TheDieselStop.Com Forums: The Warped Rotor Myth

OK, this isn't proofed.

OE Viewpoint

I respect Carroll Smith as a race car engineer, designer, and crew chief, especially since I've got 2 66 GT-350's. There is a lot of truth in a number of things that is said in his article, and some myth's as well. There is a lot that goes on within the friction material industry that is really not discussed outside of the companies, or published in SAE articles. The science is kept as a "black art" for competitive reasons. You have to sign some type of castration policy after you start. It was 25 years ago - I don't remember the details exactly.

As mentioned in the sales pitch article, I've never encountered "Warped" rotors in the way the consumer, or mechanics, perceive the term. You can stress a rotor to distort, install it with a high lateral runout due to machining tolerance stacks or a poorly machined rotor, have issues due to uneven disc castings (2 discs separated by cooling vanes assembled on a hub section is called the rotor), and a few other little funky things, but they don't warp from high temperature use, keeping the brakes on at a stop, or splashing from rain puddles.

All friction materials develop transfer to the rotor or drum. It has to happen to achieve full braking effectiveness. Cast iron (as mentioned, all rotors are made from this) by itself has a low coefficient of friction. By having a transfer layer, you develop adherent friction and a higher degree of stopping power (mue). Organic friction materials in the early 90's had a very bad case of friction transfer issues (example 90 Towncar) with caused pulsation due to the thick transfer layer of material. We were one of the many friction material suppliers tried on this platform that had the issue. It never should have had organic pads, but that was what the customer (Ford) wanted. We can be as stupid as the next supplier and make the sales until the next guy is chosen to fail.

It was actually kind of interesting. You had the driver do a sequence of specific stops until the transfer happened, then without another application, remove the rotor to see as well as measure the transfer. If you made one more brake application, the heavy transfer was gone. It was due to the rotors cooling faster then the pads, so an application with cool rotors / hot pads would generate the heavy transfer layer.

The organic compounds are only now used on light duty applications, and they are much different then they were. The majority of disc brake materials, and all on light and medium disc brake trucks, are semi-metallics. As a rule, they are very low in organic ingredients, such as cashew friction particles and the phenolic resins that bind the matrix together.

All friction materials imprint to the opposing surfaces. Always will. And this imprint wears away in short order. Friction materials are abrasive. Measure the amount of rotor wear by the time you start to feel pulsation. If this was the cause of the issue, you would feel the pulsation in short distances. You don't. You also don't keep stopping at the same exact place on the rotors. So if you ended up encountering the same conditions for a heavy imprint on the rotor, the statistics of it happening only at one point are far remote. It would occur around the entire surface. Also, many times the imprinting that one sees is actually iron oxide pitting from moisture being trapped between the pad and rotor for a number of hours when the vehicle is stopped, so don't freak out when you see this.

When cast, rotors are left in the sand form to cool down to alleviate thermal stresses. At least the OE and better aftermarket rotors are. I would never, never buy an off shore rotor. One thing that can lead to thermal stressing and the forming of carbide (hard spots) is uneven rubbing discs. You rarely find this is a Ford supplied rotor, but it has been an issue with other manufacturers (even with the same OE rotor supplier). Neon's were really bad. The thin section of the disc heats up faster then the rest of the rotor, but cools quickly. It's the rapid heating and cooling that forms the carbides.

So what type of driving conditions does pulsation or DTV normally happen in? It can happen for a number of reasons. But the majority of the time it is with vehicles that do a lot of highway driving, which is actually the coolest driving style you can have. Of course if your at GVW or towing through the mountains, you can be very hot, but vehicles that spend most of the time in city traffic usually have the hottest brakes. One or two highway stops are not going to do it.

For example of how hot our brakes get, here is what an F-250 fully loaded to GVW using the OE brake pads looks like when it runs the 2nd fade sequence during the NHTSA FMVSS 105 stopping distance certification test. First this vehicle is at GVW 8,800 lbs. The sequence of stops during this section is to run 3 baseline stops to establish the stopping ability of the braking system under average, moderate brake applications (200F Initial Brake Temperature; 40 mph; 10 fpsps (.3g) deceleration). As a reference, most people stop at 5-7 fpsps. Next, the fade sequence is performed with 15 stops from 60 mph to 0 mph; 15 fpsps; 30 sec intervals, with the first stop starting below 200F IBT. Basically as soon as you come to a full stop, you go wide open back up to 60 mph, then do the next stop. We buy a few transmissions. After the 15 stops, you immediately perform the recovery stops back at 40 mph and 10 fpsps, and compare them to the baseline stops to see if there is any delayed fade or other nasty characteristics. During the fade sequence your allowed to go up to 150 lbs pedal effort, but can't be below 5 fpsps deceleration during the last 5 fade stops.

