roll a little after any hard stop. We have not had a warped rotor on any car since starting this simple trick.

joey bagadonuts

I agree. I also value your contributions, Arjan, since most of it is based on firsthand experience.

My own experience with warped rotors mirrored those outlined in the Carroll Smith paper. I had terrible wobble while applying the brakes which I eliminated by using a more abrassive pad, Hawk Blues. So I share this information because it offers an alternative diagnosis to the problem and a relatively simple solution.

This may not be the correct answer in all situations, based on this discussion, but it is another explanation worth considering.

Bill Verburg

ALL brakes develop a series of hot spots due to thermoelastic deformation of the rotors. The first sign of permanent damage is the blue points which are a series of points where the cast iron of the rotor has been converted to cementite, a very hard mineral. The process is irreversible and will lead to further damage to the rotor and pad.


More information than most will want

Arjan B.

Oh yes, I still agree that pads can cause a kind od 'warped roter effect' by leaving material on the rotor behind. This was not in my case, I use original Porsche 964 pads front and rear, so this must be o.k.

Guys, thanks for the support and replies.

springer3

Correct, but by its very name, "thermoelastic" means that the deformation is elastic and therefore recoverable. Warping will occur only if the thermal stress rises above the yield point of the metal at the hot spot temperature. There must be plastic deformation to get warping.

I am not sure it is informative to throw out technical terms that only a metallurgist is qualified to understand. Cementite is a descriptive term for one of a number of microstructures in heat-treated medium and high carbon steels and steel alloys. It is normally produced only after heat treating and tempering. Brakes would have to be heated to around 1,800 F (1000 C), quenched, and then heated again to a lower temperature (tempered) to form cementite.

Changing the steel microstructure is an indication of seriously overheated brakes (close to white hot). Warping would be the least of your problems. You would have a cracked rotor.