Air Brake Critical Numbers
| PSI / Value | What It Means |
|---|
| 60 PSI | Low air pressure warning light/buzzer activates |
| ~20-45 PSI | Spring brakes (emergency) automatically apply |
| ~100 PSI | Air compressor governor cuts IN (compressor starts) |
| ~125 PSI | Air compressor governor cuts OUT (compressor stops) |
| 90-120 PSI | Normal operating pressure range |
| 0.4 sec | Air brake lag time (adds ~32 ft at 55 mph) |
Yellow knob = parking brake. Red octagonal knob = trailer supply valve. Test brakes before every trip.
Hours of Service Rules
| Rule | Limit |
|---|
| Driving limit | 11 hours after 10 consecutive hours off duty |
| On-duty window | 14 hours from first duty (cannot extend) |
| Mandatory break | 30 minutes off duty after 8 hours of driving |
| Weekly limit (7-day) | 60 hours in 7 consecutive days |
| Weekly limit (8-day) | 70 hours in 8 consecutive days |
| 34-hour restart | Reset weekly limit with 34+ hours off duty |
| Off-duty required | 10 hours off before new driving shift |
Federal Weight Limits
| Category | Maximum Weight |
|---|
| Gross Vehicle Weight (GVW) | 80,000 lbs |
| Single axle | 20,000 lbs |
| Tandem axle group | 34,000 lbs |
| Steer axle (typical) | 12,000 lbs |
Bridge formula limits weight based on axle spacing. States may impose lower limits than federal standards.
CDL Endorsements
| Letter | Endorsement | Requirement |
|---|
| H | Hazardous Materials | Federal background check (TSA) |
| N | Tank Vehicle | Knowledge test |
| P | Passenger (16+) | Skills + knowledge test |
| S | School Bus | P endorsement + school bus test |
| T | Doubles/Triples | Knowledge test |
| X | Tank + Hazmat combo | Both H and N requirements |
Stopping Distance Formula
Total Stopping Distance = Perception + Reaction + Braking
| Component | At 55 mph | Time |
|---|
| Perception distance | ~142 feet | 1.75 sec |
| Reaction distance | ~60 feet | 0.75 sec |
| Air brake lag | ~32 feet | 0.4 sec |
| Braking distance | ~170 feet | varies |
| TOTAL | ~400+ feet | ~3 sec |
Double stopping distance on wet/icy roads. Loaded trucks take longer to stop than empty ones.
Pre-Trip Inspection Key Points
- Engine compartment: fluids, belts, hoses, wiring
- Steering: free play max 10° for vehicles over 40 ft
- Tires: minimum 4/32" tread (front), 2/32" (others); check pressure
- Lights: headlights, brake lights, turn signals, hazards, clearance
- Brakes: check air pressure, test service brakes, parking brake
- Coupling: kingpin fully latched, 5th wheel secure, safety chains
- Cargo: properly secured, doors closed, lights working
- Emergency equipment: reflective triangles, fire extinguisher, spare fuses
Most common failure points: lights out, tire issues, brake defects, improper coupling.
CDL Classes & Hazmat Placards
| Class | Vehicles | Additional Privileges |
|---|
| Class A | Combo vehicles GCWR >26,001 lbs (towed >10,000 lbs) | Can drive Class B and C vehicles |
| Class B | Single vehicle GVWR >26,001 lbs; towing ≤10,000 lbs | Can drive Class C vehicles |
| Class C | Vehicles with 16+ passengers OR hazmat placards | Requires endorsements (H, P) |
Hazmat Placard Classes: Class 1=Explosives | Class 2=Gases | Class 3=Flammable Liquids | Class 4=Flammable Solids | Class 5=Oxidizers | Class 6=Poisons | Class 7=Radioactive | Class 8=Corrosives | Class 9=Misc