Emergency Vehicle OperationsUnit VI

Theory and Principals ofVehicle Dynamics

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Unit VI Objectives

The EVO will be able to identify reason why we crash

The EVO will be able to describe the physical forces that influence vehicle handling

The EVO will be able to identify the dynamics and physics behind large vehicle behavior and their relationship to vehicle speed and size

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The manner in which the EV is driven to and from the emergency scene is

the most important contributing factor that affects crash avoidance.

Excessive speed puts the EV driver in theposition of:1. Being unable to negotiate curves2. Being unable to stop before hitting another vehicle3. Being unable to stop before entering an intersection4. Loosing control due to weight shift5. Losing control due to right side wheels leaving road surface6. Losing control from poor road conditions such as pot holes, speed

bumps, wet, icy or snowy road surfaces

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Why Do We Crash Emergency Vehicles Responding and Returning from Calls?

• Drive too fast• Don’t Stop at Intersections• Don’t wear our seatbelts• Refuse to use spotters• Fail to understand the difference between

these vehicles and personal automobiles

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Vehicle Dynamics

Let’s look at:

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Physical Forces

Following Distances

Braking Distances

Physical Forces Include:

• Friction• Momentum• Inertia• Centrifugal Force

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IF THEY ARE EXCEEDED, CONTROL IS LOST!

Physical Forces

Always presentUnderstandableNegative effects

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Physical Force

Function Of: Size (mass) Speed (velocity) Highway Geometry Which do you control??

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Physical Forces

Inertia- Newton’s 1stMomentum- Newton’s 2ndReaction-Newton’s 3rd

Friction- resistance to motionCenter of gravityVelocity

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Force

Engine = 42,000 lbs.– At 25 MPH = 47,250 lbs..– At 55 MPH = 115,500 lbs..

Ladder Truck= 56,000 lbs.– At 25 MPH = 63,000 lbs..– At 55 MPH = 154,000 lbs..

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EXAMPLES:• Driving too fast for weather, road, or tire

conditions• Accelerating too hard• Braking inappropriately• Changing directions too abruptly

KNOW THE CONDITIONS AND THE LIMITS AT WHICH TO STAY!

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Changing directions too abruptly caused this rollover.

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What is What is FrictionFriction??

Friction is the resistance to slipping.Friction is the resistance to slipping.

Friction occurs whenever two surfaces Friction occurs whenever two surfaces rub together.rub together.

Why is friction important for vehicle control?

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It would be impossible to control a vehicle without friction.

Friction enables the EV to stop, accelerate, and

change direction.

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Friction at the Brakes• The brake shoes pressing on the drums The brake shoes pressing on the drums

or pads clamping the disc create friction or pads clamping the disc create friction and slow the wheels.and slow the wheels.

• The friction at the brake surfaces The friction at the brake surfaces generates heat.generates heat.

• Heat due to excessive, hard braking Heat due to excessive, hard braking creates brake fade. creates brake fade. ( scary at best - deadly ( scary at best - deadly at it’s worst.)at it’s worst.)

Coefficient of Friction

• Measures how slippery a road is• Dry asphalt has a value of 0.8 to 0.9• Wet or icy roads drop to 0.2 or 0.3• The lower the value, the longer it takes to

come to a complete stop• Can’t drive the same on warm dry days as you

can on cold rainy days

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The Total Braking Distance =

• Perception Distance• Reaction Distance • Brake Lag Distance• Braking Distance

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Perception Distance - How far the vehicle travels when the need to

brake occurs and the driver recognizes that need.

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Reaction Distance – How far the vehicle travels as the driver reacts

and applies the brake.

(Perception/Reaction Distance based on 1.60 Second Reaction Time)

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Brake Lag Distance – How far the vehicle travels from the time the

brakes are applied and the vehicle begins to slow down.

(Apparatus Equipped with Air Brakes –Air Lag Distance Based on

0.5 Seconds)

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Braking Distance – How far the vehicle travels from the brakes

starting to slow the vehicle and the vehicle coming to a complete stop.

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Total Distance

The total distance includes• Perception Distance• Reaction Distance• Brake Lag Distance• Braking Distance

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Vehicle weight and speed affect stopping distance.

The speed being equal, it will take a large tanker longer to

stop than most pumpers.

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2003 Seagrave Pumper w/Anti-Lock Air BrakesGVW 36,900 lbs.

