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The Turning Corridor

With Europe looking towards adopting the turning corridor method of exercising some control over artics and combinations other than by overall length and wheelbase, FleetWatch's correspondent on legislation, Jack Webster, leads evidence in this article to prove that wheelbase is not the only criteria in the determination of the turning ability of a vehicle or combination of vehicles. In  doing so, he suggests that Regulation 355 be replaced with an entirely new regulation.

Since the introduction of the first Road Traffic Legislation into South Africa in 1957 through the Provincial Road Traffic Ordinances and subsequently, the National Road Traffic Acts 1989 and 1996, there has been a lack of regulation of control of combinations of vehicles save only to prescribe the overall length and to restrict a combination of vehicles to a drawing vehicle plus two trailers.

Vehicles making up combinations are registered and licenced separately and are tested and certified separately for roadworthiness. One of the major concerns in assembling motor vehicles into combinations is the ability of the combinations to negotiate “turns” for which there is no prescribed norms in South Africa in spite of the fact that, in general terms, the permissible maximum length of vehicle combinations is one of the longest in the world.

In their efforts to maintain a measure of control on the turning ability of combinations of vehicles, particularly articulated vehicles, the South African policy has been to limit the wheelbase of the semi-trailer and it is most interesting to recall the history of the maximum wheelbase and overall length of articulated vehicles. See Figure 1.

Figure 1

When the overall length of an articulated vehicle was increased from 17m to 18.5m and the overall length of a semi-trailer was deleted from Regulation 351, a strong appeal was made for an increase in the semi-trailer wheelbase. However, the request was rejected on the grounds that to do so would make the manoeuvrability unmanageable, an argument which was contested without success. I now propose to lead evidence in this article to prove that wheelbase is not the only criteria in the determination of the turning ability of a vehicle or combination of vehicles.

Firstly, what is about to be discussed is not an airy-fairy scheme concocted by some whiz-kid. It is something which has been adopted in many countries throughout the world.

It has already been stated that in refusing appeals for an increase in the wheelbase of semi-trailers, the authorities are concerned about maintaining an acceptable level of turning ability, but – what is an acceptable level of turning ability?

On 9 April 1999, an attempt was made to determine an acceptable level of turning ability of an articulated vehicle when Regulation 355 was amended and the norm was based on the turning ability of a five axle articulated vehicle consisting of truck-tractor and a two axle semi-trailer having a wheelbase of 9m.

The standard vehicle was put through a test and the outside and inside radii of a turning corridor was determined. Using the same truck-tractor but coupled to a semi-trailer with a steerable axle unit and a wheelbase in excess of 9m, it was put through the same test and if it negotiated the same turning corridor, it was issued with a certificate of acceptance of compliance with Regulation 355.

But the exercise only dealt with an articulated motor vehicle and the wheelbase of the semi-trailer. No dimensions were prescribed for the turning corridor of the test unit so the test becomes nothing more than a “comparison exercise”.

And what about the turning ability of an Interlink Combination or a truck-trailer combination? They also have to negotiate corners and intersections? What then is the solution to the problem of prescribing an acceptable turning corridor?

Simple answer 

The answer is simple and it is not necessary to re-invent the wheel.

Europe, through the directives of the Economic Commission for Europe (ECE), has adopted the turning corridor route to determine the turning ability of vehicles and combinations of vehicles, and there is no logical reason why South Africa cannot do the same. However, due to the fact that the maximum overall length of a combination of vehicles is longer in South Africa (and SADC) than in Europe, the ECE dimensions of the turning corridor have to be adjusted accordingly.

Thought has been given to the determination of acceptable dimensions for a South African turning corridor and Table 2 shows the relative dimensions of the ECE turning corridor and the proposed corridor for South Africa.

 Figure 2

The procedure for testing a vehicle or combination is explained in Figure 3. The test area is marked out as illustrated, using 15m radius to the outside of the corridor and 5m radius to the inside of the corridor.

Figure 3

The test procedure is simple: The vehicle/combination to be tested is parked in the position “S” and proceeds forward, keeping the corner of the front bumper on the 15 m radius line, completing 180° to finish in position “F”. The rear of the vehicle “off tracks” and wheels of the rear axle / axle unit must not cut-inside the 5m radius line. In other words, it must stay within the turning corridor.

Now let us check some vehicles/combinations by theoretical calculation –

1. Standard 12.5 m 4 x 2 Bus

Wheelbase                -                       6.05 m
Front Overhang          -                       2.365 m
Overall Width             -                       2.6 m

See Table 4

2. D/D Luxury Coach 13.2 m 6 x 2

Wheelbase                 -                       6.590 m
Front Overhang           -                       2.61 m
Overall Width              -                       1.3 m

3. Luxury Coach 15 m 6 x 2

Wheelbase           -                       9.160 m (in excess of the 8.5m presently prescribed)
Front Overhang     -                       2.65 m
Overall Width        -                       2.6 m

See Table 4

4. Freight Carrier – Forward Control 6 x 4

Wheelbase                  -                       5.275 m
Front Overhang            -                       1.525 m
Overall Width               -                       2.6 m

See Table 4

5. Freight Carrier – Forward Control 6 x 4

Wheelbase                  -                       6.451 m
Front Overhang            -                       0.833 m
Overall Width               -                       2.6 m

