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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)
d
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*