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All brake pads must be bedded-in with the rotor surface they are mated with. The bedding-in process involves a gradual buildup of heat in the rotors and pad compound. When properly done, the bed-in will distribute a thin layer of the brake pad compound, called transfer film, onto the rotor surface. This transfer film forms a protective coating on the rotor surface where it comes into contact with the brake pads. As it heats up, the film becomes sticky and allows the pad and rotor to generate the friction and grip required to stop the vehicle without creating hot spots on the rotor surface. The bed-in process normally takes about 300-400 miles of normal driving to complete.
When brake pad compounds are overheated, the components crystallize on the surface as the resins and binders begin to melt and break down. This crystallized surface has a lower friction coefficient that simply means it cannot grab the surface of the rotor with the same amount of force as the undamaged pad. The term glazed is used because the pad surface will look smooth, shiny, and metallic like a glazed ceramic tile. Once a pad has been glazed, it cannot be repaired and it needs to be replaced. Glazing is caused by overheating the brakes beyond the operating temperature designed for the pad and/or failure to follow the bed-in procedure for the pads. This type of damage is not a defect in the pad and is not covered under the manufacturer warranty.
New brake pads may often produce some initial noise and dusting which will diminish as the transfer film is built up on the bare rotor surface. This is normal for most brake pads and it will go away as the surfaces properly bed-in. Following the bed-in procedures provided by the manufacturer will assure a smooth, even layer of transfer film on the rotor. This helps to minimize brake judder and the possibility of warping the rotors due to uneven heating.
As long as the old rotors have been turned or resurfaced to even out the mating surface and remove any old pad transfer film, there should be no major issues reusing your old rotors. Make sure your installer has confirmed the turned rotors are still within the recommended wear limit as stamped on the rotor. Please keep in mind that resurfacing and/or turning a rotor on a brake lathe removes metal and therefore mass from the rotor. It will not be as effective at absorbing and dissipating heat as a new, full thickness brake rotor. It will therefore not be quite as effective as a heat sink or last as long as a new, full thickness rotor.
Itís never a good idea to install new brake rotors with used brake pads. As brake pads bed-in and wear down, the surface will become uneven and irregular. This is normal with all pads, but it does mean you should never install new rotors with used brake pads. The irregular surface of the used pads will not mate well with the smooth surface of your new brake rotors. This will cause problems with noise, dusting, and rapid overheating of the brake rotors.
All brake pads are going to release some amount of particles and dust. The amount and color of the particles and dust will vary depending on the pad compound but in general, the more aggressive the compound is, the more dust and noise it will generate. Many ceramic compound pads release a lighter color dust that does not show up as easily on the surface of your wheels. The trade off is often a pad with a lower friction coefficient so itís important to keep in mind you will be giving up some of the initial bite and grip when using a ceramic pad over a more aggressive compound pad.
Bedding-in new pads and rotors should be done carefully and slowly. Rapid heat buildup in the brake system can lead to warped rotors and or glazed brake pads. Most brake pad compounds will take up to 300-400 miles to fully develop an even transfer film on the rotors. The specific bed-in procedure varies depending on the compound type. These bed-in procedures are typically found printed on the brake pad package itself or in the instruction sheet enclosed with the pads.
The recommended bed-in procedures vary for each manufacturer and can be found here: Pad Bed-In Procedures
Overheating brake pads beyond their designed temperature range releases large amounts of gas and uneven, sticky deposits of melted binders/fillers that coat the rotor surface. Every time the brake system heats up, those uneven deposits will grip the pad more aggressively than the areas without the deposits. The brake pad will slip over the non-coated areas because it canít grip as well on those bare areas. These heavy concentrations of compound material are called hot spots because they cause uneven overheating on the rotor surface. This can actually alter the molecular structure of the cast iron rotors forming hard spots. Severe overheating forms a compound within the cast iron structure called cementite which is harder and more abrasive than the surrounding iron. Itís also not as effective as a heat sink as the surrounding cast iron. The compound cannot wear down these hard spots and the result is more heat building up in those areas and a thumping you can feel whenever you apply the brakes. Continued use of the rotors will expand these areas of cementite and uneven heat distribution. Eventually this will distort and warp the rotor. Once cementite forms in the rotor, it cannot be removed. The rotor needs to be replaced.
Installing new brake pads with used, unturned, or worn rotors can also cause warping due to the uneven overheating. Most used rotors will have irregular, grooved wear patterns on the surfaces. Mating this with the flat surface, a new brake pad will generate heat and friction only on the raised areas of the brake rotor causing those sections to heat faster than the recessed areas not touching the pads.Any brake pad transfer film from the previous set of pads can also affect the new padís ability to bed-in properly. The residue on the rotor can quickly lead to hot spots on the surface that can result in noise, vibration, dust, and warping problems.
Sticking calipers or other components can also lead to overheating rotors and excessive wear of the brake pads.
Brake pad compounds are produced by compressing multiple components utilizing high pressures and heat to form the pad shape. These components can include carbon, steel wool, copper fibers, aramid (Kevlar) fibers, along with ceramic and/or petroleum hydrocarbon adhesives/binders.
As the pads get hot, the heat and friction cause the metallic particles in the dust to statically charge as they are worn off the pad surface and thatís what makes them stick to the surface of the wheels and other parts of the vehicle. The petroleum hydrocarbon binders also break down releasing an oily, greasy film that clings to the wheels and other components of the car. This is typically more noticeable on the front wheels since the front brakes normally do a higher percentage of the work compared to the rear.
