Loads and forces (6)
Page 44
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• The factor responsible for any artic actually over-turning towards the centre of a roundabout—a less common occurrence than those I have dealt with earlier in this series—is excessive speed at entry. The left-hand entry turn is made but the driver is unable to hold the vehicle within the radius of the kerb, and the nearside steered wheel bites at the kerb with enough kick to take control from the driver's hands, placing the vehicle instantly into full left-lock.. I know of one case where a driver's thumb was dislocated by the whip of the steering wheel spokes.
The effect of this full ten turn places the tractive unit almost at right angles to the trailer. The load reaction from this move is to apply such power on the return sway to the right that the load is invariably displaced to the right, that is towards the centre island. Often the trailer will go with the load, aided to some extent by the fore-and-aft action at the turntable hinge pins.
This is one instance where things are really twisted about by the viciousness of the stop, which is first a jacknife as the tractive unit is jerked to the left, followed by the fearful leverage and twist as the trailer and load seek relief by toppling to the right.
Quite apart from the general mess, damaged and spilled load, the blocked roundabout and disruption to other traffic, this is one form of overturning that has the most expensive results. The term "joggled" which is used in the steel trade as meaning, kinked, cranked, and generally bent, can be applied to both tractive unit and trailer after such an incident. Everything from the turntable outwards is shifted and twisted about-end needs a full strip and rebuild to put back in order. This form of accident, the direct result of speed and poor judgment, is worth a thorough investigation and general discussion, because it need never happen.
Where a vehicle has placed its nearside steered wheel or wheels on a soft verge, grass, clay or soil, the kick from this has to be experienced to be appreciated. Therefore I must point. out that although a seeded grass verge alongside a trunk road looks very nice, it can cause a great deal of mischief if touched by the steered wheel of a large vehicle. Experienced drivers all prefer a hard shoulder by the roadside. It can save many costly incidents by permitting control to be maintained.