Drill holes and slots in rotors can both improve braking, but under different braking scenarios. To understand how these rotor modifications can improve stopping power, it is first necessary to understand the three forms of heat transfer:
All three methods of heat transfer occur when you apply the brakes. During a typical stop, the heat transfer is about 25% conductive, 35% convective, 40% radiation. For a high temperature, high speed stop, the heat transfer is about 15% conductive, 40% convective, 45% radiation. At high speed, convection heat transfer is very important. This is why drill holes can help reduce the brake temperaure. The drill holes help air flow through the vanes. The brake temperature can drop up to 180 degrees. Brake pads work better at lower temperatures, and you reduce the risk of pulsating brakes as well.
Slotted rotors do not improve any heat transfer. However, the slots can improve brake output by removing gas and dust that is trapped between the pad and rotor. This gas and dust reduces the friction force by preventing the pad from fully contacting the rotor.
Given the choice between drill holes and slots, the drill holes will give you better braking power over slots for normal city/highway driving. This is why high end BMW, Porsche, Corvette, and Mercedes rotors are drilled, not slotted. However, for track racing (high speed stops), slotted rotors are the better choice.
Brake pads contain various metals and minerals that are bound together using a resin. At high temperature, this resin turns to liquid and can bleed out onto the rotor. Glazing is dangerous because it prevents normal contact between the pad and the rotor so you lose pad bite. Glazing can also cause brake pulsation most commonly called "rotor warping." Drilled rotors prevent pad glazing as reported in SAE paper 2006-01-0691. This is one of the reasons why drilled rotors provide 12% to 37% more brake torque over stock rotors.
Pad bite is called the coefficient of friction. At high temperatures, all brake pads have a reduced coefficient of friction. This is called brake fade. Power Stop engineers all of their friction compounds to resist brake fade. This assures safe, consistent stopping power. Different grades of pads will have different fade characteristics. The Z36 pads are by far the most fade resistant pad that is commercially available.
All rotating objects have vibration. This vibration can become amplified at specific natural harmonic frequencies. Have you ever noticed a little vibration in your car as you travel at a certain speed on the highway, and then it goes away as you go a little faster? The wheels were passing through one of these harmonics. Brake noise is caused by vibration primarily from the rotor, pad and caliper. The causes are complex and include things like the compressibility of the brake pad, the surface finish of the rotor, the stick-slip oscillation at certain temperatures, rotor dimensional run-out, rotor harmonic frequency and many other factors. Noise attenuation is a complex part of friction development. Power Stop Evolution and Posi-Mold pads feature dual active rubber coated constrained layer shims. The rubber is placed on both the pad side and caliper side of the shim. This helps reduce vibration that can be transmitted from the pad to the vehicle. Noise can be reduced by putting a silicon based material on the back of the plate where the caliper piston makes contact. It is also important to turn the rotor and make sure the rotor flange is parallel to the hub mounting surface. One sure fire way to make noise is when a pad overhangs the edge of the rotor or encroaches on the groove between the rotor hat and smooth flange surface. It is important that you check how the pad seats on the rotor when you change the brakes.
It is hard to imagine the energy required to stop a vehicle traveling at 110 mph. Kinetic energy is 1/2 mass times the velocity squared KE = mv2/2. Stopping a 2500 lb car requires 352,800 ft-lb of energy, the equivalent of dropping the car from an 11 story building. When you stop, almost all of this energy is converted into heat through the rotor in a very short time. Brake Rotors are designed to get hot, often in excess of 1400 degrees F. Contrary to popular belief, the temperature does not cause the rotors to warp, and this is not the reason why you get pulsating brakes. So what causes this brake judder?
The answer lies in understanding how friction works, and how high spots form on the rotor to cause the pulsating brakes. There are two primary mechanisms of friction at work during the stop: 1) adherent friction and 2) abrasive friction. For adherent friction, a thin layer of material is continuously transferred between the brake pad and the rotor. The breaking of the chemical bonds creates a resistant force to stop the vehicle. Abrasive friction is the mechanical wearing of the rotor and friction surface, like sand paper on wood. Brake pads use both mechanisms, but at higher temperature, adherent friction is what stops the car. The brake pad deposits material on the rotor, and as the pad gets hot, more material gets deposited. These deposits are caused by the resin (glue) in the pad. All brake pads use a binding resin that holds the friction component materials together. This resin turns to liquid as the pads get hot, and the resin material along with some other material in the pad gets deposited on the rotor. On overheated rotors, sometimes you can see the glazed resin material. It can look like someone brushed syrup on the rotor. The resin glazing forms a high spot on the rotor surface.
