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The Care And Feeding Of Brake Rotors

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Camo:
From the October, 2010 issue of Corvette Fever
By Richard Newton
Photography by Richard Newton




 
 
Eighty percent of the brake rotors sold in this country are made outside the United States. A lot of the companies making these rotors are only concerned about fit. Their thinking: If the rotor fits on your Corvette, then it's a good rotor. The actual performance of the rotor is secondary. Then we have any number of Corvette owners who shop for the lowest price.
This brake rotor thing has gotten so bad that rotor companies are starting to sue each other. The Bingham Farms-based Affinia Group Inc. and its subsidiary Brake Parts Inc. recently filed suit in U.S. District Court in Richmond, Virginia, claiming Dura International is falsely advertising its brake rotors as meeting the specifications of automakers. Dura responded by stating, "No government standards exist specifically for rotors. Each and every manufacturer is independently responsible for producing quality product meeting acceptable standards in fit, form, and function." This one could be in court for years.

Eric Bolton, a spokesman for the National Highway Traffic Safety Administration, said the agency is aware of the controversy and is monitoring complaints and field data through its Early Warning System. Right.
Even Raybestos has jumped in. It's running ads that show the difference between its rotor and the rotors produced by the competition. Basically what you have to remember is that just because a brake rotor fits your Corvette doesn't mean that it's the equivalent of the original rotor. There can be significant differences in quality.

Some of the cheap brake rotors are much thinner than name-brand aftermarket and original equipment rotors. These cheap rotors are being sold to unsuspecting consumers as standard replacement rotors. They simply can't absorb the heat that's generated from aggressive braking forces.

Increasing the air gap between the rotor faces often reduces the thickness of the discs in these lightweight rotors. This saves about 4 to 5 pounds of cast iron per rotor and reduces the manufacturing cost per rotor. Unfortunately, this trick also reduces rotor strength and its ability to absorb and dissipate heat. None of these things is good.



The arrows indicate the air gap in this brake rotor. This is a critical dimension. Cheap, inferior rotors will have a wide air gap, as less steel is needed for a rotor with a wide gap. For a manufacturer, less steel means significant savings over millions of brake rotors. For a Corvette owner, though, this is a problem. The rotor won't have enough steel to properly absorb the heat from stopping. It also means the rotor does not meet original equipment standards. If you're buying new rotors to replace your originals, compare the measurements. If a shop is doing the work, explain that you intend to compare the measurements before you pay the bill. You'll need to say this nicely, though, or you might have to find a new shop (which may not be a bad idea if they get upset over your knowledge of brake rotors)


Cheap rotors are made from damped steel. You can test them by hitting them with a hammer and listening for a ringing sound. You might have to tap on a few rotors to hear the difference, but once you hear it, you'll know what I mean. It's sort of like tapping on a forged crankshaft and a cast crankshaft. Keep in mind that original Corvette rotors are made from better steel than the rotors in an S-10 pickup. That may not be the case with offshore brake rotors. Try tapping on several brake rotors as a test.
Some of you don't actually get involved in the purchase. You outsource the task to your local shop. Most people have never given much thought to the quality of the brake rotors on their cars. I can almost guarantee you spent more time thinking about the price than you did about the quality. Not good.



The center hat here is aluminum. This means the rotor is much lighter than it would have been had the entire rotor been constructed of steel. Two-piece rotors are becoming much more common on Corvettes.



What Does a Brake Rotor Do?
Brake rotors provide a friction surface for the brake pads to rub against when the brakes are applied. The friction created by the pads rubbing against the rotor generates heat and brings the vehicle to a stop.

The basic scientific principle here is that friction between the pads and the rotor converts mechanical energy into heat energy. Just how much heat depends on vehicle speed, vehicle weight, and how hard the brakes are applied. For example, when I'm at Sebring my brake rotors normally reach about 1,200 degrees F.

Your Corvette brake system must be capable of absorbing a lot more mechanical energy (horsepower) than your engine produces. Remember the heat energy that's generated under braking happens over a very short period of time. Sixty to zero happens a lot quicker than zero to sixty. Corvettes stop faster than they accelerate. That means the brake system has to shed horsepower, or mechanical energy, very quickly.



The purpose of a brake system is to convert mechanical energy into heat energy and then dissipate all of the heat into the atmosphere. If you have a lot of horsepower you're going to have a lot of heat. That's why you see three rather large cooling hoses on this C6 racecar



Slotted or Drilled?
Drilled brake rotors became a huge fad a few decades ago. Today drilled rotors are generally considered a cosmetic item. They gained favor because of the belief that they prevented outgassing. Outgassing occurs at extreme temperatures when the bonding agents that hold the pad material together break down into a gas. This gas creates a pneumatic barrier between the rotor and the pad, reducing the amount of friction between them. Cross drilling, or slotting, creates a path for the outgassing that occurs during extreme braking conditions. These conditions are seldom reached on the street.

