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Trailer Eye Shafts: UK Types, Sizes & Eye Shaft Safety Guide

Trailer couplings come in all shapes and sizes — and one of the most important parts of any heavy-duty towing setup is the eye shaft. It might look like a simple metal loop, but get it wrong, and you risk coupling wear, bent drawbars, or worse — a trailer detachment.

Here’s what every trailer operator and hire outlet needs to know about eye shafts, UK standards, and why using the right match between eye and pin is so critical.


Recommended Standards

While there’s no specific legislation governing trailer eye shafts in the UK, the use of products complying with British Standard BS AU 24a:1989 is strongly recommended.

This standard sets out the dimensions for pins, hooks, and eyes, including the throat clearance between the jaw and the eye. That clearance is crucial — it ensures proper articulation (movement) in both vertical and horizontal planes.

Without the correct clearance, you risk overloading the coupling and introducing stress fractures in the drawbar or chassis.


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Common UK Eye Shaft Sizes

In the UK, the most common internal diameters for trailer eyes are:

  • 30 mm

  • 40 mm

  • 50 mm

These are typically used with:

There’s also the larger 76 mm NATO eye, mainly used by the military with heavy-duty pintle hooks — designed for rugged, off-road towing applications.


British vs European Eye Designs

British Eyes

British-style eyes have a round or toroidal section and are designed to be used with a parallel pin. This setup provides just enough clearance for:

  • Towing load to be taken by the pin, and

  • Braking load to be transferred through the jaw.

If the pin is too large or the clearance between the jaw and pin is too wide, both the towing and braking forces end up on the pin alone. That’s a recipe for premature wear and potential shear failure.


40 mm DIN Eyes

Across Europe, the 40 mm DIN eye is common. It has a hardened steel bush fitted to the inside and is designed to rotate around a special bulbous pin, not a flat one.

This difference is crucial. Using a 40 mm DIN eye with a British parallel pin causes:

  • Rapid pin wear from the bush’s sharp edges

  • Restricted articulation within the jaw

  • Accelerated coupling and chassis wear

In short, never mix the two systems — it’s a quick way to ruin your coupling and risk a failure on the road.


Towing with HGVs

If a trailer under 3,500 kg gross weight is towed by an HGV, it’s best to use an eye-type coupling instead of a 50 mm ball.

Why? Because the overrun coupling in an eye setup is better suited to handle:

  • The harsher suspension of an HGV

  • The extra overhang behind its rear wheels

  • The stronger braking and pulling forces involved

It’s all about matching the right components to the job — not just what fits on the day.


Brent’s Take

At Brent, we know that towing reliability comes down to small details — and the eye shaft is one of them. Choosing the right size, type, and standard-compliant coupling can mean the difference between a long, smooth hire life and costly downtime.

If your trailer uses a jaw, hook, or combination unit, make sure your eye shafts comply with BS AU 24a:1989 and match your towing setup exactly. And if you’re ever unsure — replace the coupling before the coupling replaces itself.

Find fully compliant, road-ready trailers and properly matched components on Brent — built for safety, strength, and confidence on every tow.


-B.R.E.N.T


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