Another way to form a high spot on the rotor is if you clamp down on the brake after a hot stop. When the pad gets hot, it wants to transfer friction material to the rotor and it does not care if the rotor is stationary or rotating. If you are at a stand-still, the pad will imprint material on the rotor in the shape of the pad. The next time you brake, as the pad passes over the high spot, the temperature increases relative to the rest of the rotor. This causes more material to get deposited on this spot until it gets high enough to cause pulsating brakes. It does not take a lot of material to cause this brake judder, only .0007 inch is enough to start to feel the pulsation. Power Stop friction is engineered with lower resin content to maintain a uniform boundary layer of friction on the rotor. Another way to prevent glazing or non-uniform friction deposits, is to install drilled rotors. Drilled rotors help keep the pad cooler to prevent pad glazing.Key Steps To Prevent Brake Pedal Pulsation:
In order of effectiveness: 1) Get really good tires that grip the road, 2) select the right brake pads with a high coefficient of friction, 3) use cross-drilled rotors to improve convection heat transfer.
Please avoid using DOT 5.0 synthetic fluid unless specifically recommended by the vehicle manufacturer. Some synthetic fluids can cause rubber piston seals to break down. Power Stop recommends replacing brake fluid on every pad change with a DOT 3.0 or 4.0 fluid as specified in your owner's manual.
Power Stop does not recommend turning down the rotors. However, drilled or slotted rotors can be machined using a sharp bit and very light cuts (to prevent bit skipping near the holes). Under no circumstances should a rotor be turned below the hole chamfer depth.
The silver or gold zinc dichromate plating will be removed by the brake pad contact. So the area underneath the pad will be natural gray iron after the first few stops.
The purpose of painting or plating a rotor is to resist corrosion. Rotors can get hot, more than 1,000 F. At these temperatures, paint will peel or burn off. Zinc-dichromate plating is more expensive but it lasts much longer and keeps the rotors looking new for a long time.
The term Thermal Scorched brake pads refers to the curing of the first millimeter of the brake pad material in an oven. This process actually simulates a professional high-speed break in procedure used by professional mechanics and race teams. It heats the surface of the friction material to a very high temperature, which cures the compound and makes sure it is 100% ready to perform. This process makes the coefficient of friction consistent and predictable right out of the box; it also prevents pad glazing. Many manufacturers do not go through this extra step, but Power Stop knows the importance of proper break-in of new brake pads, so they make sure that the pads are thermal scorched during the production process.
If the engineers of your vehicle decided to use aluminum for the body of the calipers on your vehicle, they were probably more focused on performance than they were on the cost to manufacture. Aluminum calipers enhance the performance of a vehicle by reducing what they call un-sprung weight, which helps acceleration and handling. Aluminum calipers also retain a cleaner look, as they are less prone to corrosion. You have already seen rusty cast iron calipers on other vehicles. There are really not many downsides of having aluminum calipers on your vehicle aside from cost of manufacture and care that needs to be exercised while installing them. Other than those 2 items, the life expectancy and their function are pretty much the same as their cast iron counterparts.
One thing to keep in mind is that aluminum is much softer than cast iron or other metals that calipers are more commonly made with. The main precaution that you need to take when installing aluminum calipers is to make sure to only finger-tighten all fittings (Banjo bolt, bleeder screw, etc),
and then use a torque wrench to finish-tighten to the vehicle manufacturer’s specifications.
This is very critical because those fasteners are made from hardened steel and will very very easily strip out threads in an aluminum caliper if overtightened.
IMPORTANT: BREAK IN NEW BRAKE PADS/ROTORS USING THE PAD BEDDING PROCEDURE AS FOLLOWS. PROPER PAD BEDDING CAN PREVENT ROTOR WARPING.
