Driving tests

How to prevent overloading on a truck’s axles by sliding the fifth wheel

Getting your load’s centre of gravity as low as possible and having it restrained securely are two of the three main tasks when transporting a load. The third is ensuring that you’re not overloading an axle (if you don’t know how to calculate your maximum legal load, you need to do this axle weight limits course). You can check the relative axle weights using a weighbridge, but what can you do if you’re overloaded? An overloaded axle is illegal, it will compromise your braking performance, affect your steering, put more wear on your tyres and increase the risk of a rollover. If you’re unsure of the legal requirements for restraining a load, take this load security course.

If you’re driving a tractor unit with a moveable fifth wheel, it’s possible to change the weight distribution between the drive axle(s) and the steering axle(s).

If there’s too much weight over the front axle, the fifth wheel can be moved backwards to shift more weight over the drive axles, and vice versa. This does not change the weight balance on the rear axles. It also won’t help if you are carrying too much for both axles or if you are overloaded across all axles. If you are measuring your truck with its fuel tanks empty, bear in mind that 10 litres of diesel weighs 8.4kg.

Be sure to check when moving the fifth wheel backwards that you don’t exceed the maximum length of a heavy vehicle.

Ideally, the fifth wheel should be as close to the cab as possible to help with aerodynamics. Reducing the gap between the cab and the trailer reduces turbulence and will improve fuel efficiency.

How do you move the fifth wheel on a tractor unit?

First, you need to ensure that you know how to couple a truck and semitrailer and be familiar with the instructions on your sliding fifth wheel. These are often on a panel on the truck.

Fifth wheel is locked. Flick the switch left to unlock it. If it’s outside the cab, it is an important thing to check when you do your walkaround, just in case someone has tampered with it.

The switch locks and unlocks the slider lock plunger from the fifth wheel assembly. It usually sits in one of the notches you can see below. The blue air hose provides the air pressure to move the plunger.

If the plunger won’t move, you may need to reduce the pressure on it between the tractor and trailer.

Process for moving the sliding fifth wheel

  1. Apply the trailer brakes and ensure the trailer is securely connected (i.e. locking jaws are around the fifth wheel and you don’t have a ‘high hook’)
  2. Ensure that the tractor unit and trailer are in line with one another – trying to move the fifth wheel with the tractor at an angle causes huge stresses on the assembly that can damage it
  3. Unlock the fifth wheel using the locking switch. If it doesn’t unlock, there’s too much pressure between the tractor unit and trailer
  4. Move the tractor unit backwards to put more weight over the steering axles or forwards to put more weight over the drive axles.
  5. Apply the parking brake on the tractor unit
  6. Lock the fifth wheel, then check by doing a tug test.

Checks when moving the fifth wheel

Before moving, check both slide plunger pins are out (there’s one on either side). As mentioned above, reducing the tension between the trainer and tractor should work, but keep this area properly lubricated and maintained.

Before setting off, double-check both slide plunger pins are correctly seated back into the notched rail. Again, reduce the pressure between the cab and trailer if they are not and try unlocking and locking again.

This assembly should be checked as part of your pre-trip inspection and serviced regularly according to the manufacturer’s instructions.

Do a tug test before moving away. An easy way to shear the kingpin off the semitrailer skidplate and bend components in your fifth wheel is for the slide plunger to not be engaged properly causing the whole trailer to slide forwards with the fifth wheel when you brake.

Can all fifth wheels be moved?

Not all fifth wheels can be moved without undoing bolts and repositioning it on the rail.

Some fifth wheels are dual-height and can only be moved up and down.

Dual height fifth wheel down
Dual height fifth wheel up
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Darren has written over 3000 articles about driving and vehicles, plus almost 500 vehicle reviews and numerous driving courses. Connect with him on LinkedIn by clicking the name above

Posted in Advice