2nd Fade sequence (You can look at it best at full screen, or better yet download it and print out the jpeg).

So why is this fool showing me this data. By reading the article, you think you get these brakes really hot. They do (the rotors are about 200F hotter then the pads), but think about the sequence that this vehicle just went through. Seven miles of 0 to 60 to 0 almost as fast as you can with a fully loaded vehicle. Now if this was a disc drum vehicle, like the pre 99's, you would be up in the 1,400F range - Rear drum brakes don't work hard. But coming off an off-ramp with brakes in the 100F normal cruising temp will only get you up into the 300-400F side of things.

What does get the vehicle into trouble is cold or off-brake rotor wear. When you going down the highway and one brake pad touches a section of a rotor due to some lateral rotor runout, that section wears down. Eventually there is enough wear that a difference of about 0.0005" exists between the inner and outer rubbing surfaces (where the inner and outer pads touch). This out of parallelism by itself can cause the pulsation that drivers feel.

Different vehicles have different sensitivities, even within a platform. The early 99 vehicles are more sensitive then the 01 vehicles. There are no magic parts that can be changed, it has to due with a number of items that are matured during production. A sensitive driver with a 99 may notice the issue with 0.0003" DVT, but the average driver / average vehicle will have the issue at about 0.0006" DTV.

Sounds like the best way to stop this is with rotors that run straight and true. Ford has tightened the machined specs of the hubs, rotors and wheels bearings over the years. It is rare for me to see a new production line rotor measuring over 0.0010" installed TIR. The service install spec is 0.0015". This is really one tight spec. It was not that long ago that rotors with 0.0050" TIR were considered OK. But even when you look at a spec of 0.0010", and realize that 0.0005" DTV wear can be felt, there is still a potential problem.

But wait a minute. Calipers pull the pistons back. It's called seal rollback (the piston square seal in the caliper's tangential groove rolls the piston back into the caliper on release) and it's usually about 0.0200". There's plenty of room between the pads and the rotor - 0.0100" on each side. In an ideal world.

First, the seal looses some elasticity over time and temperature. And there is some dirt and oxidation that can get under the boots. If you push in a new caliper's pistons with your thumbs, then try the same with an old used caliper, you'll see a big difference. Over time, that rollback may be down to 0.0100" or less. And a caliper or O-ring that was not made correctly can be worse.

But wait, 0.0100" is still plenty of room. If the sliding pins are working well the rotor will "knock back" the outer pad for clearance. If the pins hang up, the inner pad is getting all of the clearance, and the outer pad can touch the rotor.

The other factor here is the normal distortion of the wheel bearings and hub while the vehicle is turning and hitting pot holes. The rotor at the radius of the brake pads can move laterally by 0.0200" when new and to spec under these conditions. It will even move that much if the wheel / tire assembly is out of balance (there's a thought - gee, wouldn't the out of balance always be at the same plane of reference with the rotor, so the rotor keeps hitting the inner and outer pads at the same point going straight down the road). Of course there is also the acceleration force of moving that heavy caliper to one side when the outer pad and disc meet, throwing the caliper to the point that the inner pad hits the rotor, then it gets thrown back the other way. What would that do to rotor wear?

Also, these calipers use tension clips against the pad ends in the torque brackets to reduce noise. When dry and dirty, the pads may not slide back very well. In comes the concept of V-Springs about 2001. Still not as good as being cleaned and coated with silicon caliper grease.

Now that we have worn a thick and thin area of the rotor, constant stops from high speed will cause those thick areas to heat and cool rapidly, forming the carbides mentioned, along with the heat growth issue in that region. This is when you feel the pulsation getting stronger during an off-ramp brake application.

