Thursday, January 31, 2008

Repair Guide: Disc/Drum Brakes

Disc brakes are used on the front wheels of most cars and on all four wheels on many cars. A disc rotor is attached to the wheel hub and rotates with the tire and wheel. When the driver applies the brakes, hydraulic pressure from the master cylinder is used to push friction linings against the rotor to stop it.
The rotor is usually made of cast iron. The hub may be manufactured as one piece with the rotor or in two parts. The rotor has a machined braking surface on each face. A splash shield, mounted to the steering knuckle, protects the rotor from road splash.

A rotor may be solid or ventilated. Operation of the master cylinder if there is a rear system failure. Ventilated designs have cooling fins cast between the braking surfaces. This construction considerably increases the cooling area of the rotor casting. Also, when the wheel is in motion, the rotation of these fan-type fins in the rotor provides increased air circulation and more efficient cooling of the brake. Disc brakes do not fade even after rapid, hard brake applications because of the rapid cooling of the rotor.

The hydraulic and friction components are housed in a caliper assembly. The caliper assembly straddles the outside diameter of the hub and rotor assembly. When the brakes are applied, the pressure of the pistons is exerted through the shoes in a 'clamping' action on the rotor. Because equal opposed hydraulic pressures are applied to both faces of the rotor throughout application, no distortion of the rotor occurs, regardless of the severity or duration of application. There are many variations of caliper designs, but they can all be grouped into two main categories: moving and stationary caliper. The caliper is fixed in one position on the stationary design. In the moving design, the caliper moves in relation to the rotor.

Most late-model cars use the moving caliper design. This design uses a single hydraulic piston and a caliper that can float or slide during application. Floating designs 'float' or move on pins or bolts. In sliding designs, the caliper slides sideways on machined surfaces. Both designs work in basically the same way.

The single-piston caliper assembly is constructed from a single casting that contains one large piston bore in the inboard section of the casting. Inboard refers to the side of the casting nearest the center line of the car when the caliper is mounted. A fluid inlet hole and bleeder valve hole are machined into the inboard section of the caliper and connect directly to the piston bore.

The caliper cylinder bore contains a piston and seal. The seal has a rectangular cross section. It is located in a groove that is machined in the cylinder bore. The seal fits around the outside diameter of the piston and provides a hydraulic seal between the piston and the cylinder wall. The rectangular seal provides automatic adjustment of clearance between the rotor and shoe and linings following each application. When the brakes are applied, the caliper seal is deflected by the hydraulic pressure and its inside diameter rides with the piston within the limits of its retention in the cylinder groove. When hydraulic pressure is released, the seal relaxes and returns to its original rectangular shape, retracting the piston into the cylinder enough to provide proper running clearance. As brake linings wear, piston travel tends to exceed the limit of deflection of the seal; the piston therefore slides in the seal to the precise extent necessary to compensate for lining wear.

The top of the piston bore is machined to accept a sealing dust boot. The piston in many calipers is steel, precision ground, and nickel chrome plated, giving it a very hard and durable surface. Some manufacturers are using a plastic piston. This is much lighter than steel and provides for a much lighter brake system. The plastic piston insulates well and prevents heat from transferring to the brake fluid. Each caliper contains two shoe and lining assemblies. They are constructed of a stamped metal shoe with the lining riveted or bonded to the shoe and are mounted in the caliper on either side of the rotor. One shoe and lining assembly is called the inboard lining because it fits nearest to the center line of the car. The other is called the outboard shoe and lining assembly.

The caliper is free to float on its two mounting pins or bolts. Typical mounting pins are shown in the exploded view of the floating caliper. Teflon sleeves in the caliper allow it to move easily on the pins. During application of the brakes, the fluid pressure behind the piston increases. Pressure is exerted equally against the bottom of the piston and the bottom of the cylinder bore. The pressure applied to the piston is transmitted to the inboard shoe and lining, forcing the lining against the inboard rotor surface. The pressure applied to the bottom of the cylinder bore forces the caliper to move on the mounting bolts toward the inboard side, or toward the car. Since the caliper is one piece, this movement causes the outboard section of the caliper to apply pressure against the back of the outboard shoe and lining assembly, forcing the lining against the outboard rotor surface. As the line pressure builds up, the shoe and lining assemblies are pressed against the rotor surfaces with increased force, bringing the car to a stop.

The application and release of the brake pressure actually causes a very slight movement of the piston and caliper. Upon release of the braking effort, the piston and caliper merely relax into a released position. In the released position, the shoes do not retract very far from the rotor surfaces.

As the brake lining wears, the piston moves out of the caliper bore and the caliper repositions itself on the mounting bolts an equal distance toward the car. This way, the caliper assembly maintains the inboard and outboard shoe and lining in the same relationship with the rotor surface throughout the full length of the lining.

