Chapter 6
The Speed Factor: Negotiating
Curves and Braking
6.1 Physical Laws That Affect Driving
 Speed
 Refers to how fast something is moving/How quickly position is changing over time
 Measured in m/h or km/h
 Acceleration
 Measures the rate of change of an objects speed
 Increasing speed over a given period=acceleration
 Decreasing speed over a given period=deceleration
Example…
 A car is traveling from point A to point E. At the beginning of point A, you
are traveling 0mph. At the end of point C you are traveling 60mph. When
you reach point D you are still traveling 60mph. When you stop at point E,
you are traveling 0mph.
A
B
C
D
E
Starting Point
1
2
3
4
Gravity
 Force exerted by massive objects on another object to “attract”, or “pull”
toward its center
 In terms of driving:
 When traveling uphill, gravity pulls your car towards the ground causing you to use more
power for the engine to maintain speed
 When traveling downhill, it requires less power because you are moving in the direction
that gravity is pulling you
Traction
 Friction between your vehicle’s wheels and the surface of the road
 Necessary to move, change direction, and stop
 Keeps control of your vehicle
 Traction increases when the amount of tire tread touching the road
increases
Tread
 The grooved surface of the tire that grips the road.
 Water flows through the grooves
 1/16 inch tread
 Police used a penny to test tread depth
Inflation and Traction
 Check owner’s manual for best pressure
 When pressure is right, you get your best control, better gas mileage, and
tire wear.
 Under-inflation
 Over-inflation
 Split Traction
Reduced Traction
 Two things to maintain ideal levels of traction
 Vehicle must be in good condition
Tires, shock absorbers, steering system
 Road must be smooth
Snow, Ice, Rain, Gravel
Reduced Traction
Torque
 Ability of a force to cause an object to rotate
 In driving terms, it is the engine’s ability to apply more force to turn your
wheels
 When the force applied to your wheels exceeds the force resisting(traction),
your wheels WILL spin
Inertia
 An object in motion stays in motion
 An object at rest will remain at rest
 Unless acted upon by a force
 Examples of inertia:
 Accelerating
 Decelerating/Braking
 Turning
 Crashing
Kinetic Energy and force of Impact
 Kinetic Energy
•

The energy an object in motion has

For a vehicle to stop, it must lose kinetic energy
•
Braking
•
Coasting
•
Impact
Force of Impact
•
The force of a collision as determined by the magnitude of kinetic energy of the objects that
collide and the distance over which the kinetic energy is lost
•
Softer objects spread have more “give” and will spread the force of impact over a greater distance
•
3 Factors
•
Speed
•
Weight
•
Distance between impact and stopping
Reducing Force of Impact
 Guard Rail
 Dashboards
 Bumpers
 Body of Car
 Air Bags
6.2 Negotiating Curves
 Centrifugal Force
 The outward force or “pull” experienced when traveling in a circular path
 In terms of driving, the inertia of your body attempting to continue traveling in a straight
line as the vehicle changes direction
Center of Gravity (COG)
 Point at which an object’s weight is centered
 Vehicles that are wide have a lower COG
 SUVs and Jeeps have higher COG
 Vehicles with higher COG have higher potential to roll or flip on sharp turns
Loading
 Adding weight to your vehicle’s weight as measured when it is empty
 Can affect a vehicle’s COG
 You are loading your vehicle anytime you get in the car
 Increased loading will cause your vehicle to work harder
Road Slopes
 Level
 Banked
 Crowned
Banked Road
 Dips down in one direction so that one side is higher than the other
 Dips in the direction of the curve to reduce loss of traction
 Reduces incidence of rollover
 Some are banked the wrong way, must be aware
Crowned Roads
 Higher in the center than the sides
 Promote runoff to reduce hydroplaning
 Can help or harm you, depending on the direction of travel
Entering a Curve
 SPEED is only thing we can control
 Must reduce speed before entering the curve
 Reducing during the turn is extremely dangerous
 Road conditions
 Should be a last resort
 Maintain lane position
 Do not understeer or oversteer
 Slow in, Fast out
 Brake going into the turn and accelerate out
6.3 Braking
 How to brake
 Get to know how “touchy” the brakes are before you begin driving at high speeds
 Keep heel of right foot on the floor
 Move from accelerator to brake without lifting foot
Anitlock Braking Systems (ABS)
 Sensors mounted on each wheel that determines whether the wheel locks
or stops rotating during braking
 Sensor sends a signal to the microprocessor that controls the brakes
 Microprocessor tells the brakes to release enough pressure on that
particular wheel to allow it to skid
 The microprocessor then tells the brake to increase pressure
 This is repeated many times a second
 You may feel a “pulsing” sensation
Emergency Braking
 E-Brake
 Pumping the brakes
 For vehicles without ABS
 If done on a vehicle with ABS, it can send false information to the sensors and can cause
the car to skid
Stopping Distance
 Strong correlation between the speed you are traveling and the time
required for you to stop.
 Your ability to stop smoothly, accurately, and rapidly depend on two things:
 Reaction distance
•
Distance traveled by your vehicle during the time it takes you to identify the need to stop
•
Depends on speed, visibility, and your mental and/or physical state
•
Faster you are moving and the harder it is to see, the longer it takes to react
 Braking distance
•
Distance needed to come to a complete stop once the brakes have been applied
•
Depends on vehicle type and weight, type and wear of tires, and condition of brakes
Reaction distance + Braking distance =
 Stopping distance
 Total distance required to stop from the first time you recognize the need to brake to the
time the vehicle is no longer moving
 SPEED is the most important factor
•
The faster you go, the longer it takes to stop
6.4 Speed Limits
 What is a safe driving speed?
 Factors that affect “safeness”:
•
Posted speed limits, weather conditions, visibility, surface condition of the roadway, traffic
conditions, lane width, and any special speed laws (depends on the type of vehicle you are
driving)
• Posted Speed Limits
•
Maximum speed limit
• Not there to recommend a driving speed
•
Minimum speed limit
• Prevent drivers from moving at such slow speeds that they impede or block the normal flow of
traffic
Basic Speed Law
 Everyone has one
 It says that a driver should operate his/her vehicle at a speed that is
reasonable for existing conditions
 Examples would include inclement weather, poor visibility, poor traffic
conditions, or poor roadway surface
Other Speed Laws
 Uniform Speed Zones

•
Provide speed limits in commonly encountered locations even where no max. or min. speed
limits are posted
•
School zones
•
Railroad crossings
•
Alleys
•
Business districts
•
Residential districts
•
Rural highways
•
Interstate freeways
Special Speed Laws
•
Slow moving vehicles
•
Trailers
•
Trucks or tractors with three or more axles