Speed / Speed / MPHMPH

Reaction Reaction

DistanceDistance

Air LagAir Lag Braking Braking DistanceDistance

TotalTotal

DistanceDistance

1010 23.4723.47 7.347.34 6.546.54 37.3437.34

2020 46.9446.94 14.6714.67 26.1426.14 87.7687.76

3535 82.1582.15 25.6725.67 80.0780.07 187.89187.89

4545 105.62105.62 33.0133.01 132.35132.35 270.98270.98

5555 129.10129.10 40.3440.34 197.71197.71 367.15367.15

This information obtained from: Stephen A. Coulon Fire Service Instructor, Collision Reconstruction Specialist, Adjunct Instructor N.Y. City Fire Department 181 Quinnipiac Ave North Haven, CT 06473 203-773-9829.

Reaction Time: 1.6 Seconds Air Lag Time: 0.5 Seconds

How Far is 367 Feet?

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Emergency Vehicle Operations

Stopping DistancePerception DistanceReaction DistanceBraking Distance

Speed, Weight, Road Surface60 MPH = 88ft/sec30 MPH = 44 ft/sec

Variables

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What Happens to this Distance if the Coefficient of Friction drops from 0.8 to 0.2

because of ice?

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Antilock Brakes

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AIR BRAKES

Air brakes have many common applications in vehicles today.

They can be found on school busses, dump trucks, fire trucks and most any vehicle over 15.000 pounds.

Let’s first cover some basic terminology.

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AIR BRAKESAir compressor and governor

The air compressor pumps air into the storage tanks (reservoirs) this compressor can be driven by gears or most common a V-belt

The governor controls the pressure going to the storage tanks. When the air pressure reaches 130 psi depending on the system requirements the governor will shut the compressor off until a pressure loss is detected.

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AIR BRAKES

Air storage tanks

Storage tanks will hold enough air to allow the brakes to be used in the event the compressor stops working

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AIR BRAKESSafety valve

A safety relief valve is installed in the first air tank (quick build up tank) this protects the braking system from over pressuring. This safety valve in most vehicles will pop off at 150 psi, then reset itself.

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AIR BRAKES

Drain valve

The tank drain valve, when opened, will allow any moisture or water to be drained from the air tank at its lowest point. This valve should be opened at least once a week for 2 to 3 seconds then closed.

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AIR BRAKESBrake drums, shoes, and linings.

Brake drums are located at the end of each axle and the wheels are bolted to the drums.

The brake shoes and linings are located inside the drums and push against the inside of the drum to provide friction needed for stopping.

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AIR BRAKESAir chamber

Holds the parking brake spring and applies pressure to a piston that will push on the s-cam. This will put pressure on the slack adjuster then to the brake pads.

Rear air chambers

Front air chamber

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AIR BRAKESAir supply gauge

All air-braked vehicles have an air supply pressure gauge. This gauge allows the driver to see the pressure in the system at all times. This is also used to check for air leaks when the system is off.

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AIR BRAKESLow pressure warning buzzer.This warning device will give the driver an audible warning in the event the system air pressure falls to the danger zone. In most fire trucks this alarm will sound at 60 < psi. Should this buzzer sound when the vehicle is in motion locate a safe location to pull the vehicle to a stop and check the system for damage.

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AIR BRAKE INSPECTIONNFPA 1915-10-3 3.12 THROUGH 3.12.10.10

The breaking system shall be inspected and maintained in accordance with the manufacture’s service recommendations .

If no service recommendation exist the shortest interval recommendation by the braking system manufacture, based on time or mileage, shall be adhered to.

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AIR BRAKE INSPECTIONThe braking system should be inspected at least once a week. Many full time fire departments require there emergency vehicles to be checked at the start of each shift.

In the volunteer sector emergency response vehicles should be inspected at least once a week. The most important point to remember is to document each inspection and keep the records for not less than 5 years.

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AIR BRAKE INSPECTIONWet dry switch (valve)

This switch (valve) will reduce the braking capacity of the front wheels to about 50% of normal. This will help to control the vehicle on wet or icy roads.

This switch should be in normal mode on dry roads.

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AIR BRAKE INSPECTION

The entire braking system should be inspected by an ASE certified mechanic once a year or every 3000 miles.

There are some simple test that should be completed during all in-house inspections.

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AIR BRAKE INSPECTIONAIR LEAK-DOWN RATE

With a fully-charged air system (typically 125 psi) turn the engine off and the wheels chocked and the barking brake off. Note the pressure and the time.

For single system, unit there should not be more than a 3 psi drop in one minute. If the pressure drop is more than 3 psi the system should be inspected for damage or leaks.

NFPA 1915-10-3.12.10.9

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AIR BRAKE INSPECTIONPARKING BRAKE TEST PROCEDURE.

With the system air pressure in the normal operating range and the driver in the vehicle with seatbelt on and the shifter in natural; the parking brake off, allow the vehicle to roll forward. Apply the parking brake and the vehicle should come to a rapid stop and prevent vehicle movement.

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AIR BRAKE INSPECTIONCompressor system

The compressor system should also be inspected for safe operation. Areas to be inspected will be the drive belt, hose fittings, and mounting brackets.