Sliding calipers are made to slide back and forth on the steering knuckle support to which it is mounted. There is a V shaped surface, sometimes called a rail, on the caliper that matches a similar surface on the steering knuckle support. These two mating surfaces allow the caliper to slide in and out. The internal components of the caliper are the same as those previously described.

The stationary or fixed caliper has a hydraulic piston on each side of the rotor. Larger calipers may have two pistons on each side of the rotor. The inboard and outboard brake shoes are pushed against the rotor by their own pistons. The caliper is anchored solidly and does not move. The seals around the pistons work just like those already described. The main disadvantage of the stationary caliper is that it has more hydraulic components. This means they are more expensive and have more parts to wear out.


My brakes are squealing. Does that mean I need a brake job?

Not necessarily. A certain amount of brake noise is considered "normal" these days because of the harder semi-metallic brake pads that are used in most front-wheel drive cars and minivans. This type of noise does not affect braking performance and does not indicate a brake problem. However, if the noise is objectionable, there are ways to eliminate it.
Brake squeal is caused by vibration between the brake pads, rotors and calipers. Pad noise can be lessened or eliminated by installing "noise suppression shims" (thin self-adhesive strips) on the backs of the pads, or applying "noise suppression compound" on the backs of the pads to dampen vibrations. Additional steps that can be taken to eliminate noise are to resurface the rotors and replace the pads.

Some brands of semi-metallic pads are inherently noisier than others because of the ingredients used in the manufacture of the friction material. Strange as it may sound (pardon the pun), cheaper pads are sometimes quieter than premium quality or original equipment pads. That's because the cheaper pads contain softer materials that do not wear as well. For that reason, they are not recommended. Premium quality pads should cause no noise problems when installed properly and will give you better brake performance and longer life.

Conditions that can contribute to a disc brake noise problem include glazed or worn rotors, too rough a finish on resurfaced rotors, loose brake pads, missing pad insulators, shims, springs or anti rattle clips, rusty or corroded caliper mounts, worn caliper mounts, and loose caliper mounting hardware. Drum noise may be due to loose or broken parts inside the drum.

Most experts recommend new caliper and drum hardware when the brakes are relined, a thorough inspection of the calipers and rotors for any wear or other conditions that might have an adverse affect on noise or brake performance, and resurfacing the rotors (and drums) if the surfaces are not smooth, flat and parallel.

If you hear metallic scraping noises, on the other hand, it usually means your brake linings are worn out and need to be replaced -- especially if your brake pedal feels low or if you've noticed any change in the way your vehicle brakes (it pulls to one side when braking, it requires more pedal effort, etc.).

Some brake pads have built-in "wear sensors" that produce a scraping or squealing noise when the pads become worn. In any event, noisy brakes should always be inspected to determine whether or not there's a problem. And don't delay! If the pads have worn down to the point where metal-to-metal contact is occurring, your vehicle may not be able to stop safely, and you may score the rotors or drums to the point where they have to be replaced.


How can I tell if a rotor or drum really needs to be replaced?

A rotor must be replaced if it is at or below the minimum thickness specification or discard thickness stamped on the rotor (this same information can also be found in brake service manuals). Replacement is also necessary if a rotor cannot be resurfaced without exceeding the minimum thickness specification or the discard thickness specification. Replacement is also required if the rotor is cracked or damaged. Replacement may be recommended if a rotor has hard spots, is warped, or has been previously resurfaced for a warped condition.
A drum must be replaced if it is at or beyond the maximum inside diameter specification or discard diameter stamped on the drum. Replacement is also necessary if a drum cannot be resurfaced without exceeding the maximum diameter specification or discard diameter specification. Replacement is also required if a drum is cracked, damaged, bell mouthed or too far out of round for resurfacing.


Is it always necessary to resurface the rotors and drums when the brakes are relined or to rebuild or replace the disc brake calipers and drum brake wheel cylinders?

No. The rule here is resurface when necessary, don't resurface when it isn't necessary. If the rotors and drums are in relatively good condition (smooth and flat with no deep scoring, cracks, distortion or other damage), they do not have to be resurfaced. Resurfacing unnecessarily reduces the thickness of these parts, which in turn shortens their remaining service life.
According to the uniform inspection guidelines developed by the Motorist Assurance Program (MAP), "friction material replacement alone does not warrant rotor reconditioning." Whether or not the rotors or drums need resurfacing or replacing depends entirely on their condition at the time the brakes are relined.

Even so, many mechanics prefer to resurface rotors and drums when relining the brakes to restore the friction surfaces to "like-new" condition and to minimize any chance of brake squeal.

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