According to Baer Brake Systems, while cross-drilled or slotted rotors produce a strong visual appeal behind a modern open wheel, they have only a very slight performance edge when pad outgassing occurs. In other words, the holes don't matter all that much.

But rotors can crack under very intense heat. I cracked even my rear rotors before I gave up on drilled rotors. Basically you don't need drilled rotors on the street, and they don't work very well at the track. Got the idea here?
The best explanation of why you don't need drilled brake rotors is found in the advertising for Brembo, which states, "Brembo Sport drilled brake rotors provide excellent stopping power in everyday traffic, as well as more spirited, high performance street and highway driving." It then goes on, "Brembo Sport slotted brake rotors provide excellent stopping power in everyday traffic, as well as high performance street and track driving." Notice that there was no mention of using drilled brake rotors for track use?



This plastic lip (red arrows) helps cool the brakes by setting up a vortex in the wheelwell, which in turn sucks air out of the wheel. You can actually trace the airflow in this picture. The air is shoved into the backside of the brake rotor and is then forced out the vanes. From there the hot air flows through the spokes of the wheel and into the atmosphere.



Two-Piece Brake Rotors
There are two main reasons for choosing a two-piece rotor over a standard one-piece. The first is weight savings. A two-piece brake rotor's center aluminum hat reduces the overall weight by an average of 3 pounds versus a one-piece, cast-iron rotor.

Remember how I said earlier that lightweight rotors are generally cheap junk? Well, with a two-piece rotor you can have light weight and strength at the same time. The actual rotor can be substantial enough to dissipate heat properly, while the aluminum hat section saves weight. This is the best of both worlds.

The other big advantage of the two-piece rotor is that it's easier to vent air through the rotor's center. The ideal brake duct is one that pushes air into the center of the brake rotor, thus allowing the heated air to exit through the vanes. Two-piece rotors are ideal for this ducting and cooling.
Two-piece rotors can be very expensive when you first install them. Just the rotor alone for my C4 is $183.54. The hat section is going to be an additional cost, but only once. This means that a one-piece brake rotor is cheaper, but not by a lot. I might add that I've never cracked a Wilwood brake rotor, but then again I've never cracked a NAPA premium rotor. I have, though, broken just about everything else.

A lot people also prefer two-piece rotors for cosmetic reasons. The black aluminum center hat bolted to the rotor ring gives the brakes an aggressive, race look. Just be prepared to pay the price.



This is a front brake on a C3 Corvette vintage racer. Even though the rotor is much smaller than what we see in racing today, the braking principles remain the same. That's why you see the cooling duct, to blow heat into the atmosphere. Braking is all about energy conversion.



Rotor Preparation
No one talks about brake rotor prep, but it's a really big deal. You can't just take a brand new rotor out of the box and throw it at the car. Even worse is to take a used brake rotor and install new brake pads without any rotor preparation. This is just asking for trouble. You run a huge risk of brake squeal, and you're not getting the maximum effectiveness out of the new brake pads.

I sand brake rotors all the time. Any number of people will tell you it's unnecessary, but I've installed thousands of brakes on a wide variety of cars, and I can count the number of noise problems I've had on one hand.

Most suppliers of bench lathes and on-car lathes say sanding after turning isn't necessary as long as proper feed rates and sharp lathe bits are used. The finish should be within specifications and cause no problems. Proponents of sanding claim that sanding improves the finish by making rotors smoother. Sanding knocks off the sharp peaks as well as torn and folded metal left on the surface by the lathe bits. Sanding may improve the surface finish 2 to 5 microinches. This will give you better pedal feel, quicker pad seating, and improved overall brake performance.
Rotors can be sanded in various ways. One is to apply a non-directional finish with 80- to 120-grit sandpaper on a flat sanding block. Hold a pair of sanding blocks against the brake rotor for about 60 seconds while the rotor is turning at normal speed on the lathe (assuming you have a machine for turning brake rotors in your home garage).

For the rest of us, the most common method is to use an abrasive pad on a drill. This technique is risky because it's difficult to do an even job, and metal buildup on the pad may actually make the surface rougher unless a fresh sanding pad is used for each side of the rotor. If you do sand, do it carefully, evenly, and remember that you're not trying to remove metal. A minute or less on each side should be enough. You'll see the scratches develop as you use the sanding disc. You really don't want to remove any material; just simply scratch the surface.

