AN UNDERSTANDING OF REGULATION 219

THERE ARE STILL many in the Road Transport Industry, Suppliers and Operators, who are apparently unaware that it is no longer legal to fit an independent "Tag" axle to a Semi-Trailer.

Road Traffic Regulation states -

The importance of this Regulation is that a semi-trailer can have only one axle or one axle unit and in the case of an axle unit all the axles in the unit must be interconnected, and reference must be made to the definition of "Interconnected" which states - 

"Interconnected" - means, for the purpose of the definition of "axle unit", the design is such that an upward force on one axle in an axle unit transmits a downward force to the remaining axle or axles in the axle unit. 

Figure 1 illustrates the ramifications of Regulation 219 which applies to all semi-trailers registered for the first time after 1 July 1999.

It is important to remember that the fitting of independently suspended "Tag" Axles, in front of or behind other axles in an axle unit is no longer permitted, also, all axles must be fitted with this same suspension, i.e. conventional steel springs, air or hydraulic. This does not mean that a semi-trailer cannot have an axle unit with the axles positioned as illustrated on Figure 2, as it is permissible to fit an axle unit, shown on Figure 2, providing all the requirements prescribed in Regulation 219 are complied with. But there are application problems. 

To achieve complete interconnection of all axles with steel/spring suspension is extremely difficult and complicated, and if all axles are fitted with air suspension, care must be taken to ensure that the suspension incorporates adequate stroke (up and down). 

In the case of an axle unit, as depicted on Figure 2, it is essential that the rear most axle be a steered axle (mechanical or self-steered) and care must be taken to ensure that the steered axle provides adequate angle of steer.

Figure 3, shows a six axle artic consisting of a standard 6x4 truck-tractor and a three axle semi-trailer, as shown on Figure 3.

With the front steering wheel of the truck-tractor, the turning radius set at 13.1 m to the outside of the outer wheel, it will be noted that theoretically the angle of turn is 24° but...

The maximum angle of turn of the outer wheel of available steered axles is ± 15° when the angle of turn is prevented trom further turning by limiting stops. This of course means that the very purpose of the steered axle is negated. Theoretically, when the turning radius of the outside of the outer front wheel of the truck-tractor reaches 18.322 m, the outside radius of the turning corridor is 18.733 m and the inside radius 13.922 m and, most important, the turning angle of the outer steered wheel of the semi-trailer will be 15°.

Any "tighter" turning radius will result in "scuffing" of the tyres of the axle unit of the semi-trailer.

For the record, the turning radius of a standard 6x4 truck-tractor is 7.5 m.

The question must be asked - "Is the three axle semi-trailer with an axle unit, as illustrated on Figure 3, acceptable in practice if tight turning conditions are involved.

The purpose of Regulation 219 was to include the fitting of a steered axle to gain additional loading on Tridem Semi-Trailers. The purpose of this Regulation is still applicable to existing semi-trailers, but in view of the amendment to Regulation 225 increasing the wheelbase of a semi-trailer from 9 m to 10 m, virtually negates the advantage gained from Regulation 219.

Therefore, Regulation 219 does not apply to new semi-trailers manufactured since the amendment to Regulation 225 on 25 September 2003 and only to the pre-existing semi-trailers.