IRTE compatible code
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• Following extensive vehicle trials the Institute of Road Transport Engineers has produced a Code of Practice to help solve braking compatibility problems encountered by artic and drawbar operators. It will be launched at the IRTE Telford Show next week at £5.
Braking Compatibility of Combinations covers all airbraked vehicles and stipulates
that there should be no more than 0.3 bar (4.51b/sq in) pressure difference between the truck and trailer for the brakes to contact.
According to the IRTE code the "threshold pressure" (the pressure needed to overcome the flow resistance and friction in an air-brake system before the brakes first contact) must be between 0.6 and 0.9 bar(just under 9 to just over 131b/sq in) to achieve balanced braking.
Pressure rules currently laid down by the EC allow for a wide band of threshold pressures of 0.2 to 1 bar. However, following extensive tests since 1991 involving BTAC and IRTE member vehicles, the IRTE says this range is too wide and that many trailer brakes are coming on too soon.
The IRTE says: "The effect was that because most braking is done at low pressures, just for slowing down (90% of stops are at less than 2 bar air pressure), trailer brakes were being overworked, getting hot, wearing out too quickly and causing an imbalance between the powered and trailed portions of a truck-trailer combination. This raised the risk of skidding on a slippery road or diminishing maximum efficiency on a dry road."
IRTE investigators also found that many tractive units had thresholds that were not within EC limits.
"Analysis showed that a variation of more than 5% either side of the ideal threshold pressure for each brake was likely to produce unsatisfactory performance in operation. It is this tightened 5% tolerance that forms the basis of the IRTE code's conclusion that the threshold pressure should lie between 0.6 and 0.9 bar."
Copies of the code can be bought at Telford or ordered from the IRTE on 071-589 3744.