It should also be noted in the vehicle inspection form the pressure that the low air warning alarm starts and stops.

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BRAKING TECHNIQUES Air brakes and steep grades (Hills)

If you were faced with a 6% grade that went for a total distance of 6 miles and if you did not use any braking action during your decent.

With a vehicle with a GVW (gorse vehicle weight) of 38,000 pounds your speed would climb to 238 mph this is called terminal velocity. Therefore we must learn how to brake on hills.

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AIR BRAKE OPERATIONDown hill braking

For many years drivers of fire vehicles were taught to apply light steady brake pressure to keep vehicle speed in check during down hill braking.

This method is still taught to this day however after much testing and research this theory has been proven WRONG.

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AIR BRAKE OPERATIONDown hill braking con’t

SNUB braking is now the recommended method for downhill braking.

This method works by allowing the vehicle to speed up to the desired or posted safe speed then applying the brakes and slowing the vehicle down 5 to 10 mph below the posted speed.

Then allowing the speed to pick up and the process is repeated.

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AIR BRAKE OPERATIONDown hill braking con’t

By applying the brakes in this method there is time to allow the brakes and the brake drums to cool to help control heat buildup as well as to reduce the chance of brake fade .

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AIR BRAKE OPERATIONApplying the parking brake.

Whenever you bring the vehicle to a total stop with the transmission in neutral, you must set the brake. If you are on an incline, turn the steer wheels towards the curb on a decline.

On an incline the wheels should be turned away from the curb.

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Key points to remember1. Inspect vehicle brake systems not less than once a week.2. Document each inspection and keep records for 5 years.3. Have brake systems inspected by an ASE certified mechanic every 3000 miles or once a year.4. Insure a good driver training program is in place and train all new drivers on air brake systems.

Following DistancesFollowing distance is influenced by:

Reaction Distance =1.6 seconds and is the distance the EV travels from the time the operator recognizes the need to brake until the brake pedal is applied. (ADD .5 seconds for air lag)

Braking Distance =7.02 seconds@55mph for a Pumper. The distance traveled from brakes being applied until the EV comes to a full stop is 367.15 feet.Other factors are the WEIGHT and MECHANICAL CONDITION of the EV.

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Four SECOND RULEFour Second Rule - Keep a separation of at least two seconds between the EV and the vehicle being followed

When to increase following distance:

• 2 Seconds might be adequate for smaller trucks and speeds under 40mph

• 3 seconds should be used if traveling over 40mph• 4 seconds should be used for emergency vehicles. • Greater distance is required under poor road

conditions, overloaded vehicles, etc.

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Following Distancein

The Emergency Mode• Many operator’s reactions and performance

get worse under stress.• Motorist may react in unusual ways to lights

and sirens. If they stop abruptly, the EV Operator will need as much distance as possible to respond.

• A greater following distance permits the EV Operator to get “The Big Picture” of the traffic situation.

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Centrifugal Force

Centrifugal force is the force that tends to push a vehicle away from the center of the turning radius.

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Centrifugal Force

The force that tends to make a body move away from the center of rotationPushes a vehicle out of a curve4500lb Engine, 500’ radius @ 25 MPH = 3844 lbfSame Engine @ 55 MPH = 18606 lbf

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Obey posted speed limits in curves and

exit ramps from highways.

Skid marks should be a clue, others have made this mistake. DON’T REPEAT IT!

Critical Speed of a Curve• The speed at which the vehicle will loose

control rounding a curve• The experience of the driver does not matter• Need to know the radius of the curve and the

coefficient of friction to figure• If this speed is broken the vehicle will not stay

on the road!

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Ensure Safety at the Training

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Emergency Vehicle Operations

Impact ForceEngine = 42,000 lbs.

– At 25 MPH = 439 tons– At 55 MPH = 2125 tons

Ladder Truck= 75,000 lbs.– At 25 MPH = 784 tons– At 55 MPH = 3794 tons

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Big Bang Theory• The bigger it is and the faster it goes:• The BIGGER THE BANG WILL BE

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Center of Gravity

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Center of Gravity

Center of Gravity

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Center of Gravity

Community Expectations

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Center of Gravity

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Center of Gravity

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Center of Gravity

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Center of Gravity

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When Center of Gravity is exceeded

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Know What you are Driving

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It is important to train EV drivers on the vehicle they will be expected to

operate under emergency conditions.

Unit VI Review

The EVO will be able to identify reason why we crash

The EVO will be able to describe the physical forces that influence vehicle handling

The EVO will be able to identify the dynamics and physics behind large vehicle behavior and their relationship to vehicle speed and size

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Review Exercise is located on Page 68 of the student Training Manual.