TRRL tips at on-board scales
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• On-board axle weighing systems are neither accurate nor reliable, says the Transport and Road Research Laboratory, after testing the three accredited "most accurate" systems available in the UK.
AB three arrive at axle weights by measuring either the bending moment of the axle or the load between the body and chassis, using the fifth wheel mounting for articulated vehicles, and the rear pivot of a tipping vehicle.
The systems, tested over a period of six months with various loads, were compared with measurements obtained using a slow speed dynamic axle weigher.
The effects of different load conditions, ground slopes and tyre pressures were also examined, but in between tests the vehicles were used normally by their owners.
Report 103 concludes: "Under legal conditions none of the systems measured axle, bogie or gross weights with errors less than plus/minus 2% of the true weight." In some instances accuracy varied by more than plus/minus 6%, only achieving the target accuracy of plus/minus 2% for 38% of non-bogie axle and 40% of bogies.
Sensors fitted to measure the bending of twin-tyred axles are influenced by unequal tyre pressures.
The full report, priced at £4, can be obtained from the Transport and Road Research Laboratory, Old Wolcingham Road, Crowthorne, Berkshire RG11 6AU.