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Physics, Mechanics and Maintenance

“Who let the apple fall out of the tree?”

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Physics

• Natural Laws you can’t change• Measurable and predictable

– Friction – Inertia– Gravity– Kinetic energy– Force of impact– Skid control and Stopping distance

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What happens when you break the laws of nature?

• The best of intentions cannot change the laws of nature.

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Friction

• Resistance to motion

• Necessary to control vehicle

• Control of vehicle depends on friction to tire on the surface of the road.– AKA….traction

• Must know condition of road– Effect on your tires

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Influences on Traction

• Wet, snow, ice, mud, loose gravel– Condition of road surface

• Bumpy road– Reduced friction thus control

• Hydroplaning– Water between tires and road surface

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Hydroplaning

• Factors that influence– Standing water– Depth of tread on tires– Design of tire tread– Speed of vehicle– Demands of vehicle

• Corners• Hills• Braking

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What is the surface made of?

• Dirt

• Gravel

• Asphalt

• Cement

• Brick

• Misc.

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Dirt

• Dusty

• Soft

• Moisture effect

• Rough

• Limit speed

• Not usually mapped

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Gravel

• Rough, can be washboard

• Hard to steer

• Hard on tires

• Limit speed and traction

• Potholes

• Rock chips

• Dusty

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Asphalt

• Most common• Usually smooth• Sloped for drainage• Good traction• Speed limited by other factors• Slick in adverse weather (snow, ice, rain)• Oil leeches out• Potholes are sharp edged• Rutted

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Cement

• Smooth with expansion joints

• Good traction

• Sloped for drainage

• Speed limited by other factors

• Slick in adverse weather

• Different slab heights

• Less pliable

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Brick

• Mostly decorative

• Less friction

• Rough ride

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Other Misc

• Metal

• Wood

• Mixture

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Bridges

• May be narrow

• No shoulder

• Freeze first

• Metal are very slippery, especially when wet or oily

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Tire influence on traction

• Tread Wear

• Tread pattern

• Studs, siped

• Tire pressure

• Chains

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Inertia

• An object in motion tends to stay in motion unless acted upon by a force applied to change its rate or direction.

• Traction is your way to overcome inertia– Not enough traction and you

crash• Stopping • Turning

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Corners

• Centrifugal force wants to go straight• Traction needed to turn

– Reduced traction, reduce speed– Snow, ice, etc.

• Banking for improved speed turn– Lean into curve– Reverse banking makes high hazard

• Guard rails present?• What is your visibility around corner

– Can you stop in time?

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How fast to go around corner?

• Use good judgment to judge– Curve radius– Bank– Surface– Condition– Speed– Visibility– Traffic volume

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Curve Radius

• Low speed curves (<40 mph)

– Constant curve

– With or without banking

• High Speed Curves (>40 mph)

– Gradual onset

– Increasing curve

– Gradual return to straight

– Banking

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Curve Radius (cont.)

• Combinations are Dangerous– Require changes in speed and adjusted steering– Heavy braking causes vehicle spin

• Posted speeds– Recommended to be comfortable – 15mph over usually OK– Not if <25mph

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Let’s talk about Visibility!

• How far can you see?– Are you overdriving your visibility?– Stopping distance for conditions present

• Corners

• Hills

• Buildings

• Vehicles (Trucks in particular)

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Gravity

• “Why did the apple fall?”• Weight of vehicle is a big factor

– Acceleration and braking

• Uphill and Downhill– Changes vehicle actions

• Change of weight distribution – Center of gravity– More on front or rear tires

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Kinetic Energy

• AKA momentum

• At rest……….stored energy

• In motion……….kinetic energy

• K=1/2 mass X velocity squared (KE=½ MV2)

• What is the most important factor?

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Let’s look at some examples:

• You are driving your ambulance 20 mph• Now you accelerate to 40 mph

– How much more kinetic energy?– 4 times the energy

• Now accelerate to 80 mph– Now how much more kinetic energy– 16 times more– 16 times more potential for damage

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Force of Impact

• Influenced by– Speed*****– Weight – Suddenness of stop

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Force of Impact

• What are you hitting?• Is it going to give any?• Is it adding KE to the equation• Who is bigger, moving faster?• Mechanical Engineering

– Vehicles are made to absorb energy– Energy absorbed in vehicle is not in patient or

you!

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Skid Control

• Loss of traction=skid

• What do you do if you skid?– Stop braking– Turn into direction of skid

• Usually the back end of vehicle

– Be prepared for secondary skids

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Skid Prevention

• Causes of skids– Over-accelerating

• Ease up on gas pedal

– Over-steering• Inertia greater than traction

• Slow down and turn slower

– Over-braking• Breaks lock up

• Release brakes– Re-establish traction

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Stopping Distance

• “48 vehicle pile-up on I-80 due to fog”

• Common problem on freeways• Stopping Distance Factors:

– Perceive potential hazard

– Identify unsafe situation

– Make decision to stop

– React by pressure on brake pedal

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Breaking Distance?

• Reaction time– Mental alertness

– Anticipation

– Foot on brake before

• Brakes applied• Friction stops vehicle

– Most variables

– Tires, Road surface, etc.

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Examples tell the story

• At 55 mph– Perception+reaction+braking=228feet

• At 65 mph– Perception+reaction+braking=305feet

• That’s 100 yards!

• Many things can influence the braking distance………….

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Influences to Stopping Distance

• Vehicle Speed• Perception time#• Reaction time# #driver condition• Weather• Road conditions• Weight of Ambulance• Condition of tires *• Brake condition * *Preventative Maint.

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Influences to Stopping Distance

• Brake ability– Heat– Dust– Water

• Brake fade– 700o liner – shoes melt– Disc better than drum– Less surface friction

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Type I Ambulance

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Type I Ambulances

Truck chassis with mounted patient care box

· Box and cab are separated

· No walk through area between patient compartment and drivers compartment

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Type II Ambulance/Rescue

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Type II Ambulances

Van body

· Raised roof

· Walk area between cab and patient compartment

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Type III Ambulance/Rescue

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Type III AmbulancesPatient care box mounted on Van Chassis

· Walk through between cab and patient care compartment

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Following Distance

• Distance needed to stop in time!• Less traction=greater distance• Less visibility=greater distance• Heavier vehicle=greater stopping distance• Two easy rules (under ideal conditions)

– 4 second rule…• Time to reach spot where lead vehicle is

– Car length rule• 1 car length for each 10 mph

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Clues to help judgments

• Look for oil discoloration on road– Usually means dip in road

• Scrapes on guardrails– Wrecks, vehicle contact

• Shade on road– Slick in frost conditions

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Read the Signs

• Highway Department Signs

• Use as guide to speed

• Pay attention to them!– Loose gravel– Congestion Ahead– Slippery when wet– Safety Corridor

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A new line of work

• Painted surfaces are helpful– Fog lines– No Passing lines (and lanes)– Solid lines versus intermittent– Merging lanes and Arrows– Reflectors

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Let’s talk just briefly on Maintenance

• Brakes

• Tires

• Visibility (windows)– Washer fluid– Wiper blades

• Sirens and horns

• Panel lights

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What about the driver

• Effect on Perception and Reaction time– Fatigue– Excitement– Illness– Medications– Pre-occupation/distraction

• Thinking about the call

– Horseplay– Experience

• Know road and vehicle better

– Complacency********

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How to make the best out of a bad situation!

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