the speed factor: negotiating curves and braking chapter 6
TRANSCRIPT
The Speed Factor: Negotiating Curves and Braking
Chapter 6
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
1 2 3
E
4
Starting Point
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