Your not going to machine those hard spots out and the pulsation can come back in a short time, sometimes right after you've left the shop. This is why some dealers do not turn rotors at all. A good mechanic who listens to the rotor being cut can tell if it has hard spots and will toss it if it does. Most will be taking the rotor off the other side of the vehicle while the rotor is being turned and miss it. Often in a machined or sanded rotor, you can see the hard stops as shiny areas.

I probably missed about a dozen things.

When the situation of organic material transfer came up, a number of companies developed ways of measuring the transfer layer. In some cases, doing the surface to surface thickness measurement, then dissolving the transfer layer or iron rotor away and measuring what remained. There still are aftermarket friction materials that have the issue, but your paying $20 for a set of those pads.

And abrasive is not always the best answer. Ask mechanics who worked on the 94-95 F-150's with the Performance Friction pads. Pads or rotors are not always the solution. And consider the out of balance tire situation would still cause more havoc with an abrasive pad.

Paul,

The Federal-Mogul OE pads are made in Orangeburg, SC, and the Federal-Mogul Motorcraft pads are made in Smithville, TN. Other then that, I get a higher voice.

Part 2, OE perspective.

I had mentioned that the disc / disc vehicles ran cooler rotors then the disc / drum vehicle would. The best example I can give in comparison to the earlier shown current F-250 disc / disc is an 1997 F-250 with disc / drum brakes. Here is how hot the front brakes can get with this vehicle running the same procedure. The braking bias is more the normal 70 / 30 that people are accustomed to. This is due to the fact that the vehicle has a mechanical proportioning valve. Rear drum brakes are self energizing in these applications (Duo-servo) and do not match up well in brake factor vs hydraulic pressure with a disc brake. A disc brake is linear, the duo-servo will get more aggressive at the higher pressures. Great if you are running 4 wheel manual brakes like in the 50's. Disc / drum is not a good marriage. And drums cool poorly.

For those not familiar with this, compare this and the earlier JPEG's hydraulic pressure graphs and you see that the rear brake hydraulic pressure is proportioned lower at the higher pressure applications with the disc / drum vehicle. This is due to prevent rear wheel skids under moderate to high stops and light load applications. You might say that with ABS on the rear, why is this a problem. A drum brake is slow to react in apply and release during an ABS stop. You have mechanical factors as well as normal hydraulic considerations.

With a disc / disc setup, you get good linearity. And with 4 wheel ABS and dynamic proportioning (controlled by the ABS system), you can run a more balanced system. Brakes last longer and the vehicle stops better under all conditions. You can still have software issues, but that's another problem.

Getting back to DTV. I don't have permission to show any equipment that my company uses, so here are some public web sites.

Here is the type of equipment used to measure rotors. This example is done off the vehicle with the rotor in a free state. Not my favorite. Basically we take the same equipment, but just set the probes in brackets that mount to the brake knuckles, measuring the rotors on the vehicle. This is important due to the distortions that occur in the rotor when it is mounted. Ah, lug nut torque and pattern.

If we work with a steel wheel, we can vary the torque from one (or two) nuts from the others. Each situation is different, but 10 lbs of differential can distort a rotor 0.0001"-0.0002". Depending on the assembled tolerance stack, and the amount that a shop mechanic impact hammers the lug nut, this can add up. Also tightening one nut after another in a circular pattern can throw a lot more stress and distortion into a rotor.

Aluminum wheels are different. They are inherently stiffer due to the design, so the nut to nut torque variation does not have as much of an issue. But due to the stiffness, it can whack out a rotor more if the nuts are tightened in the circular, not star pattern. As long as the rotor has not seen significant heat or use, loosening then properly tightening the nuts will correct the issue.

So why would we do this stuff. To run a DTV test of course. These are generally done in the Detroit area. If you run the 275 / 94 routes, often you will see vehicles with instruments on the dash. DTV tests run 24 hr. a day to complete 25,000 miles of testing in short order.

But before you run the test, you also need to check out the calipers for seal rollback and knock back (how well the slides work). You also check them at the end of the test, and sometimes during specified intervals. Not many dealerships have these.

jswilliams,

The pulsating squeak is from the rotor touching the pad at a certain point during the rotation. This is how you develop the thin areas resulting in the pulsation. If the rotor does not have a large amount of runout (over 0.0015" TIR), then the pins may not be working as well as they should, or a pad is hanging up in the tension clips. You probably do not have the V-Springs which push the pads away from the rotor. These were not available in 2000, and some mechanics toss them during a rebuild.

