How Cars Work: Brakes
by Kevin Schappell
The braking system is the most important system in your car. If the brakes fail, the result can be disastrous. The brakes are in essence energy conversion devices, which convert the kinetic energy (momentum) of your vehicle into thermal energy (heat).
When you step on the brakes, you command a stopping force ten times as powerful as the force that puts the car in motion. The braking system can exert as much as 1,000 pounds of hydraulic pressure on each of the four brakes. In modern systems, the master cylinder is separately power-assisted to activate the front and rear brakes. If one set fails, the other can provide adequate braking power. Many such safety systems within the braking system make modern brakes very complex, but much safer than earlier braking systems.
High-performance disc brakes originally were developed for racing, but are now used on many newer cars. On most cars, the front brakes are of the disc type, and the ones in the rear are the drum type. The parking brake is a cable operated system, which usually is attached to the rear wheels.
In almost all braking systems, the brake pedal is connected to a "master cylinder" by a push rod. The master cylinder is connected to the brake cylinders ("slave cylinders") at each wheel by steel brake lines and flexible rubber hoses. The entire hydraulic system is filled with a special brake fluid, which is forced through the system by the movement of the master cylinder pistons. The front disc brakes use friction "pads" which are mounted in "calipers". The pads are forced against machined surfaces of a rotating disc called the "rotor". The rear brakes are usually of the "drum" type. In these, the internal expanding brake "shoes" are forced against the inside machined surface of a rotating drum.
In recent years, brakes have changed greatly in design. Disc brakes, due to their lighter weight and better performance, are replacing drum types on the rear wheels. Instead of linings which press outwards against the inside of a drum, a disc attached to the axle is gripped from either side by friction pads attached to the calipers. The greatest advantage of disc brakes is that they are essentially "fade" free. That is, repeated application does not result in excessively high temperatures developing in the linings and drums, lowering the stopping power of the brake. Commonplace on newer cars are "anti-lock" brake systems, (ABS) which prevent the wheels from completely stopping when the brakes are applied in a panic stop.
As impressive as these advances are, the basic process of converting a vehicle's momentum into (wasted) heat has not changed since the days of horse and buggy. To stop carriages, the driver would pull on a lever which would rub on the wheel. But with the advent of brake-charging electric vehicles, a new braking equation is opening up the possibility of recapturing this lost energy, instead of warming the air with it. In modern electric cars, when you step on the brake the motor switches into "generator mode", and stores the car's momentum as chemical energy in the battery, to be used when the light turns green!
In 1923, a Packard was the first car in America with four-wheel brakes. In 1927, four-wheel brakes were introduced in the Lincoln production cars.
Wear: The braking system does allot of work and the brake pads take the brunt of the punishment. It is a good idea to have you brake pads checked every 6 months or when you suspect a problem. Symptoms include squeaking, grinding, or increased stopping distance. Most pads have a thin metal tab which vibrates against the rotor when the pads wears down to a dangerous level. Some pads do not have this and if not checked periodically can wear down far enough to ruin the rotors. A modern trend is to make the brake pads very hard thus extending life. This harder material can squeak and sound like the wear indicators. Brake dust can also cause squealing but can be fixed by spraying brake cleaner on the brake system to remove the dust.
Warped Rotors: More common in newer cars, but possible on all disc brake systems. Rotors warp due to being overheated or incorrect tightening of the wheel. A warped rotor will give a pulsing feeling when applying the brakes. This pulsing can be annoying and dangerous. Most newer cars have rotors which are very thin and warp very easy. Furthering the problem, the manufacturer does not leave enough material to resurface the rotor. Check with you mechanic to make sure you can safely have the rotors machined or replace with new rotors. To resurface, the rotor is placed in a lathe and a cutting tool removes a few thousandth's of material from the braking surface. This restores the flatness of the rotor and eliminates the pulsing sensation in the pedal. Make sure when your mechanic puts everything back together that he torques the lug nuts to proper specifications and never uses an impact wrench. If the lug nuts are not tightened evenly the rotor can warp and you are back to square one. Note: Some shops use a torque stick, which attaches to an impact wrench and does not allow the torque wrench to tighten more than it should. This is acceptable. If your mechanic does not use a torque wrench or torque sticks, find another mechanic.
- Avoid "riding" your brakes. It's better to slow down with moderate pressure and then releasing the brake to cool, than riding the brakes and overheating them.
- On steep grades consider downshifting to save your brakes. Only do this when traction conditions are good. In ice, snow, or even rain, downshifting into too low of a gear may cause a skid. Downshifting lets you engine do some of the braking instead of your brakes.
- Keep your wheels and braking system clean. Clean brakes work better and keep temperatures down. Use a good wheel cleaner which you know if safe for your wheel finish.
What to discuss with your mechanic:
- Be weary of low priced brake jobs advertised in the paper or TV. Some shops will try a bait and switch or find other parts which "need" to be replaced. Salesmen will try to make you feel guilty for putting your families safety on the line. They claim you need the premium pads and rotors, of course at a higher price.
- You mechanic should clean all the components of the brake system to ensure a dust and squeak free job.
- All bolts including lug nuts should have anti-seize compound on the threads to prevent them from rusting fast and causing headaches down the road.
- Have your mechanic use an anti-squeak compound on the back of the brake pads. This keeps the pads from vibrating and annoying you to no end. There are spray, and paste forms, with the paste working better for me.
- Insist on seeing the pads they removed from your car. There is no use paying to replace something that doesn't need to be replaced.
- National brake shops are not all bad. Some stores only do brakes so they should be pretty good at it. Ask around and get recommendations before you get work done. Quality depends on the owner of the national chain store, not the parent company so shop carefully.
- Do you need the lifetime brake pads? Well that depends on how long you will keep the car and how many rotors you plan on buying in the next few years. This initial cost is a little higher due to the fact the manufacturer knows he will most likely have to give you another set when yours wear out. Also these pads are made from a harder material and tend to wear down the rotors instead of themselves. You would be better off buying the basic pads and replacing them periodically instead of costly rotors every year or two.
- Make sure your mechanic uses a torque wrench or torque sticks on their impact guns. See above for the explanation.
Safety is important to you and your family. Stay safe by educating yourself and not by falling pray to the salesman. When you think you have a brake problem, take it to be checked by a mechanic you trust, for your families sake
Kevin maintains http://www.autoeducation.com where he gives advice on car maintenance, buying, selling, insurance, and financing. A mechanical engineer and car guy, Kevin has decided to spend his online time helping others learn about automobiles.
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