Braking

BRAKING SYSTEM

Braking With Brake Rotors or Discs
Braking with brake rotors (also called discs) is accomplished by the brake pads being forced against the surface of the brake rotor [torque]. The brake pads skate across the surface of the brake rotor squeezing the brake rotor between the two brake pads. Friction is the mechanism that converts dynamic energy into heat. Just as there are two sorts of friction between the tire and the road surface (mechanical gripping of road surface irregularities by the elastic tire compound and transient molecular adhesion between the rubber and the road in which rubber is transferred to the road surface), so there are two very different sorts of braking friction - abrasive friction and adherent friction. Abrasive friction involves the breaking of the crystalline bonds of both the pad material and the cast iron of the disc. The breaking of these bonds generates the heat of friction. In abrasive friction, the bonds between crystals of the pad material (and, to a lesser extent, the disc material) are permanently broken. The harder material wears the softer away (hopefully the disc wears the pad). Pads that function primarily by abrasion have a high wear rate and tend to fade at high temperatures. When these pads reach their effective temperature limit, they will transfer pad material onto the disc face in a random and uneven pattern. It is this "pick up" on the disc face that both causes the thickness variation measured by the technicians and the roughness or vibration under the brakes reported by the drivers.

With adherent friction, some of the pad material diffuses across the interface between the pad and the disc and forms a very thin, uniform layer of pad material on the surface of the disc. As the friction surfaces of both disc and pad then comprise basically the same material, material can now cross the interface in both directions and the bonds break and reform. In fact, with adherent friction between pad and disc, the bonds between pad material and the deposits on the disc are continually being broken and some of them are continually reforming.
With many pad formulas, the pad material must be abrasive enough to keep the disc surface smooth and clean. As the material can cross the interface, the layer on the disc is constantly renewed and kept uniform - again until the temperature limit of the pad has been exceeded or if the pad and the disc have not been bedded-in completely or properly. In the latter case, if a uniform layer of pad material transferred onto the disc face has not been established during bedding or break-in, spot or uncontrolled transfer of the material can occur when operating at high temperatures.
The friction material that is quiet and functions well at relatively low temperatures around town will not stop the car that is driven hard. If you attempt to drive many cars hard with the OEM pads, you will experience pad fade, friction material transfer and fluid boiling. The true racing pad, used under normal conditions will be noisy and will not work well at low temperatures around town.

Mechanically, the brake pads skate across the brake rotor's surface, the rotor's surface must be smooth and flat . If the surface of the brake rotor is not flat it will transmit the surface variations to the vehicle's steering wheel or brake pedal via the hydraulics systems.There are three primarily surface variations:
1) Lateral runout (sometimes call Warpage) is a variation of the lateral surface position, starting with a measurement of 0.004"(0.1mm) and greater. Lateral runout or Warpage may be caused by several factors; over tightening the lug nuts (over-torquing), inaccurate lathe setups, and rapid temperature changes during normal driving [very hot brake rotors are subjected to a splash of very cold water multiple times]. The driver feels lateral runout when he applies the brakes and feels the brake pedal "pulsate" or "judder".
2) Parallelism is a variation in the thickness of the rotor starting with a measurement of 0.004"(0.1mm) and greater. Parallelism or thickness variation is caused by a slight wobble in the rotor allowing the rotor to come into contact with the brake pads at the wobble peaks. This wobble may create significant parallelism in as few as 5000km. The driver feels parallelism by as a "shake" in the steering wheel.
3) Martensitic transfer, or hard spots occurs as a result of a temperature build-up due to the inability of the rotor surface to dissipate heat evenly resulting in a "hard spot" on the rotors surface.Martensitic transfer occurs when the brake rotor is flat and its does not dissipate heat rapidly and uniformly. The internal rotor heat increases to a point where at critical temperatures a Martensitic transfer takes place producing a "hard spot(s)". The Martensitic spot may be slightly raised from the rotors surface. Excessive heat also results in increasing the surface hardness and glazing of the friction brake pad materials. The driver may find this similar to lateral runout, depending upon the severity of it and the number of hard spots on a rotor.

Problems occurring in the braking system

Air in the system This is usually caused by air getting into the brake fluid area, usually from the master cylinder. As the brake pads wear, the caliper pistons ride farther out of the caliper, allowing more fluid to remain in the calipers. Over time this can add up to almost as much fluid as there is in the master cylinder reservoir. If neglected, this will allow the master cylinder to pump some air into the brake lines. Air is very compressible, whereas brake fluid is not, as long as there is a solid stream of brake fluid between the master cylinder piston and the caliper piston, the brake pedal will be nice and firm. If there is air in the system, the pedal will feel spongy and will go down almost all the way to the floor, maybe all the way, depending on how much air is in the system. The standard way of dealing with air in the brake system is to perform an operation called "bleeding the brakes".
Hard brake pedal Can be caused by bad power booster, (or loss of vacuum to the booster) seized caliper pistons, seized caliper slides, pinched brake lines, and (rarely) problems with the pedal linkage under the dash. The probable best fix is new calipers, and new pads.
Brake fade Two phenomena contribute to brake fade, one is the fact that the coefficient of friction of most substances gets lower at high temperatures, and that most liquids will boil at some temperature, and that gases compress, while liquids do not. When you use the brakes to decelerate 3,000 or 4,000 or 7-8-15,000 lbs of vehicle, they get hot. Under normal circumstances this would be no big deal, the heat that builds up in the pads, rotors, and calipers will slowly radiate back to the air flowing over them as the car continues down the road. If you are back on the brakes, doing more decelerating, instead of cooling off, the brakes are getting hotter. .As the pads and rotors get hotter the friction material of the pads starts to separate. The binding agent starts to melt off from the surface of the pad, and the coefficient of friction approaches zero, as does the braking efficiency.
Brake squeal This is a high pitched squealing noise, often heard when you are going slow and are not applying the brakes. If it goes away as you apply the brakes, it could be coming from the brake wear sensors. (Also called 'squealers' by mechanics.) They are small bits of spring steel that are attached to the brake pads in such a way that when the pads are about 75% worn out, the sensors start to rub on the rotors, making the noise. Grinding noise Sometimes it is just a small stone, trapped between the brake rotor and the air deflector. The faces of the rotor should be smooth and clean. If you see large scaly rusted places on the friction surfaces of the rotor you should replace them. If your pads are worn out(less than 3/16 of an inch of friction material left) and you catch it in time, all you have to do is install new brake pads. If the surface of the rotor is damaged, you will have to replace it.
Brake pedal pulsation There are a lot of things that can cause this, from out-of-adjustment wheel bearings to rotors that are bent, brake drums that are out-of-round, rusty spots on the rotors that have a different surface smoothness. To determine whether the pulsation is coming from the front or the rear wheels, check to see if you can feel the pulsation in the steering wheel when the pedal is pulsating. If you can, the problem is coming from the front wheels.
Brake pull Mostly this one comes from either a caliper piston seized or caliper slides seized. Replace calipers and pads and service the caliper slides.
Brake grabbing When you just barely touch the brake pedal and one or more wheels locks up and skids. This one most commonly comes from contaminated friction material on one or more brakes.
Pedal goes to the floor Stop driving and check your fluid level. It might just need to be topped up to temporarily get you some braking .