Vehicle combination: coupling, load distribution and dimensions
This domain tests the physical facts of the rig: what it may measure, what it may weigh, and how the tractor and trailer are joined and checked.
Dimensions
- Width 2.55 m (2.60 m for insulated or refrigerated bodies), height 4.00 m.
- Combination length 18.75 m generally; modular combinations up to 25.25 m only on the defined modular network.
- Axle and bogie limits apply as well as the combination limit (typically 10 t per axle, 19 t per bogie); road-specific restrictions can be lower.
Coupling sequence
- Set the tractor suspension to coupling height and reverse squarely under the semi-trailer.
- Confirm the fifth wheel has locked: pull the release handle forward, inspect the locking jaw, fit the safety pin.
- Connect the red supply line, yellow service line, electrical cable and ABS/EBS line; check for chafing.
- Raise the landing legs and check mud flaps and rear underrun protection.
- Test the service brake and the parking brake before moving off, and re-check the coupling after 5 to 10 km.
Load distribution
Keep the centre of gravity low and forward of the drive axle without overloading the steer axle. Move the fifth wheel to trim axle loads, and weigh at a weighbridge rather than trusting the load plan.
Worked example
A 600 kg pallet is moved 2 m rearwards on a 4 m wheelbase. Load transferred off the front axle = 600 x 2 / 4 = 300 kg. The steer axle unloads by 300 kg and the rear axle gains it, which changes steering feel and can make an axle illegal.
Traps that cost marks
- Treating the permitted total weight as proof each axle is legal.
- Driving off before the air system is charged and the brake test done.