Turning Circle Requirements for HGV Trailers

Q: Does the choice of tractor unit affect turning circle?

Yes. Wheelbase, steering angle and fifth-wheel position affect the swept path. However, trailer type-approval compliance is determined independently of the tractor and based on the trailer’s geometry or deemed-to-comply rule.

Q: Do I need a steering axle for urban delivery trailers?

Not necessarily. Short urban trailers can meet manoeuvrability requirements with fixed axles, but self-steer or command-steer axles reduce tyre scrub and improve turning in confined areas.

Q: How does wheelbase affect rear swing-out?

Shorter wheelbases increase rear swing-out because rear overhang is proportionally longer. Longer wheelbases reduce swing-out but may slightly increase the turning radius and affect axle-load balance.

Q: How is the 8,135 mm limit derived?

It comes from the 97/27/EC Annex I turning-circle formula: sqrt((12.50 − 2.04)^2 − (5.30 + L/2)^2). For a 2.55 m trailer this gives approximately 8.135 m; for 2.60 m about 8.115 m.

Q: Is TrailerWIN the same as a physical turning test?

No. TrailerWIN is a simulation tool for assessing turning geometry. It supports design validation but does not replace the geometric or deemed-to-comply methods used for type-approval compliance.

The turning circle of a heavy goods vehicle (HGV) defines how tightly it can manoeuvre and is a key factor in type-approval compliance. For articulated vehicles, the turning-circle requirement is based on the ability of the combination to negotiate a path between two concentric circles of 5.3 m (inner radius) and 12.5 m (outer radius).

Within the type-approval framework, compliance for semi-trailers can be demonstrated either by a geometric drawing or CAD calculation showing conformity with the 5.3 m / 12.5 m envelope, or by applying the statutory “deemed-to-comply” rule defined in European legislation.

Directive 97/27/EC, Annex I 7.6.1.2

“A semi-trailer is deemed to comply with the requirements of 7.6.1 if its wheelbase is not greater than:
√((12.50 – 2.04)² – (5.30 + L⁄2)²) (metres)”

In this formula, L represents trailer width in metres. For a standard 2.55 m-wide semi-trailer, this equates to a maximum wheelbase of approximately 8 135 mm. This value remains the accepted reference within UK and EU type-approval practice for conventional semi-trailers.

Type-approval framework

The “deemed-to-comply” provision originated under Directive 97/27/EC and was subsequently carried forward in Regulation (EU) 1230/2012 (as amended by 2019/1892). The requirement applies to the semi-trailer design and must be demonstrated during type approval.

For semi-trailers placed on the market under valid type approval, compliance is generally accepted where the kingpin-to-bogie-centre distance does not exceed 8 135 mm for a 2.55 m-wide trailer. This removes the need for a physical turning test of the trailer itself.

Don-Bur typically designs wheelbases up to approximately 8 100 mm to maintain a compliance margin.
Shorter wheelbases are used where tighter manoeuvrability or tail-lift integration is required.
Turning-circle performance is validated using CAD-based swept-path analysis and specialist software such as TrailerWIN.

Design considerations

Wheelbase selection affects both manoeuvrability and payload distribution.
Longer wheelbases improve stability and load transfer to the tractor but reduce turning agility.
Shorter wheelbases improve turning but increase bogie loading and potential tyre scrub.
Axle and kingpin positions are optimised during design to meet both type-approval geometry and axle-load limits.

Conclusion

The turning-circle requirement remains a fundamental part of semi-trailer type-approval. Compliance may be demonstrated by geometric analysis or through the long-established 8 135 mm deemed-to-comply rule. Don-Bur applies these standards throughout the design and approval process to ensure that all semi-trailers meet the prescribed manoeuvrability criteria.

Disclaimer: This article provides general information on type-approval requirements for semi-trailers. It reflects Don-Bur’s interpretation of published legislation and should not be taken as legal advice.

FAQs

Does the choice of tractor unit affect turning circle?

Yes. The tractor unit’s wheelbase, front-axle steering angle and fifth-wheel position all influence how the articulated combination steers. A long-wheelbase tractor will typically increase the swept path compared with a shorter one. For trailer type-approval, however, compliance is assessed independently of the specific tractor unit and is based on the trailer’s geometry and deemed-to-comply rule.

Do I need a steering axle for urban delivery trailers?

Not necessarily. Many short urban trailers achieve the required manoeuvrability with fixed axles. However, fitting a self-steering or command-steering axle can reduce tyre scrub, improve turning radius and lower operating costs where tight access or frequent yard manoeuvres are involved.

How does wheelbase affect rear “swing-out”?

A shorter wheelbase increases rear swing-out because the rear overhang becomes proportionally longer relative to the pivot point. A longer wheelbase reduces swing-out but may slightly increase the turning radius and affect axle-load distribution. These factors are considered during trailer design to maintain compliance and safe handling.

How is the 8,135 mm limit derived?

It is derived from the turning-circle formula in Directive 97/27/EC Annex I, para 7.6.1.2:

√((12.50 − 2.04)² − (5.30 + L/2)²) metres

Where L is the trailer width in metres. For a 2.55 m trailer this produces approximately 8.135 m (≈ 8,135 mm). The value varies slightly with trailer width, e.g. L = 2.60 m → ≈ 8.115 m.

Is TrailerWIN the same as a physical turning test?

No. TrailerWIN and similar software simulate specific tractor–trailer configurations to illustrate swept paths and turning geometry. They are useful design tools but remain theoretical models. For type-approval purposes, compliance is demonstrated either by the geometric calculation or by the 8,135 mm deemed-to-comply wheelbase limit.