See Table 4

6. Six Axle Artic – T/T Normal Control with Tri-Axle Semi-Trailer

Truck-Tractor Wheelbase         -           5.613 m
Front Overhang                       -           1.273 m
5^th Wheel Position                  -           0.713 m
Semi-Trailer Wheelbase           -           9.000 m
Overall Width                          -           2.6 m

See Table 4

7. Six Axle Artic – T/T Normal Control with Tridem Semi-Trailer

Truck-Tractor Wheelbase         -           5.613 m
Front Overhang                       -           1.273 m
5^th Wheel Position                  -           0.713 m
Semi-Trailer Wheelbase           -           10.0 m
Overall Width                          -           2.6 m

See Table 4

The Six Axle Artic with a long wheelbase – bonneted truck-tractor and a semi-trailer with a wheelbase of 10m, is still within the 15.5m turning corridor.

8. Six Axle Artic – Truck-Tractor Forward Control

Truck-Tractor Wheelbase         -           3.875 m
Front Overhang                       -           1.525 m
5^th Wheel Position                  -           0.575 m
Semi-Trailer Wheelbase           -           10.0 m
Overall Width                          -           2.6 m

See Table 4

Note:    This articulated vehicle using a short wheelbase forward control truck-tractor and a semi-trailer with a 10m wheelbase is equivalent to a six axle artic using a standard long wheel normal control (bonneted) truck-tractor and a semi-trailer with a 9m wheelbase.

9. Interlink Combination – 22 m Overall Length – 3 Six Metre Containers

Truck-Tractor Wheelbase          -           3.875 m
Front Overhang                        -           1.525 m
5^th Wheel Position                    -           0.575 m
1^st Semi-Trailer Wheelbase-                 6.775 m
Semi-Trailer 5^th Wheel Position -          0.00 m
2^nd Semi-Trailer Wheelbase -                7.500 m
Overall Width                           -           2.6 m

See Table 4

The turning corridor of this Interlink Combination is the same as the six-axle artic with a semi-trailer with a wheelbase of 10 (ten) metres.

10. Truck-Trailer Combination – 22 m – Freight Carrier and Four-Axle Trailer

See Table 4

Table 4   

Note:   No Steerable Axles are necessary

Test Procedure

It is essential when conducting turning ability tests that the vehicle or in the case of a combination, that the drawing vehicle follows a predetermined course – in the examples used – entering the test as shown at Position S (Start) on Figure 3 and keeping the front corner of the front bumper on the 15m outside radius line, and continues through 180° until leaving the test area at position “F” (Finish).  The inside of the rear tyres is not to be less than the 5m inside radius of the corridor.

Conclusion

It will be noted that in the ten examples analysed, all stay within the turning corridor. Particular attention is drawn to Example 8 - the six axle artic with a semi-trailer having a wheelbase of 10m is within the turning corridor and has a better turning ability than a six axle artic with a semi-trailer having a 9m wheelbase or a Super Link Combination.

This being so, it becomes apparent that the wheelbase of the semi-trailer is not the only influencing factor in the turning ability of the artic. The wheelbase and front overhang of the truck-tractor are just as influential.

Amend the Regulation

Therefore, it is suggested that Regulation 355 be amended by deleting the entire Regulation and substituting the following:

Turning Radius and Turning Corridor

355. No person shall operate on a public road a vehicle or a combination of vehicles:

• The turning radius of which exceeds 13.1m
  
  
• The turning ability of which does not permit the vehicle or combination of vehicles to negotiate a 180° turn within a corridor having an outside radius of 15m and an inside radius of 5m.

The definition of “turning radius” is included in the existing regulations. In the event of the proposed amendment, the following simple definition could be introduced:

“TURNING CORRIDOR” means a demarcated roadway having an outside radius of 15m and an inside radius of 5m extending through 180°.

Concern has been expressed that there could be a problem enforcing legislation on the turning corridor because vehicles, artics and combinations change and are tested for roadworthiness separately. But, the position applies to many aspects of artics and combinations. For example:

• the prescribed length of an artic is 18.5m, and
• the prescribed length of a combination is 22m

But these vehicles are not checked unless monitored on the open road and either measured by a law enforcement officer or directed to a vehicle testing station for checking.

The checking for compliance with the turning corridor is no different. Furthermore, there is an incentive to an operator to voluntarily check for compliance with the turning corridor without being constrained by wheelbase.

Benefits derived from using the Turning Corridor method 

• A much improved mass distribution can be achieved, particularly in the case of artics;
  
  
• Without the semi-trailer wheelbase constraint, the six axle artic will be able to achieve the gross mass of the sum of the axle/axle unit massloads.

It is becoming more apparent that Europe is adopting the turning corridor method of exercising some control over artics and combinations other than by overall length and wheelbase. The subject was taken up in the February issue of the IRTE Transport Engineer in which John Dickson-Simpson uses the turning corridor in his article “Running Rings Around Archaic Rules and Regulations” to gain acceptance of a semi-trailer 2.5m longer than the current English law allows.

The economic commission for Europe (ECE) has adopted the turning corridor system which is being fully recognised as confirmed in literature and publications. The good news is that the IRTE Technical Committee has indicated to FleetWatch that it is supporting and promoting the suggestions made.