Vehicle combination and coupling: bus with trailer, weights, dimensions and load
Here the exam tests whether you can prove a combination is legal before you move it: length, width, height, axle loads, permitted total weights and coupling ratings. The numbers must be checked on paper, not estimated at the kerb.
Dimensions
- Maximum width 2.55 m for the bus and for the trailer.
- Maximum height 4.00 m, measured from the ground with the vehicle loaded.
- Maximum length of the complete combination - bus plus drawbar trailer, tip to tip including the drawbar - is 18.75 m.
- Loads may not project beyond the permitted outline without a dispensation, and a projecting load must be marked.
Weights and coupling ratings
- Each axle has a rated maximum (commonly 10 t, and 11.5 t for a drive axle with air suspension and twin tyres); the sum may not exceed the bus's permitted total weight.
- The trailer's permitted total weight - not its actual weight - must lie within the coupling's rated towing capacity and within the train weight shown on the bus's registration certificate.
- Drawbar load must sit inside the coupling's rating and above the trailer manufacturer's minimum. Too little nose weight makes the trailer unstable; too much overloads the bus's rear axle.
- Load the trailer forward and low, heaviest items over the axle(s), so the combination sits level and the tail does not wag.
The check before moving off
Confirm combination length, the trailer's permitted weight against the coupling rating, the actual axle loads, and that the breakaway cable, lights and coupling lock are engaged.
Worked example
A 12.30 m bus is coupled to a trailer with a 9.80 m body and a 1.20 m drawbar; the gap between them is 0.40 m. Total = 12.30 + 1.20 + 9.80 + 0.40 = 23.70 m, which exceeds 18.75 m, so the combination may not be used on the road however light the trailer is. Suppose the trailer body is 6.40 m: 12.30 + 1.20 + 6.40 + 0.40 = 20.30 m - still too long. Now check weights: bus permitted train weight 26,000 kg, bus 18,500 kg, trailer permitted total 8,000 kg gives 26,500 kg - 500 kg over, so the trailer must be replated or the load plan changed.
Traps
- Measuring only the trailer body and forgetting the drawbar.
- Using a weighbridge figure instead of the trailer's permitted total weight.
- Treating the bus's own gross weight as the limit for the combination.
- Loading the trailer tail-heavy, which lightens the drawbar and destabilises the combination.