Sanding becomes even more critical when installing new brake pads on old rotors. The surface of the brake rotor is coated with the old pad material. When you install new brake pads, the new friction material won't contact the rotor but will make contact with the old pad material. You want your new pad material to contact the steel of the brake rotor. Sanding the rotor surface assures that this will happen.



This is a carbon fiber brake rotor on a Pratt and Miller GT1 Corvette. This is the state of the braking art. A single rotor may cost more than your Corvette is worth.



Cleaning Brake Rotors
Very few people take the time to clean or wash a rotor before installing it on the car. Washing a brake rotor with hot soapy water can remove bits of metal that might otherwise end up embedded in the new pads. I finish this washing process with brake clean. If I rub a white paper towel on the rotor surface and dirt shows on the towel, the rotor isn't clean enough.



Slots are slots. It seems that every rotor company has its own version of slots. I've never noticed that the shape makes a whole lot of difference.



Break in New Brake Rotors
Some people call it rotor seasoning. I call it a break-in procedure. Gradually heating the brake rotor means that the crystalline matrix will reconfigure to relieve internal stresses. After these stresses are relieved the rotor is ready to accept the heat of normal braking. Heating the rotors before they're fully seasoned can result in material deformation due to the unrelieved internal stresses in the material. This deformation can cause a vibration from the brakes.

Find a safe location where you can bring your brakes up to operating temperature. The goal is to gradually increase brake temperatures with progressively faster stops. Start by performing four 60- to 70-mph stops. Do this the same way you would normally drive around town. Next, perform four medium-effort partial stops from 60 mph down to 15. Follow this with five minutes of driving with little to no braking to allow the rotors to cool.

Now perform four medium-hard-effort partial stops (about 75 percent) from 60 mph down to 15. Once again, follow this with 10 minutes of driving with little to no braking to cool the rotors. Now park the car and allow the brakes to cool overnight. You're almost done with the rotor bedding procedure.
On the next day, go back to your safe location where the brakes can be brought up to temperature. Once the brakes are warmed to normal operating temperature, perform four medium-effort partial stops from 60 mph down to 15. Follow this with five minutes of freeway driving with little to no braking to cool the rotors. Now make four medium to hard partial stops from 60 mph down to 15. Follow this with 10 minutes of freeway driving with little to no braking to cool the rotors.

Finally, make six hard partial stops from 60-plus mph down to 15 mph or until the rotors have reached an operating temperature of between 900 and 1,100 degrees F. If your Corvette doesn't have ABS, try to do this without locking a wheel. Follow this with 10 minutes of driving with little to no braking to cool the rotors.



The important item in this picture is the temperature strip on the caliper. This will give you an accurate reading of how hot your calipers are getting. The caliper temperature is a better indicator of brake fluid temperature than the rotor temperatur



One final note for those of you who run track events. Rotors need to be gradually elevated to operating temperatures before any severe use. Use the first lap of a session to warm the brakes as well as the engine, transmission, and tires.
When you come off the racetrack at the end of your session, drive around the pit area a couple of times to let your rotors cool. Race cars are normally placed on jack stands, or the air jacks are used, to get the same cooling effect. At track events, slowly doing a couple of laps around the pit area works nicely. Also, never use the parking brake at a track event. If you normally use the parking brake, put some duct tape on the handle to remind you not to use it at the track. The last thing you need to do is place a red-hot brake pad against a red-hot brake rotor.



This is a very basic C3 vintage brake system. The four-piston calipers work just fine if you keep fresh fluid in the hydraulic system and modern friction material in the caliper



Check Rotor Quality
Compare the thickness of the rotor plates to an OE or premium-quality rotor. If you really want to have fun, weigh the cheap rotor on a scale and compare it with a known high quality brake rotor. If the weight and plate thickness are significantly below the OE or a premium-quality rotor, buy a different rotor.

Measure run out in the rotor and wheel bearing with a dial indicator. On most vehicles, lateral run out greater than 0.002 inch will generate a pedal pulsation problem. Also, check the thickness of the rotor with a micrometer. The minimum thickness will be cast into the rotor.



Even solid brake rotors crack. That's how hot things get under racing conditions. This is a stock C5 brake rotor. The small cracks came from normal racing, but the rotor finally decided it had had enough. You'll probably hear a crack this big as you apply the brakes. This is why you need to inspect your brake rotors between every session. This applies to those of you who run track days, too. You're braking just as hard as a racer brakes



C4 Grand Sport wheels are very popular as race wheels. One reason is that the open spoke design allows the hot air from braking an easy exit



The idea is to direct the air into the center of the brake rotor. This hot air then exits through the rotor vanes and into the atmosphere.

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