A semi‑trailer is an unpowered freight unit with no front axle. It connects to a tractor (truck) via a kingpin–fifth wheel coupling, so roughly 30–50% of its weight rests on the tractor, while the rear axles carry the rest. It has landing gear (support legs) to stand alone when disconnected.
Main Types of Semi‑Trailers
① Flatbed Trailer
- Open, flat platform (48–53 ft long, 102 in wide)
- Best for: Steel, lumber, machinery, oversized loads
- Payload: ~44,000–48,000 lbs
- Pros: Easy loading/unloading; versatile
② Dry Van Trailer
- Enclosed, box‑shaped (standard 53 ft)
- Best for: General cargo, electronics, retail goods
- Pros: Weatherproof, theft‑resistant; most common for dry freight
③ Reefer (Refrigerated Trailer)
- Insulated + cooling/heating unit (–22°F to 77°F)
- Best for: Food, pharmaceuticals, perishables
- Pros: Precise temperature control; protects sensitive goods
④ Lowboy / Lowbed Trailer
- Extremely low deck (18–24 in ground height)
- Best for: Heavy machinery (excavators, cranes), overheight loads
- Pros: Ultra‑low center of gravity; legal for tall/heavy equipment
⑤ Step Deck (Drop Deck) Trailer
- Two‑level deck (upper + lower)
- Best for: Loads too tall for flatbeds (up to 10 ft)
- Pros: No oversize permit needed for most high cargo
⑥ Dump Trailer
- Hydraulic tilting bed
- Best for: Sand, gravel, coal, construction debris
- Pros: Fast, automated unloading
⑦ Tanker Trailer
- Sealed cylindrical tank
- Best for: Liquids (fuel, water, chemicals), gases
- Pros: Leak‑proof; designed for safe liquid transport
⑧ Container Chassis
- Frame for 20 ft / 40 ft / 45 ft containers
- Best for: Intermodal shipping (ship → truck → rail)
- Pros: Standardized; fast container swapping