The break in procedure is critical to brake performance. The reason for a proper break in is to establish an even layer of friction material deposited on the rotors from the brake pads. It is very important that this initial layer of friction material is evenly distributed.
Break in the pads as follows:
PLEASE NOTE: ALL VEHICLES ARE DIFFERENT. THE PROCEDURE BELOW IS INTENDED AS A GENERAL GUIDE AND IS NOT A SUBSTITUTE FOR THE VEHICLE SERVICE MANUAL WHICH SHOULD BE REFERENCED THROUGHOUT THE PROCESS. THE IMPORTANCE OF PROPER BRAKE FUNCTION CANNOT BE OVERSTATED! IF YOU ARE UNSURE OF YOUR ABILITY TO PROPERLY PERFORM THIS PROCEDURE STOP AND CONSULT A PROFESSIONAL TECHNICIAN.
What is brake bleeding? Bleeding the brakes is a process by which fluid is forced through a hydraulic system with the intent of purging trapped air and/or replacing the used fluid with new.
Why should I bleed my brakes? Liquid cannot be compressed which makes it ideal for applications like vehicle brakes where tremendous pressure must be generated. Air on the other hand is compressible and when it is introduced to a hydraulic system the result is a loss of efficiency. As a driver you feel that inefficiency as a spongy or mushy brake pedal. Inefficient or soft brakes are dangerous and need to be bled to remedy the problem.
Brake fluid is also hygroscopic by nature, which means that it absorbs water. Replacing your brake fluid regularly will ensure that moisture doesn’t build up and corrode your lines from the inside out. Rust can also clog the small orifices in the system which can cause stuck calipers, non-functioning calipers, clogged proportioning valves and other issues. Moisture in the lines also poses a problem because of its 212* F boiling point. Brake fluid can easily exceed 300*F during normal operation which boils water and creates vapor pockets in the fluid.
For the above reasons, it is good practice to bleed your brakes during routine brake service and flush the system at least every two years as part of your normal vehicle maintenance. More aggressive drivers should bleed the brakes annually and racers should bleed their brakes before every event.
Please note that you will require an assistant for the bleeding process.
Put on your safety glasses and gloves and refer to your vehicles service manual for the proper lifting and support techniques. Once the vehicle is supported properly remove all four wheels to gain access to the brake calipers. The bleeder screws are always located at the top of the caliper and are usually covered with a rubber cap. Most vehicles only have one bleeder screw per caliper, however some high performance multi piston calipers may have two or more.
WARNING! BRAKE FLUID IS VERY CAUSTIC AND WILL LIFT VEHICLE PAINT LIKE CHEMICAL STRIPPER. TAKE GREAT CARE TO AVOID SPILLS OR DRIPS AND IF IT COMES IN CONTACT WITH YOUR PAINT AT ANY TIME REMOVE IT QUICKLY AND CLEAN THE AREA THOROUGHLY.
Before you go any further, locate your vehicles brake fluid reservoir (usually atop the master cylinder) and check the fluid level. Make sure the fluid is at the MAX fill line and monitor the fluid level throughout the bleeding process to ensure it does not dip below the MIN fill line. If the fluid in your reservoir goes empty while you’re bleeding the brakes, it will complicate the process and require a more involved procedure to correct the problem. So pay attention and check it often!
You always want to bleed the lines starting with the caliper furthest from the master cylinder and work your way forward. On most cars the process goes; RR, LR, RF,LF. Starting with the first caliper, place the flare nut wrench of the proper size over the bleeder screw and attach the hose. Secure the waste container and run the end of your hose to it. Now ask your assistant to enter the vehicle and turn the key to the II position (don’t start the car).
