CCAS 3381 CCAS 3381 AUTOMOTIVE SKILL IAUTOMOTIVE SKILL I

TIRES & BRAKES

OBJECTIVESOBJECTIVESTo understand the operational

principles and basic mechanisms of braking system

To understand the importance of wheels & tires

Lecture – 1 hourWorkshop – 1 hour

BRAKE SYSTEM (1)BRAKE SYSTEM (1)When brake pedal depressed

force is transmitted from your foot to brakes through hydraulic fluid

Force multiplied by:–Mechanical advantage

(leverage) –Hydraulic force

multiplication Brakes transmit force to

tires using friction & tires transmit force to the road using friction

BRAKE SYSTEM (2)BRAKE SYSTEM (2)

Master cylinder with slave cylindersFriction–coefficient of static friction–coefficient of dynamic friction

For a car tire coefficient of dynamic friction is much less than coefficient of static friction - greatest traction when contact patch is not sliding relative to the road

BRAKE SYSTEM (3)BRAKE SYSTEM (3)To increase safety

most modern car brake systems are broken into two circuits with two wheels on each circuit– relatively failsafe

BRAKE SYSTEM (4)BRAKE SYSTEM (4)Disc brake pad is normally in contact

with the discDrum brake shoes are normally pulled

away from the drumMetering valve compensates & make

drum brakes engage just before the disc brakes

Pressure differential valve alerts you if you have a leak in one of your brake circuits

Proportioning valve only lets a certain portion of the pressure through to the rear wheels so that the front wheels apply more braking force

BRAKE SYSTEM (5)BRAKE SYSTEM (5)Most modern cars have

disc brakes on the front wheels (some have disc brakes on all four wheels)

Most common - single-piston floating caliper - self-centering and self-adjusting

BRAKE SYSTEM (6)BRAKE SYSTEM (6) Most common brake service

required - changing pads–wear indicator (squealing

sound) – inspection opening in the

caliper Deep scores worn into brake

rotors/disc (worn-out brake pad left on for too long)

Brake rotors can also warp (not flat) - brakes may shudder or vibrate when you stop (fixed by refinishing)

BRAKE BRAKE SYSTEM (7)SYSTEM (7)

Drum brakes - shoes press against a spinning surface

Many cars have drum brakes on the rear wheels (drum brakes have more parts than disc brakes - harder to service)–Self-actuating (wedging

action)–Auto adjusting

BRAKE SYSTEM (8)BRAKE SYSTEM (8)Most common service -

changing brake shoes– Inspection hole to see how

much material left on the shoe–Riveted shoes – 0.8 mm of

friction material–Bonded to the backing

plate - 1.6 mmDeep scores get worn into

brake drums from worn-out shoes – refinishing (max. allowable diameter)

BRAKE SYSTEM (9)BRAKE SYSTEM (9)On FWD cars system is usually split

diagonally–one circuit works the right front & left

rear brake–other works the left front & right rear

brakeMost cars use a cable to

actuate the emergency brake - a cable pulls on the lever, which forces the two shoes apart

BRAKE SYSTEM (10)BRAKE SYSTEM (10)Vacuum booster – needs

a vacuum source (engine) to operate –check valve (one-way

valve that only allows air to be sucked out of the vacuum booster)

BRAKE SYSTEM (11)BRAKE SYSTEM (11)Anti-locking brake system

(ABS) – to avoid wheels from skidding–Stop faster–Able to steer while you stop

Speed sensors Valves in each brake (positions

I, II & III) Controller – computer watching

the speed sensors & controls the valves

Pump - to restore pressure in brake line

BRAKE SYSTEM (12)BRAKE SYSTEM (12)Different variations & control algorithms

–Controller monitors speed sensors at all time (decelerations in the wheel right before a wheel locks up)–Controller reduces pressure to brake

until it sees an acceleration then it increases the pressure until it sees the deceleration again

ABS working - pulse in brake pedal (cycle up to 15x/sec)

4-channel, 4-sensor/3-channel, 3-sensor

BRAKE SYSTEM (13)BRAKE SYSTEM (13)One-channel, one-sensor ABS

–You absolutely should not pump the brake pedal in a car with ABS during slippery conditions

–Anti-lock brakes really do help you stop better by preventing wheels from locking up and providing the shortest stopping distance on slippery surfaces

BRAKE SYSTEM (14)BRAKE SYSTEM (14)Three main types of brake fluid available: –DOT3 & DOT4 - glycol-based fluids

(absorb water)Absorb water from air (b.p.

decreases)Eat paint

–DOT5 - silicon-based (corrosion due to pure water pockets) –Boiling point – very important –Brakes lost when boiling occurs

(prolonged braking downhill) Do not mix different types of brake fluid

WHEELS & TIRES (1)WHEELS & TIRES (1)Sidewall markingsBead bundle - loop of high-strength

steel cable coated with rubber Body - made up of several layers of

different fabrics/plies –Cords in radial tire run perpendicular

to the tread (tire's strength described by no. of plies it has - most car tires have two body plies)

Steel-belted radial tires - to reinforce the area under the tread

These belts provide puncture resistance & help tire stays flat (best contact with the road)

WHEELS & TIRES (2)WHEELS & TIRES (2)

Tread

Sidewall

Water channel to prevent hydroplanning

Pressure gage

WHEELS & TIRES (3)WHEELS & TIRES (3)Cap plies - an extra layer or two of polyester fabric

to help hold everything in place at high speedsSidewall provides lateral stability for the tire,

protects body plies & helps keep the air from escaping

Tread - mixture of many different kinds of natural & synthetic rubbers with patterns that give the tire traction