I am the worst person to ask what to do because I feel I can't tell without checking and measuring all of the above items mentioned in these posts. But if you have pulsing, the rotors need addressing.

The OE pads will not dust up like the current ones you are using. This formulation was first used on the Expedition. It had such a low warranty that Ford had us try it on this vehicle (successfully) and then the F-150 (successfully). It is low mue, low abrasion. However, with the new NHTSA FMVSS 135 rules for booster failure, most vehicles have to go to a high mue material. This results in more dust, as if you own a current 03 Excursion you know very well. It now has Bendix / Jurid up front. Owners come back after 400 miles complaining.

The best grease for the pads is the Ford Silicone Caliper grease, and I use it on the end of the pads and a layer of grease sandwiched between the inner pad and heat shield. The heat shield is the Stainless Steel shim that clips on the back of the inner pad to assist in keeping the brake fluid cool. Think Space Shuttle. Then rethink fluid boil.

Paul,

I have to get that part number tomorrow from work.

RichS,

I don't doubt that you can get fade, especially with aftermarket materials. Even if they come from my own company. And you always have to bring up the issues I don't want to discuss.

All brake materials need bedding or burnishing in. You need to change the composite material (even low organic) into a graphitic composite at the pad / rotor interface, and get a transfer layer on the rotor. Your situation is not normal driving and here I will lend more credence to Mr. Smith's story. [Before I get hammered here, let me state they is never an absolute in brakes].

You may have had a hard spot in those rotors already (if they were reused) or due to one condition or another, you developed a hard spot. Can also be in the drums. Drum linings are very organic and will easily follow Mr. Smith's transfer / hard spot development. Heavy transfer will also get the drum brake very aggressive, but so will a poorly ground aftermarket drum shoe.

But JUST FOR YOU, I've added another graph. Or two. Here's what you went through. Your driving, and driving, and going fast. Your going through the mountains like a Bat out of Hell. Your running the FMVSS 105 fade sequence, just like it was conceived. High thermal energy dissipation with short cooling times. Hello Mr. Fade. And your screaming .....

I'll show an example of a disc drum C-2500 pickup running the first fade section with a well known and not really cheap aftermarket material. I'm not going to name who's it was, but we run not only our own, but do competitive analysis as well. But first, you need to look at what an FMVSS 105 test lays out. The example here states the pass car distances (light truck and heavy truck are longer) and also shows the new current FMVSS 135. It's what I got on my computer, your getting it.

Now this vehicle has gone though a whole lot of high energy stops (most GVW) - 30, 60, 80 mph. 200 brake applications from 40 to 0 mph, again GVW 8800 lbs. And a series of partial hydraulic system stops (Front Brakes Only; Rear Brakes Only - GVW & Light Weight)have stopped the vehicle. The pads and shoes should be well burnished or bedded in by now.

Not really as shown by this first fade . By the 4th stop in the 10 stop sequence, the front brakes are at about 600F. Keep in mind that the first stop only had a temp rise to about 350-400F, which is the max the brakes experienced during ALL of those previous stops. During this 4th stop, you can see the in stop pressure characteristic change from an increasing mue (decreasing pressure, decreasing pedal effort while maintaining even deceleration) condition to fade with increasing efforts by the driver. As the stops progress, so does the effort in pedal force and hydraulic pressure.

Now the pedal travel graph ends up during the last 5 fade stops showing the brake pedal about 1/2" from the floor. But if you look at the front pressure traces, you can see that the pressure falls off even though the driver increase the pedal effort. The front pads have compression (deformation) and used up all the available displacement out of the front brake master cylinder chamber (you have front and rear on this vehicle). Now our driver is trained that he still is working off the front brakes, but most civilians running in the hills think they are on the floor with the brake pedal and give up. Notice that the vehicle maintains a good amount of deceleration during all of the stops since the driver keeps up the effort.

Now according to the FMVSS rules, this passes requirements because the driver never exceeded 150 lbs pedal effort, and the last stops of the fade never went below 5 fpsps. In 25 years, I've only known of one OE situation where this was deemed acceptable. Here's the kicker. After performing 35 burnish stops to clear off the charring that occurred during the 1st fade, the second fade is run and shown here.

So the reason I don't usually answer brake questions in detail is because I don't know how without doing a thesis.