Can you advise if there is any regulation preventing the
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carrying of one axle of a tandem axled semi-trailer when the vehicle is running empty? We have seen 32-ton gross outfits with "wide spread" tandem suspension on the trailers. What is the reason for this?
AThe retracting of a trailer axle when the
outfit is running empty is permissible providing that the axle so retracted is an inner one. Generally the retraction of a rearmost axle would put the vehicle outside the regulations relating to maximum overhang. It is important when running in this condition that the outfit remains safe and roadworthy. Widespread trailer bogies are sometimes fitted to four-axle 32-ton gross combinations to comply with provisions relating to axle loading and outer axle spacing. Where sufficient outer axle spread 138ft) is to be obtained with a four-axle 32-ton gross outfit, the dimension between the semi-trailer kingpin and the centre of a close-coupled bogie is such that excessive imposed load may be thrown upon the coupling and single-drive axle.
In order to reduce fifth-wheel loading it is necessary to move the leading trailer axle forward so that a greater proportion of the imposed load can be absorbed by the bogie. It is still necessary, however, to .retain the 38ft outer axle spread, hence the unorthodox bogie formation.