The brake rotors are produced from cast iron. As they wear down, the iron particles wearing off of the rotor and drum surfaces also tend to statically charge and stick on surfaces. In extreme conditions, the hot iron particles can imbed and fuse in the clear coat surface of the wheels. When exposed to moisture, these iron particles will oxidize and the result is rust stains on the wheel surface.
Brake fade occurs when the brake pads are operated in excess of their normal operating temperature. Overheated pad compounds melt and break down, releasing gases that build up between the pad and rotor contact surfaces. This gas layer between the pad and rotor can keep the pad and rotor from making contact and generating friction. Itís very much like a tire hydroplaning in standing water or a puck floating over the surface of an air hockey table. When this occurs the trapped gasses under the pad literally cause it to float over the surface of the rotor without making physical contact. If the pad canít contact the rotor surface, it canít generate friction and it canít slow down the spinning rotor. The result is a soft feeling brake pedal that will often go all the way to the floor without stopping the vehicle! This can occur very quickly and without warning especially when the brake pads are new and not completely bedded-in.
Using new brake pads without bedding them in can lead to excessive dust, vibration, noise, and potential brake failure! Heavy braking during the first few hundred miles of the padís life can overheat and glaze the pad surface. It can also overheat and warp the rotors.
Itís not uncommon for the front brake pads and rotors to wear out faster than the rears. This is because the front brakes do most of the work stopping the vehicle. As the brakes are applied, the weight distribution of the vehicle shifts foreword on the suspension putting more strain on the front brakes. This is why on most vehicles the front brakes are larger than the rears. Itís also why most front brakes wear out about twice as fast as the rears. This is normal.
Worn out springs, shocks, or struts can allow the vehicle weight distribution to shift faster and more dramatically, overloading the front brakes. This can result in premature front brake wear.
Sticking caliper pistons, caliper slides, or a damaged parking brake can also cause premature brake wear. When this happens, the caliper cannot retract the pad properly when you let off the brakes and it remains in contact with the rotor. In situations like this, itís typical for the inboard pads next to the caliper piston to wear out very quickly while the outer pad is hardly worn at all. This will generate excessive dust, noise, and heat. It will also wear out the inner surface of the rotor quickly. The tell-tale sign is one wheel with excessive, heavy dusting and noise.
This is not a defect in the pad or the rotor; itís a failure of the caliper and or the related components such as the piston, slides, or the parking brake assembly.
Keep in mind: the pads donít move by themselves! The caliper is what forces the pads into the rotor surface and if the caliper sticks, it canít retract the pads properly which causes excessive pad wear.
New brake pads need a smooth, clean surface to lay down an even transfer film. Residue from the previous pad compound on the surface or an uneven transfer film from overheating the new pads will cause the pads to grip-slip-grip-slip as they pass over the rotor surface under pressure. The resulting vibrations and noise telegraph and amplify as they pass through the suspension and steering components of the vehicle. This vibration and noise is known as brake judder or brake shimmy. This is often misdiagnosed as a warped rotor. If caught early enough, this can be fixed by cleaning the rotor surfaces with 200 grit garnet paper followed by spray brake cleaner to remove any compound buildup on the rotors. Then the pads should be re-bedded following the manufacturerís instructions. Normally this will eliminate the judder and noise issues.
Excessive run-out of the hub could indicate worn wheel bearings that need to be replaced or debris trapped between the rotor and the hub surface. Both of these can cause excessive run-out and pulsations in the brake pedal. Once the hub and bearings are checked, the new rotor should also be checked for excessive run-out using a dial indicator gauge. This should be done before the caliper and pads are installed. If a rotor has excessive run-out of over .004" (.10mm) it should be replaced.
Brake rotors donít warp unless overheated. Itís very common to misdiagnose brake problems and attribute them to a warped brake rotor. Warping is caused by excessive heating of the brake rotor and the resulting distortion of the cast iron.
The main symptom of warped rotors is a pulsation felt in the pedal when brakes are applied. If the vibration is felt in the steering wheel or in the dash of the vehicle, itís more likely a completely different problem: brake judder. A simple run-out measurement will confirm the diagnosis.
The only way to confirm a rotor is warped is to measure run-out with a dial indicator gauge. Most shops are equipped with this measuring tool and can check the bare rotor run-outs on the vehicle. If your installer says a rotor is warped, be sure to ask them for the run-out numbers on the rotor as well as the run-out numbers on the hub under the rotor. The new rotor should also be checked for excessive run-out using a dial indicator gauge once it is mounted and before the caliper and pads are installed. If a rotor has excessive run-out of over .004" (.10mm) it should be replaced.
All brake rotor manufacturers will require run-out numbers when processing a warranty claim.
Brake rotors are checked at the factory for machining flaws and excessive run-out before they are packaged and shipped. It is rare for a new rotor to have excessive run-out right out of the box.
If your new rotor does have excessive run-out, please contact our customer service department at 888-541-1777, ext. 360, for a replacement. Do not install or drive using the rotor! Rotor manufacturers will not warranty a used rotor for excessive run-out.
The dust produced by brake systems is highly corrosive. If left on the surface of the wheels or on other painted parts of the vehicle long enough, it will begin to etch and erode the paint and other protective coatings. Regular washing and waxing of the wheels will help eliminate brake dust buildup and the resulting damage.
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