A traditional two person bleeding procedure relies on good communication and timing to go smoothly. Instruct your assistant to depress the pedal several times then hold it down. Your assistant should give an audible confirmation once the pedal has been fully depressed. Then open the bleeder screw a quarter to half turn briefly and let the fluid fill the attached hose. If your system has air in it you will see bubbles come out with the fluid as well. The bleeder screw should only be in the open position for a brief moment. As pressure drops the flow will slow down, as it slows close the bleeder screw. After you have snugged the bleeder screw (don’t over tighten!) tell your assistant to release the brake pedal. Stress to your assistant the importance of not releasing the pedal before you give confirmation that the bleeder is closed. Doing so will draw a large amount of air in to your system making your job more difficult. Observing the fluid as it comes out for any sediment or debris will give you an idea of the condition of your brake lines, ideally you don’t want to see any. Repeat the process until air bubbles are no longer visible. If you are flushing your system, continue till the fluid turns clear or the color of the new fluid being used.
Work your way around the vehicle one caliper at a time until all air or old fluid has been purged. Use some brake cleaner and a towel to wipe any spilled brake fluid off your calipers and cover the bleeder screws with the rubber caps you removed earlier.
Do a final check on your fluid level and cap off the reservoir. You can now replace your wheels and lower the vehicle. Be sure to tighten your lug nuts to the proper torque and in the proper sequence (progressive star pattern).
Start the vehicle and while in park/neutral depress the brake pedal. After a couple pumps the pedal should feel nice and firm. Check for leaks at the calipers and if everything looks good perform a couple low speed stops to ensure the brakes are functioning properly.
Congratulations, you now have a properly bled brake system that will provide confident, consistent and reliable stopping until your next service interval!
The installation guide is for reference only. Please refer to the vehicle’s service manual or professional installer for complete instructions.
Ninety percent of the brake pad changes you make during the life of your vehicle will be to the front brakes because they do 60% to 70% of the braking. On most cars, you can look through the openings on the outside of the wheel/rim assembly to see the pads. On some vehicles, you may have to remove the wheel to see the pads. Look at it from above or the side. The pad will be pressed against the shiny metal rotor. The best way to inspect a pad is through visual inspection. If the pad depth is less than 3/16 inch, plan on replacing it soon. If it’s less than 1/8 inch, you should change the brakes immediately.
You can also tell if the brakes should be replaced if you hear noise from the wear sensor on the pad. However, some parts do not have wear sensor clips, i.e. noisemaking clips that are mounted on the brake pad to tell you when the pads need to be changed. If you hear a screeching sound when you apply the brakes, then you are due for a brake change. (If the sound is more like a metal rasp or grinding sound, then you’ve already damaged your rotors and need to fix the brakes immediately.)
STEP 1: Have the following parts available before you start the brake change: 1) Power Stop brake pads such as the Evolution ceramic pads, 2) Power Stop cross-drilled or slotted rotors, 3) a bottle of DOT 3 or 4 brake fluid as recommended by your owner's manual, 4) brake pad grease 5) a c-clamp. You should also have a car jack and jack stands.
STEP 2: Block the rear wheels so the car won't roll once you jack it up. Put the car in park and set the parking brake firmly.
STEP 3: Before jacking the car up off the ground, loosen the lug nuts on the wheels just enough to break them free. Work them off just enough until they loosen their resistance and become easy to turn with the tire iron. Raise the car and support with safe jack stands under flat spots on the frame. Usually the frame support area is immediately to the rear of the front wheels.
CAUTION: Always use jack stands. Never attempt to work on an elevated vehicle held in place only by a hydraulic jack.
STEP 4: Remove the lug nuts and the wheel. It is best to work on one wheel at a time, leaving the other side intact as a point of reference. As a safety precaution, roll the wheel/tire assembly under the front-center of the car, between the jack stands, and set it down beneath the engine's K-member. In the event of a faulty jack stand.
STEP 5: Review the brake components. A disc brake assembly is composed of a caliper, two brake pads, and a rotor. The caliper works on pressurized brake fluid through a piston in the caliper. The caliper has a fixed part bolted to the spindle knuckle and a slide part that holds the brake pads. The caliper is mounted with two bolts. These bolts usually have dust boots. When the brakes are applied, the caliper piston squeezes the pads against the rotor creating friction.
NOTE: If you are not replacing or turning down the rotor, then install 2 lug nuts backwards to hold the rotor in place when removing the caliper (fig. 2).
