Chapter 12 - Commercial Vehicles

Commercial Motor Vehicle (CMV) — DOT Definition

A vehicle (or combination) used in commerce that meets ANY of the following:

• GVWR ≥ 26,001 lb (11,794 kg)
• Towed unit GVWR ≥ 10,000 lb (4,536 kg)
• Designed to transport 16+ passengers (including driver)
• Any size vehicle transporting hazardous materials

CMV Weight Classifications (GVWR)

Class 1: 0–6,000 lb (light duty)
Class 2a: 6,001–8,500 lb (light duty)
Class 2b: 8,501–10,000 lb (light duty)
Class 3: 10,001–14,000 lb (light duty)
Class 4: 14,001–16,000 lb (medium duty)
Class 5: 16,001–19,500 lb (medium duty)
Class 6: 19,501–26,000 lb (medium duty)
Class 7: 26,001–33,000 lb (medium/heavy duty)
Class 8: over 33,000 lb (heavy duty)

Commercial Truck Anatomy — Major Systems

Drive Train (power train)

Transfers power from engine to wheels.

Cab Types

• Cab over engine (COE/tilt cab) — driver sits over engine, cab tilts forward
• Conventional cab — driver sits behind engine and front axle
• Cab beside engine (CBE) — driver sits next to engine

Sleeper

• Compartment attached behind cab for resting
• Often aluminum/fiberglass with insulation and wiring
• Requires multiple tools for cutting

Glass on CMVs

• Windshields = laminated safety glass in rubber gasket
• Side windows = tempered safety glass (thicker than passenger vehicles)
• Some windshields sealed with mastic (must be cut out)

Chassis

• Main structural frame of CMV
• Supports cab, cargo, suspension, engine
• Built with heavy steel rails and crossmembers

Suspension Systems

Types:

• Leaf spring systems
• Air ride systems (air bags + air pressure tanks)
• Coil springs (in some designs)

Air ride effects:

• Uses onboard compressed air
• When parking brake engaged → air released but stability maintained
• If air bag ruptures → vehicle may drop several inches
• Air bags usually interconnected by axle (one rupture can drop entire axle)

Cargo Area Systems

• Box truck — cargo attached to same frame as cab
• Semi-tractor trailer — cargo on separate trailer frame

Fifth wheel

• Locking hitch connecting tractor to trailer

Landing gear

• Manual support legs for trailer when detached

Dromedary (drom box)

• Separate storage box in front of fifth wheel behind cab

Axles

• Live axle — drives wheels (propulsion)
• Dead axle — load support only (steering axle)
• Lift axle — raised/lowered to distribute weight

Typical tractor:

• Front steering axle
• Two rear tandem axles

CMV Doors

• Heavier steel construction
• Hinges may be piano hinges, strap hinges, hidden hinges
• Latches stronger than passenger vehicles

Door removal philosophy remains:

Expose → purchase point → cut latch or hinges

Air Brake Systems

Three brake types:

• Service brake — normal driving brake
• Emergency brake — activates during air failure
• Parking brake — holds vehicle when stopped

System components:

• Onboard air compressor
• Air tanks (120–125 psi normal)
• Service air line (blue)
• Emergency air line (red)
• Glad hands connect tractor to trailer

ABS systems standard on modern trucks

Battery Systems

• Usually 4+ 12-volt batteries wired in parallel
• Located under step, behind cab, or frame rails

Fuel Systems

• Aluminum or steel saddle tanks
• Usually diesel fuel
• Equipped with shutoff valves (varies by manufacturer)

Hybrid/Electric CMVs

• Use combustion engine, electric motor, or both
• High-voltage battery systems
• Regenerative braking

HAZARDOUS MATERIALS & CARGO TANKS

Hazard Classes (9 total)

1 — Explosives
2 — Gases (flammable, nonflammable, toxic)
3 — Flammable liquids
4 — Flammable solids / spontaneous combustion / water-reactive
5 — Oxidizers & organic peroxides
6 — Toxic & infectious substances
7 — Radioactive
8 — Corrosives
9 — Miscellaneous hazardous materials

Common Cargo Tanks

MC-306/DOT-406
• Flammable liquids (gasoline)
• Nonpressurized

MC-307/DOT-407
• Flammable & corrosive liquids
• Higher internal pressure

MC-312/DOT-412
• Corrosives (acids)
• Heavy reinforcing rings

MC-331
• Pressurized gases (propane, ammonia, butane)
• ~300 psi operating pressure
• Explosion hazard if relief valves fail

MC-338
• Cryogenic liquids (liquid oxygen, nitrogen)
• Insulated tank
• Pressure relief venting common and normal

Tube trailers
• Multiple high-pressure gas cylinders

Dry bulk tanks
• Powders, grain, fertilizer
• V-shaped bottom discharge

Placards

Required when:

• Single package ≥ 1,001 lb aggregate
• ANY explosives, poison gas, radioactive material
• Word “DANGEROUS” allowed when mixed hazards > 1,001 lb

Displayed on all sides and ends

UN/NA Identification Numbers

Used when:

• Large bulk shipments
• Displayed on placard or orange panel

Conditions:

• Same material only
• ≥ 8,820 lb (4,000 kg)
• Loaded at one facility

Shipping Papers Must Include:

• Shipper
• Receiver
• Hazard class/division
• UN/NA number
• Packing group
• Quantity/weight
• 24-hour emergency contact
• Certification statement
• Signature

CMV SCENE OPERATIONS — REQUIRED ACTIONS

When CMV involved:

• Identify hazardous cargo (placards/UN numbers)
• Dispatch hazmat teams + heavy tow units
• Establish hazard zones (hot/warm/cold)
• Disconnect batteries
• Deploy hose lines for protection
• Stabilize all vehicles
• Confirm shipping papers BEFORE opening cargo
• Follow IAP with Plan A / Plan B contingencies

NFPA TEST FACTS — CAUSE → EFFECT

If CMV transports hazardous materials → placards required

If air bag ruptures → vehicle may suddenly settle

If parking brake applied → air released but vehicle remains stable

If emergency air line fails → emergency brakes engage

If CMV involved in MVC → assume hazardous cargo until confirmed

If relief valve vents on cryogenic tank → usually normal operation

If MC-331 exposed to fire → extreme explosion hazard

If UN/NA number present → bulk shipment of single hazardous material

SAFETY AWARENESS (FROM TEXT)

• Secure tilt-cab COE before extrication
• Disconnect battery systems before entry
• Cargo shift during lifting can destabilize scene
• Never manually release air brake springs

KEY TERMS — CONSOLIDATED

Commercial Motor Vehicle (CMV)
Vehicle used in commerce meeting DOT weight/passenger/hazmat criteria

GVWR
Gross vehicle weight rating — max allowed vehicle weight

GCWR
Gross combined weight rating — tractor + trailer capacity

Cab over engine (COE)
Cab tilts forward over engine

Conventional cab
Driver behind engine and front axle

Cab beside engine (CBE)
Driver sits next to engine

Fifth wheel
Locking hitch connecting tractor and trailer

Landing gear
Trailer support legs when detached

Air ride system
Suspension using air bags and compressed air

Live axle
Drives wheels

Dead axle
Load support only

Lift axle
Weight-distribution axle

MC-306/406
Nonpressurized flammable liquid tanker

MC-307/407
Higher-pressure chemical tanker

MC-312/412
Corrosive tanker

MC-331
Pressurized gas tanker

MC-338
Cryogenic tanker

Placard
Hazard warning signage

UN/NA number
Hazardous material identification number