Tire type–P ~ passenger vehicle–LT ~ light truck–T ~ temporary/spare

WHEELS & TIRES (4)WHEELS & TIRES (4)Tire width - width of tire in mm (measured from

sidewall to sidewall) when it is on its intended rim size

Aspect ratio - height of tire from bead to top of the tread (as a percentage of tire width)–Aspect ratio of 75 means tire's height is 75% of

its width (the smaller the aspect ratio, the wider)Two tires with different aspect ratios but same

overall diameter High performance tires have a lower aspect ratio

(better lateral stability due to stiffer sidewalls) Tire construction – R (radial), D (diagonal) & B

(bias)

WHEELS & TIRES (5)WHEELS & TIRES (5)

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DOT_Pagxml_no_dDOT_PagDOT_Pag1DOT_Pagxml_no_dDOT_PagDOT_Pag1

WHEELS & TIRES (6)WHEELS & TIRES (6)Rim diameter in inchesUniform tire quality grading–Tread wear - the higher the no, the longer

you can expect the tread to last –Traction - AA (top of scale)

Uniform tire quality grading–Temperature - A, B or C (measure of how

well tire dissipates heat & how well it handles buildup of heat)

Underinflation, overloading or excessive speed lead to more heat buildup (cause tires to wear out faster tire wear or tire failure)

Service description– Load ratings - higher no indicates higher load

capacity

WHEELS & TIRES (7)WHEELS & TIRES (7)Service description–Speed rating – max. speed allowable for

this tire (as long as the weight is at or below the rated load)

Hydroplaning occurs when car drives through puddles of standing water–Water cannot squirt out from under tire

quickly enoughContact patchUnderinflated/overloaded tire needs more

force to push it down the road (so it generates more heat)

Problems with tires–Underinflation causes tires to wear more

on the outside than the inside

WHEELS & TIRES (8)WHEELS & TIRES (8)Problems With Tires–Causes reduced fuel

efficiency & increased heat buildup in the tires (check the tire pressure with a gauge at least once a month)

Overinflation causes tires to wear more in the center of the tread

Misalignment of the wheels causes either the inside or the outside to wear unevenly, or to have a rough, slightly torn appearance

MISCELLANEOUS (1)MISCELLANEOUS (1)Brake job– replacing front disc brake pads– resurfacing rotors or discs– replacing rear drum brake shoes– resurfacing drums–bleeding brake lines (replacing old brake fluid

with new & getting all the air out of the lines)– inspecting system for leaks or other

problems that might require additional repairs–checking & adjusting the parking brake

MISCELLANEOUS (2)MISCELLANEOUS (2)Noisy brake–Semi-metallic pads–Debris on pads–Grease on pads

Caliper service–Leaked–Frozen piston–Caliper causing uneven pad wear–Rubber seal hardened

Dragging brake Low brake pedal needs pumping repeatedly to bring a

vehicle to a stop:– low fluid level

MISCELLANEOUS (3)MISCELLANEOUS (3)Low brake pedal needs pumping repeatedly to bring

a vehicle to a stop:–drum brakes need adjustment–air in the lines

Brakes squeal –Caused by vibration between brake pads, rotors &

calipers (lessened/ eliminated by installing "noise suppression shims“ on backs of pads)

Brake booster failure (require increased pedal effort)Bleeding brakes - flushing old brake fluid out of

master cylinder, brake lines, calipers & wheel cylinders & replacing it with fresh fluid

MISCELLANEOUS (4)MISCELLANEOUS (4)Balancing of new tiresHigh speed shimmy caused by out of balance or

bent wheelRadial tires improve fuel economy, better traction

& corneringDirectional & asymmetrical tiresWheels need to be realigned after struts have

been replacedCar pulls one side:–Underinflated front tire on one side –Mismatched tires –Dragging brake

MISCELLANEOUS (5)MISCELLANEOUS (5)Normal wear - tread wears evenly across entire

surface of tire (No bumpy, directional, feathered or cupped wear patterns developed on tread - rotating your tires frequently (every 12,000 miles or so or every 6 months) can help to equalize this kind of wear between tires)

Abnormal wear - inside or outside edge or shoulder of the tire shows extreme wear:–Camber wear caused by suspension

misalignment, a bent strut, a mislocated strut tower (often result of unrepaired collision damage), a weak or broken spring, a bent spindle, or collapsed or damaged control arm

MISCELLANEOUS (6)MISCELLANEOUS (6)Cupped wear pattern caused by a wheel

& tire that are out of balance or by weak shock absorbers or struts (wheel bounces up & down as it rolls

Feathered or directional wear pattern (tread feels smooth when you run your hand across it one way but feels rough when you rub it in the opposite direction) - toe wear caused by toe misalignment, worn tie rod ends, worn idler arms, bent steering linkage or bent steering arms

MISCELLANEOUS (7)MISCELLANEOUS (7) Changing a flat tire Step 1: Choose your spot

well Step 2: Remove tools from

vehicle Step 3: Loosen the lug

nuts  Step 4: Jack up the vehicle Step 5: Remove the flat

tire  Step 6: Put on the spare

tire  Step 7: Lower the vehicle Step 8: Put the tools away

Spare Tire, inflated properly Tire Blocks (optional) Gloves (optional) Flashlight (optional)

MISCELLANEOUS (8)MISCELLANEOUS (8)

Second-rib Wear One Side Wear

MISCELLANEOUS (9)MISCELLANEOUS (9)

Feathering Under Inflation

Over Inflation Cupping

THE ENDTHE ENDIQ + EQ + SQ = TQ*