Driver Knowledge Tests

What is a lift axle or retractable axle?

A retractable axle or lift axle is a way in which the axle can be raised so that its tyres aren’t touching the road surface. This improves fuel economy, and reduces maintenance and tyre wear, although it increases road wear.

The advantage of a lift axle is that it provides more scope for carrying heavier loads without having negative impacts when the truck and/or trailer are empty.

This American tanker truck has a lift axle ahead of the tri-axle set

Every tyre has a rolling resistance – it’s the force required to make it roll on the road. Burning fuel is the way this is overcome. The rolling resistance heats the tyre up and that heat plus friction from the road surface is what degrades the tyre.

On a tri or quad trailer (i.e. three or four axles) the rearmost axle scrubs sideways around corners. Trailers may have a rearmost axle which turns to reduce this, but lifting is a better option to reduce the scrubbing which wears the tyre out more quickly.

Trucks have multiple axles with two or four tyres per axle to spread the weight over the road surface. There’s a maximum weight per type of axle, therefore to carry heavy weights, trucks need multiple axles. But, when the truck or trailer is empty, there’s no reason to have those axles rolling when the weight is not there to justify it.

While this does create slightly more damage to the road surface than spreading the weight over a large number of axles, road authorities allow for retractable axles under certain conditions which are explained here.

This heavy rigid truck has a tag axle at the back

What are the types of lift axle?

If the axle is behind the primary drive axle, it’s a pusher axle. If the axle is the rearmost axle on a trailer, it’s a tag axle. Retractable axles are sometimes called dead axles or drop axles.

The rear axle of this crane truck is raised. The additional axle provides extra stability for heavy loads but isn’t required once the load has been removed.

Does a retractable axle provide any power?

Retractable axles are freely rotating – they only spin when they are in contact with the road’s surface and don’t have any connection to the drive shaft.

Are the benefits of a retractable axle proven?

A study by the Green Truck Partnership found that a lift axle resulted in 0.823l/100km fuel consumption improvement and $1.15/100km economic benefit.

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

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