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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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