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