What is Kingpin on a Semi Trailer? (Purpose, Sizes & Safety)

Learn what a kingpin is on a semi trailer, its role in coupling, standard sizes such as 2” and 3.5”, safety checks and EU/USA regulations.

📌 Published on 2025-11-06 | ✍️ Semi Trailer News Editorial

Image: Fifth wheel and kingpin coupling system

🔹 What is Kingpin?

The kingpin is the steel pin located under a semi trailer’s front plate that locks into the truck’s fifth wheel. It is the main coupling component that transfers vertical load and ensures articulation between the tractor and trailer.

🔹 Kingpin Sizes

There are two globally accepted kingpin diameters:

• 2” (50.8 mm) → Standard trailers up to ~44 tons GCW
• 3.5” (90 mm) → Heavy haulage over +70 tons GCW

🔹 Roles of the Kingpin in Safe Transport

• Transfers static and dynamic load from trailer to tractor
• Allows articulation during cornering
• Ensures a secure mechanical lock with the fifth wheel
• Supports braking and acceleration forces

🔹 Coupling Safety Checks

Kingpin must be properly engaged with the fifth wheel lock. Common inspection procedures:

• Visual check of locked jaws
• Pull test before departure
• Check for cracks, corrosion or excessive wear

🔹 Mounting & Replacement

Kingpins are either:

• Welded to the bolster plate
• Bolt-on type for fast replacement and maintenance

Important: Worn kingpins can cause accidental decoupling — which is one of the most dangerous trailer failures. Regular inspection schedules recommended by manufacturers must be followed.

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🔹 Regulations

• EN 12195 & ISO 337 standard requirements in EU
• DOT & SAE J2638 compliance for USA transport
• Mandatory wear limit controls during roadside inspections

Conclusion: The kingpin is the heart of the semi trailer coupling system. Correct size, regular inspection and proper locking ensure maximum safety and performance on the road.

5️⃣ Kingpin Load / 5th Wheel Height vs Slope Calculator

When evaluating a trailer for international purchase, fleet managers often overlook one of the most critical safety parameters: how kingpin load changes with slope and 5th wheel height differences.

Even a small slope (2–6%) or a mismatch between trailer skid-plate height and the tractor’s 5th wheel height can significantly shift the load toward (or away from) the kingpin. This affects:

• Tractor steering traction
• Axle group loading
• Fifth wheel wear
• Stability during braking & climbing

Use the calculator below to estimate how slope and height differences affect the vertical load on the kingpin.