STEP 6: Remove the two bolts that hold the two parts of the caliper together (fig 3). Gently slide it out and away from the rotor. Hang the caliper inside the wheel well using a coat hanger, so that the hose is not stressed. Do not let the caliper dangle from the brake hose line. Inspect the inside of the caliper and remove the brake pads. Remove the two remaining bolts that hold the caliper slide to the knuckle. Use a wire brush to clean rust from the rails where the pad contacts the caliper.
STEP 7: With the caliper out of the way, remove the rotor. Sometimes the rotor rust will make it bind and you will need to use a mallet to loosen it. Tap both the front and backside alternating left and right, top and bottom of the rotor. The rotor thickness should be measured and compared to the minimum discard thickness that is etched on the rotor. If the thickness is less than this minimum, then the rotor should be replaced.
STEP 8: The next step is to move the piston back inside the caliper. The piston has extended as the pad material wears. With new thicker pads, you must return the piston back inside the caliper body to give the thicker pads room for installation. First crack open the bleeder screw on the caliper to allow brake fluid to be relieved. Place a container under the caliper to collect the fluid. Get a large C-clamp, place the used brake pad over the face of the piston to protect the surface from marring, and work it back that way. As you turn the handle on the clamp, it will increase pressure on the piston, until it becomes flush with the surrounding metal. Brake fluid will be released through the bleeder. Then loosen and remove the C-clamp. Close the bleeder screw for now. If you cannot open the bleeder screw, then push the C-clamp in slowly to prevent unsafe back pressure and damage to the ABS modulator, brake valving or master cylinder. It may be necessary to drain some fluid from the master cylinder reservoir.
NOTE: many rear brakes pistons cannot be retracted with a C-clamp as they screw in and out. This type of rear brake piston will have two recessed notches where you can use a set of needle nose pliers to retract the piston.
STEP 9: Check and replace all hardware as needed. Improper hardware can lead to noise or poor brake pad performance. Clean the caliper rails or hardware slides where the pads make contact. Clean the hub-mating surface with a wire brush. Rust or debris on the hub can cause rotor run out and lead to wheel vibration.
STEP 10: Resurface the rotor or replace the rotor. Most auto retailer stores provide rotor resurfacing. After machining, use a 120-grit sandpaper on the rotor in a light circular motion to give a non-directional finish. Clean the rotor with mild soap and wipe clean with a lint-free cloth. Do not use petroleum-based cleaners.
STEP 11: Install the new rotor and remount the caliper bracket (not the piston part) to the spindle knuckle arm. Use brake grease to lubricate the edge of the brake pad plate where it touches the caliper. Do not put grease on the friction material. The lubricant goes between the plate and the piston or caliper back. Don't over- look this. If you don't do it, you may get brake noise when you apply the brakes.
NOTE: When you install new rotors, it is recommended that you check rotor run out. Rotor run out can cause brake pulsation. First tighten the stud nuts to the manufacturer specification using spacers as required. Mount a dial indicator and rotate the rotor while measuring the run out. Run out should be less than +/-.001 inch. If run out exceeds this then mark the high spot, remove the rotor and index it two studs and check to see if the high spot moved. Make sure that the hub and rotor-mounting surface is clean.
STEP 12: With the fixed part of the caliper bolted to the spindle. Reinstall the brake pads in the caliper slide, and make sure that they are pressed back to leave clearance for the rotor to slide between them. Slide the caliper over the rotor and line up the bolt holes. Bolt the piston part of the caliper to the caliper bracket.
NOTE: sometimes the rubber boot will extend the bolt and cause interference during installation. Just push the rubber boot back to allow the caliper to slide over the rotor.
STEP 13: Bleed the brakes to remove air from the brake line. First make sure the brake fluid is full to the top. Have a buddy press down slowly on the pedal as you monitor the bleeder screw. When you see a constant stream of fluid, close the bleeder screw. It usually takes three pumps of the pedal to clear the air out of the line. Check the master cylinder reservoir and replace brake fluid to the MAX line. Do not overfill the reservoir. Pump the brakes several times to seat the brake pads to the rotor and check the reservoir one more time.
STEP 14: Replace the wheel and bolts. Drop the car to the ground to finish tightening the bolts to the manufacturer's specified bolt torque. Alternate tightening sequence by going to the opposite side of the hub. A torque gun is not recommended.