Driver Knowledge Tests

How do you move a 5th wheel to keep your load balanced?

On some prime movers, the fifth wheel or turntable can be moved backwards and forwards to achieve better load balance. Overloading an axle is illegal, plus it can cause overheating and delamination of tyres, affect the brake balance and, in extreme cases, cause jackknifing.

Moving the fifth wheel forwards in this case can change an overload axle into one that is within the acceptable axle limits.

Not all fifth wheels can be moved while in situ.

This fifth wheel is fixed – the only way to change it is to remove all the bolts and reposition it.

Some fifth wheels can be unlocked and locked using a switch behind the cab.

This switch unlocks and locks the fifth wheel

To move the fifth wheel position, line the tractor unit up with the trailer while coupled, ensure the trailer brakes are on, unlock the fifth wheel slider (not the fifth wheel locking jaws) and check both pins are out, then move the prime mover either forwards or backwards. If you move the prime mover forwards, less weight will be over the steering axle and more will be over the drive axle, and vice versa.

When the fifth wheel is unlocked, the rectangular pin (slider lock plunger) you can see at the centre of this image, comes out; it’s usually sitting in one of the notches. There’s one each side. When locked, this pin moves back in. The blue coiled line on the right provides the power to do this via air pressure.

Each notch on the fifth wheel represents a certain percentage of weight shift forwards or backwards – consult the manufacturer’s documentation to find out what this is.

Moving the fifth wheel does not affect the axle loads on the rear of the semitrailer axles.

Locked the fifth wheel and do a tug test before driving away.

You can check your load balance by stopping at a weighbridge.

Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists

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