Sunday, October 21, 2012
Car care: basics of braking -- Part 1
[Article copyright Ewe Paik Leong]
A typical modern vehicle weighs around 1.4 tonnes, has a 3.5 litre engine, and accelerates from 0 to l00 kmph in approximately t0 seconds. To do this, it has a sophisticated engine, transmission, and drive-line system. This system consists of thousands of mechanical and electronic parts, and makes up nearly half of the vehicle's weight.
Yet, to meet the expectations of today's driver, the braking system which usually consists of some 200 components and weights less than 40kg needs to be capable of stopping the car from this speed in only 3.5 seconds. The braking system is a means of con-verting the vehicle's momentum (called kinetic energy) into heat by creating fric-tion in the wheel brakes. The heat is then dissipated into the air from the brake discs and drums. The ability to absorb and dissipate heat rapidly is the controlling factor in preventing brake fade under severe conditions.
Of course, final contact between the vehicle and the road is made through the tyres - hence their importance in braking performance. Bald or defective tyes will make even the best brakes ineffective. The modern braking system consists of a pedal and booster assembly to which a master cylinder and proportioning valve are attached. At the wheels, there are either disc calipers and rotors, drums and brake shoes, or a combination of both. The system also has a park brake. Park brakes can be either part of or separate from the main system. Park brakes are usually applied with a hand brake. And that's it.
The safety of the vehicle rests largely on these few components so their condition is obviously extremely important.
Some vehicles are fitted with anti-lock brakes that prevent wheel lock-up under adverse road and conditions. These add weight and complexity to the braking system but give the driver far better control in an emergency. Most large vehicles have disc brakes all round. Smaller vehicles sometimes have drum brakes on the rear wheels.
Disc brakes dissipate heat quicker than drum brakes so they are usually fitted to the front where most of the braking happens. Wheel brakes are applied by hydraulic pressure. The pressure is produced in a master cylinder and delivered to the disc calipers or drum wheel cylinders by tubes and flexible hoses.
Whilst some master cylinder pistons are operated by a push-rod connected to the brake pedal, most have a power brake booster that helps the driver by increasing the force on the piston, and reduces the driver's pedal effort.
The individual brake disc calipers or wheel cylinders generate braking force by bringing lining material into contact with the discs or drums. The amount of force generated on each set of brakes is determined by the area of the hydraulic piston, its size and its type.
Tandem Master Cylinder
A tandem or dual master cylinder is one of the most important safety devices in any vehicle. It operates a divided or split hydraulic system so that if one circuit fails, the other will still operate. Most front engine rear-drive vehicles have one circuit connected to the front brakes and the other to the rear. If there is a failure in one circuit, the amount of available braking force corresponds to the amount of weight on the axle.
Front engine rear-wheel drive vehicles usually have 60% of their weight acting on the front wheels and 40o/o on the rear. By contrast, on front engine front- wheel drive vehicles, the weight distribution is significantly different, with about 80% of the weight being on the front wheels and only 20% on the rear. To allow for the possibility of a single circuit failure, a diagonal or cross-split layout is used. This has one front wheel and the diagonally opposite rear wheel connected to each circuit. Should either circuit fail, an equal amount of brakingwill still be available. It would still, of course be only half the normal braking force of a fully-operational braking system.
Most tandem master cylinders have a warning switch to alert the driver to a circuit failure. They also have a proportioning valve (or valves) that provides balanced braking by reducing the hydraulic pressure to the rear wheels. This helps prevent rear wheels lock-up, too.
Modern disc brakes have more space between the disc and the pad than in the past. This reduces drag and improves fuel economy. However, to avoid increased pedal travel, some master cylinders are fitted with a fast-fill valve. often called a quick take-up.